Human infection patterns and heterogeneous exposure in river blindness

Short- and Long-Range Dispersal by Members of the Simulium damnosum Complex (Diptera: Simuliidae), Vectors of Onchocerciasis: A Review

Blackfly members of the Simulium damnosum complex are major vectors of the parasite that causes onchocerciasis in Africa and Yemen, with other vector species involved in a few localized areas of Africa and in the Neotropics. Although the life cycle of these blackflies is linked to fast-flowing rivers, they can travel long distances (up to at least 500 km), calling into question how transmission zones are defined. Knowledge of the short- and long-range dispersal of these vectors could inform where control interventions and monitoring are necessary if targets for onchocerciasis elimination are to be met. Yet, research on blackfly dispersal has been limited and fragmented over the last 70 years. Here, we review the literature on the dispersal of onchocerciasis vectors, and we show the need for further research to establish how far larvae can travel downstream; the extent to which adults invade transmission zones; and whether adults migrate in a series of successive short movements or in single long-distance shifts, or use both methods.

Nodding syndrome, a case-control study in Mahenge, Tanzania: Onchocerca volvulus and not Mansonella perstans as a risk factor

BACKGROUND

Nodding syndrome (NS) has been consistently associated with onchocerciasis. Nevertheless, a positive association between NS and a Mansonella perstans infection was found in South Sudan. We aimed to determine whether the latter parasite could be a risk factor for NS in Mahenge.

METHODS

Cases of epilepsy were identified in villages affected by NS in Mahenge, Tanzania, and matched with controls without epilepsy of the same sex, age and village. We examined blood films of cases and controls to identify M. perstans infections. The participants were also asked for sociodemographic and epilepsy information, examined for palpable onchocercal nodules and onchocerciasis-related skin lesions and tested for anti-Onchocerca volvulus antibodies (Ov16 IgG4) by ELISA. Clinical characteristics of cases and controls, O. volvulus exposure status and relevant sociodemographic variables were assessed by a conditional logistic regression model for NS and epilepsy status matched for age, sex and village.

RESULTS

A total of 113 epilepsy cases and 132 controls were enrolled, of which, respectively, 56 (49.6%) and 64 (48.5%) were men. The median age in cases and controls was 28.0 (IQR: 22.0-35.0) and 27.0 (IQR: 21.0-33.3) years. Of the persons with epilepsy, 43 (38.1%) met the probable NS criteria and 106 (93.8%) had onchocerciasis-associated epilepsy (OAE). M. perstans infection was absent in all participants, while Ov16 seroprevalence was positively associated with probable NS (odds ratio (OR): 5.05, 95%CI: 1.79-14.27) and overall epilepsy (OR: 2.03, 95%CI: 1-07-3.86). Moreover, onchocerciasis-related skin manifestations were only found in the cases (n = 7, p = 0.0040), including persons with probable NS (n = 4, p = 0.0033). Residing longer in the village and having a family history of seizures were positively correlated with Ov16 status and made persons at higher odds for epilepsy, including probable NS.

CONCLUSION

In contrast to O. volvulus, M. perstans is most likely not endemic to Mahenge and, therefore, cannot be a co-factor for NS in the area. Hence, this filaria is unlikely to be the primary and sole causal factor in the development of NS. The main risk factor for NS remains onchocerciasis.

Novel epidemiological model of gastrointestinal nematode infection to assess grazing cattle resilience by integrating host growth, parasite, grass and environmental dynamics

Gastrointestinal nematode (GIN) infections are ubiquitous and often cause morbidity and reduced performance in livestock. Emerging anthelmintic resistance and increasing change in climate patterns require evaluation of alternatives to traditional treatment and management practices. Mathematical models of parasite transmission between hosts and the environment have contributed towards the design of appropriate control strategies in ruminants, but have yet to account for relationships between climate, infection pressure, immunity, resources, and growth. Here, we develop a new epidemiological model of GIN transmission in a herd of grazing cattle, including host tolerance (body weight and feed intake), parasite burden and acquisition of immunity, together with weather-dependent development of parasite free-living stages, and the influence of grass availability on parasite transmission. Dynamic host, parasite and environmental factors drive a variable rate of transmission. Using literature sources, the model was parametrised for Ostertagia ostertagi, the prevailing pathogenic GIN in grazing cattle populations in temperate climates. Model outputs were validated on published empirical studies from first season grazing cattle in northern Europe. These results show satisfactory qualitative and quantitative performance of the model; they also indicate the model may approximate the dynamics of grazing systems under co-infection by O. ostertagi and Cooperia oncophora, a second GIN species common in cattle. In addition, model behaviour was explored under illustrative anthelmintic treatment strategies, considering impacts on parasitological and performance variables. The model has potential for extension to explore altered infection dynamics as a result of management and climate change, and to optimise treatment strategies accordingly. As the first known mechanistic model to combine parasitic and free-living stages of GIN with host feed-intake and growth, it is well suited to predict complex system responses under non-stationary conditions. We discuss the implications, limitations and extensions of the model, and its potential to assist in the development of sustainable parasite control strategies.

Relationship between skin snip and Ov16 ELISA: Two diagnostic tools for onchocerciasis in a focus in Cameroon after two decades of ivermectin-based preventive chemotherapy

Background

Onchocerciasis elimination currently relies on repeated ivermectin-based preventive chemotherapy. Current World Health Organization’s guidelines strongly recommend, though with low evidence of certainty, the use of Ov16 serology testing in children younger than 10 years old to assess whether mass drugs administration can be safely stopped. Therefore, more evidences are needed to support the use of this marker as sero-evaluation tool. This study aimed at determining the relationship between microfilaridermia and anti-Ov16 IgG4, and their variation according to age, gender and ivermectin intake history.

Methodology

A cross-sectional survey was conducted in an area where ivermectin-based MDA has been implemented since more than 20 years. A questionnaire was used to record ivermectin intake history for the last 5 years. All volunteers aged ≥2 years were tested for microfilaridermia. IgG4 antibodies against Ov16 antigen were determined using the Standard Diagnostic Ov16 IgG4 ELISA kits and the recombinant anti-Ov16 AbD19432 antibodies. Prevalences, microfilaridermia counts and IgG4 concentrations were compared with regards to age, gender and history of ivermectin intake.

Principal findings

The prevalence of skin microfilariae was 23.4% (95% CI: 23.4–30.8), whereas Ov16 seroprevalence was 53.2% (95% CI: 47.9–58.4). A moderate positive percentage agreement (50.4%) and a high negative percentage agreement (69.2%) was found between skin snip and Ov16 serology in the whole population, while in children aged <10 years, the agreements were higher (positive percentage agreement: 62.6%; negative percentage agreement: 83.5%). In addition, no associations were found between ivermectin intake, Mf counts and estimated IgG4 concentration of participants. Anti-Ov16 IgG4 were higher in individuals harboring microfilariae than their negative counterparts (p<0.0001), though a negative correlation was found between skin microfilarial counts and anti-Ov16 IgG4 levels (r = -0.2400; p = 0.03). No variation in microfilarial counts according to age and gender was observed. Though positively correlated with age (r = 0.4020; p<0.0001), IgG4 was significantly different between the different age classes (p<0.0001).

Conclusion/Significance

Our results revealed moderate positive and negative agreements between parasitological and immunological parameters of onchocerciasis infection after several rounds MDA. Anti-Ov16 IgG4 levels increased with age but decreased with microfilarial counts, suggesting a variation of anti-Ov16 IgG4 as a result of constant exposure and accumulation of infection. This brings evidence sustaining the use of Ov16 serology in children as evaluation tool. However, additional investigations are needed to further reshape the appropriate age range among children aged <10 years old.

The elimination of onchocerciasis places high demands on monitoring and evaluation. The current WHO’s guidelines recommend the use of serological test (ELISA) to determine the presence of IgG4 antibodies to the O. volvulus specific antigen Ov16 among children aged <10 years old, thought with low evidence of certainty. In this paper, we explored the relationship between anti-Ov16 IgG4 antibodies and microfilaridermia counts, and assessed their variation according to age, gender and history of ivermectin intake. Our findings revealed no variation of Mf count and IgG4 with ivermectin intake. However, we observed that anti-Ov16 IgG4 decrease with microfilaridermia counts, but an increasing trend was observed with age. This brings evidence sustaining the use of Ov16 serology testing as exposition marker in children younger than 10 years. However, children age 2 to 4 years seem to have a very low anti-Ov16 IgG4 concentration, this finding should be considered when defining the age class for seroprevalence evaluation.

High infection rates for onchocerciasis and soil-transmitted helminthiasis in children under five not receiving preventive chemotherapy: a bottleneck to elimination

BACKGROUND

The current mainstay for control/elimination of onchocerciasis and soil-transmitted helminthiasis (STH) relies on ivermectin- and mebendazole/albendazole-based preventive chemotherapies. However, children under five years of age have been excluded in both research activities and control programs, because they were believed to have insignificant infection rates. There is therefore a need for up-to-date knowledge on the prevalence and intensity of STH and onchocerciasis infections in this age group. This study aimed at assessing the rates and intensities of onchocerciasis and STH infections in children under five years of age who are excluded from ivermectin- or mebendazole/albendazole-based preventive chemotherapies.

METHODS

A series of cross-sectional surveys was conducted in four Health Districts in the Centre and Littoral Regions of Cameroon between 2018 and 2019. All subjects aged 2 to 4 years, were screened for prevalence (or infection rate) and intensity [number of eggs per gram of stool (epg) or number of microfilariae per skin snip (mf/ss)] of STH and onchocerciasis infections respectively using the Kato-Katz and skin snip methodologies. Chi-square and the non-parametric tests (Mann Whitney and Kruskal Wallis) were used to compare infection rates and intensities of infections between Health Districts and genders, respectively.

RESULTS

A total of 421 children were enrolled in this study. The overall prevalence of onchocerciasis was 6.6% [95% confidence interval (CI): 4.3‒9.9], ranging from 3.6% (in the Ntui Health District) to 12.2% (in the Bafia Health District). The intensity of infection ranged from 0.5 to 46 microfilariae per skin snip [median: 5; interquartile range (IQR): 2.25‒8.5]. The overall prevalence of STH was 9.6% (95% CI: 6.5‒13.9), with a high infection rate (29.6%) in the Akonolinga Health District. Two STH species (Ascaris lumbricoides and Trichuris trichiura) were found among infected individuals. The median intensities of STH infections were 1,992 epg (IQR: 210‒28,704) and 96 epg (IQR: 48‒168) for A. lumbricoides and T. trichiura, respectively.

CONCLUSIONS

This study reveals that children < 5 years of age are highly infected with STH and onchocerciasis, and could contribute to the spread of these diseases, perpetuating a vicious circle of transmission and hampering elimination efforts. These findings reveal the urgent need to provide (or scale) treatments (likely pediatric formulations) to these preschool-aged children, especially in areas of high transmission, to accelerate efforts to reach WHO 2030 target.

Anti-Th17 and anti-Th2 responses effects of hydro-ethanolic extracts of Aframomum melegueta, Khaya senegalensis and Xylopia aethiopica in hyperreactive onchocerciasis individuals’ peripheral blood mononuclear cells

Hyperreactive onchocerciasis (HO) is characterized by a severe skin inflammation with elevated Th17-Th2 combined responses. We previously demonstrated the anthelminthic activity of Aframomum melegueta (AM), Xylopia aethiopica (XA) and Khaya senegalensis (KS) used by traditional healers to treat helminthiasis in the endemic area of Togo. However, their effect on severe onchocerciasis is poorly investigated. The present study aimed to investigate the anti-Th17 and anti-Th2 effects of hydro-ethanolic extracts of AM, XA and KS during HO. Onchocerca volvulus-infected individuals were recruited in the Central region of Togo in 2018. Isolated peripheral blood mononuclear cells (PBMCs) from both generalized onchocerciasis (GEO) and HO forms were activated with anti-CD3 and anti-CD28 monoclonal antibodies in the presence or absence of the hydro-ethanolic extracts of AM, XA and KS as well as their delipidated, deproteinized and deglycosylated fractions. After 72 hours, cytokines were assayed from cell culture supernatants. Then, flow cytometry was used to investigate the effects of the extracts on cell activation, proliferation, intracellular cytokines and T cells transcription factors. The production of both Th17 and Th2 cytokines IL-17A and IL-5 were significantly inhibited upon T-cell receptor (TCR) activation in the presence of the hydro-ethanolic extracts of AM, XA and KS in HO individuals’ PBMCs in vitro. AM and XA inhibited CD4⁺RORC2⁺IL-17A⁺ and CD4⁺GATA3⁺IL-4⁺ cell populations induction. This inhibition was not Th1 nor Treg-dependent since both IFN-γ and IL-10 were also inhibited by the extracts. AM and XA did not interfere with T cell activation and proliferation for their inhibitory pathways. Lipid and protein compounds from AM and XA were associated with the inhibition of IL-17A. This study showed that in addition to their anthelminthic effects, hydro-ethanolic extracts of Aframomum melegueta, Xylopia aethiopica and Khaya senegalensis could downregulate both Th17 and Th2 responses and prevent the severe skin disorder observed.

Severe form of human onchocerciasis also called hyperreactive onchocerciasis (HO) is characterized by skin disorders such as dermatitis. Our previous study showed that concomitant Th2 and Th17 responses play a role in the pathophysiology of the disease. In Onchocerca volvulus endemic areas such as Togo, traditional healers (TH) used to treat this disease with medicinal plants such as Aframomum melegueta (AM), Xylopia aethiopica (XA) and Khaya senegalensis (KS). Mass drug administration including ivermectin and albendazole has been used over the past decade to control and eradicate helminths infections. Despite its successes, resistance to ivermectin has been reported and therefore, alternative drugs are urgently needed. We previously confirmed in vitro the anthelminthic effects of Aframomum melegueta (AM), Xylopia aethiopica (XA) and Khaya senegalensis (KS). However, their effect on severe onchocerciasis has not been demonstrated. Here, the anti-Th17 and anti-Th2 effects of hydro-ethanolic extracts of AM, XA and KS during HO were investigated. The data showed that in addition to their helminth-killing effects, plant-derived molecules AM, XA and KS downregulated Th2 and Th17 profiles and therefore, could be candidates for the development of new drugs not only for the treatment of helminth-induced inflammatory pathologies but also auto-immune Th2/Th17-dependent inflammatory diseases in general.

Demographic patterns of human antibody levels to Simulium damnosum s.l. saliva in onchocerciasis-endemic areas: An indicator of exposure to vector bites

BACKGROUND

In onchocerciasis endemic areas in Africa, heterogenous biting rates by blackfly vectors on humans are assumed to partially explain age- and sex-dependent infection patterns with Onchocerca volvulus. To underpin these assumptions and further improve predictions made by onchocerciasis transmission models, demographic patterns in antibody responses to salivary antigens of Simulium damnosum s.l. are evaluated as a measure of blackfly exposure.

METHODOLOGY/PRINCIPAL FINDINGS

Recently developed IgG and IgM anti-saliva immunoassays for S. damnosum s.l. were applied to blood samples collected from residents in four onchocerciasis endemic villages in Ghana. Demographic patterns in antibody levels according to village, sex and age were explored by fitting generalized linear models. Antibody levels varied between villages but showed consistent patterns with age and sex. Both IgG and IgM responses declined with increasing age. IgG responses were generally lower in males than in females and exhibited a steeper decline in adult males than in adult females. No sex-specific difference was observed in IgM responses.

CONCLUSIONS/SIGNIFICANCE

The decline in age-specific antibody patterns suggested development of immunotolerance or desensitization to blackfly saliva antigen in response to persistent exposure. The variation between sexes, and between adults and youngsters may reflect differences in behaviour influencing cumulative exposure. These measures of antibody acquisition and decay could be incorporated into onchocerciasis transmission models towards informing onchocerciasis control, elimination, and surveillance.

Why onchocerciasis transmission persists after 15 annual ivermectin mass drug administrations in South-West Cameroon

Introduction

Onchocerciasis is targeted for elimination mainly with annual community-directed treatment with ivermectin (CDTI). High infection levels have been reported in South-West Cameroon, despite ≥15 years of CDTI. The aim of this study was to assess factors associated with continued onchocerciasis transmission and skin disease.

Methods

A large-scale cross-sectional study was conducted in 2017 in 20 communities in a loiasis-risk area in South-West Cameroon. A mixed-methods approach was used. Associations between infection levels, skin disease and adherence to CDTI were assessed using mixed regression modelling. Different community members’ perception and acceptability of the CDTI strategy was explored using semi-structured interviews.

Results

Onchocerciasis prevalence was 44.4% among 9456 participants. 17.5% of adults were systematic non-adherers and 5.9% participated in ≥75% of CDTI rounds. Skin disease affected 1/10 participants, including children. Increasing self-reported adherence to CDTI was associated with lower infection levels in participants aged ≥15 years but not in children. Adherence to CDTI was positively influenced by perceived health benefits, and negatively influenced by fear of adverse events linked with economic loss. Concern of lethal adverse events was a common reason for systematic non-adherence.

Conclusion

CDTI alone is unlikely to achieve elimination in those high transmission areas where low participation is commonly associated with the fear of adverse events, despite the current quasi absence of high-risk levels of loiasis. Such persisting historical memories and fear of ivermectin might impact adherence to CDTI also in areas with historical presence but current absence of loiasis. Because such issues are unlikely to be tackled by CDTI adaptive measures, alternative strategies are needed for onchocerciasis elimination where negative perception of ivermectin is an entrenched barrier to community participation in programmes.

Serological Evaluation of Onchocerciasis and Lymphatic Filariasis Elimination in the Bakoye and Falémé Foci, Mali

Background

Ivermectin-based onchocerciasis elimination, reported in 2009–2012, for Bakoye and Falémé, Mali, supported policy-shifting from morbidity control to elimination of transmission (EOT). These foci are coendemic with lymphatic filariasis (LF). In 2007–2016 mass ivermectin plus albendazole administration was implemented. We report Ov16 (onchocerciasis) and Wb123 (LF) seroprevalence after 24–25 years of treatment to determine if onchocerciasis EOT and LF elimination as a public health problem (EPHP) have been achieved.

Methods

The SD Bioline Onchocerciasis/LF Ig[immunoglobulin]G4 biplex rapid diagnostic test (RDT) was used in 2186 children aged 3–10 years in 13 villages (plus 2 hamlets) in Bakoye and in 2270 children in 15 villages (plus 1 hamlet) in Falémé. In Bakoye, all-age serosurveys were conducted in 3 historically hyperendemic villages (1867 individuals aged 3 -78 years).

Results

In Bakoye, IgG4 seropositivity was 0.27% (95% confidence interval [CI] = .13%–.60%) for both Ov16 and Wb123 antigens. In Falémé, Ov16 and Wb123 seroprevalence was 0.04% (95% CI = .01%–.25%) and 0.09% (95% CI = .02%–.32%), respectively. Ov16-seropositive children were from historically meso/hyperendemic villages. Ov16 positivity was <2% in ≤14 year-olds, and 16% in ≥40 year-olds. Wb123 seropositivity was <2% in ≤39 year-olds, reaching 3% in ≥40 year-olds.

Conclusions

Notwithstanding uncertainty in the biplex RDT sensitivity, Ov16 and Wb123 seroprevalence among children in Bakoye and Falémé is consistent with EOT (onchocerciasis) and EPHP (LF) since stopping treatment in 2016. The few Ov16-seropositive children should be skin-snip polymerase chain reaction tested and followed up.

A systematic review and an individual patient data meta-analysis of ivermectin use in children weighing less than fifteen kilograms: Is it time to reconsider the current contraindication?

BACKGROUND

Oral ivermectin is a safe broad spectrum anthelminthic used for treating several neglected tropical diseases (NTDs). Currently, ivermectin use is contraindicated in children weighing less than 15 kg, restricting access to this drug for the treatment of NTDs. Here we provide an updated systematic review of the literature and we conducted an individual-level patient data (IPD) meta-analysis describing the safety of ivermectin in children weighing less than 15 kg.

METHODOLOGY/PRINCIPAL FINDINGS

A systematic review was conducted using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) for IPD guidelines by searching MEDLINE via PubMed, Web of Science, Ovid Embase, LILACS, Cochrane Database of Systematic Reviews, TOXLINE for all clinical trials, case series, case reports, and database entries for reports on the use of ivermectin in children weighing less than 15 kg that were published between 1 January 1980 to 25 October 2019. The protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO): CRD42017056515. A total of 3,730 publications were identified, 97 were selected for potential inclusion, but only 17 sources describing 15 studies met the minimum criteria which consisted of known weights of children less than 15 kg linked to possible adverse events, and provided comprehensive IPD. A total of 1,088 children weighing less than 15 kg were administered oral ivermectin for one of the following indications: scabies, mass drug administration for scabies control, crusted scabies, cutaneous larva migrans, myiasis, pthiriasis, strongyloidiasis, trichuriasis, and parasitic disease of unknown origin. Overall a total of 1.4% (15/1,088) of children experienced 18 adverse events all of which were mild and self-limiting. No serious adverse events were reported.

CONCLUSIONS/SIGNIFICANCE

Existing limited data suggest that oral ivermectin in children weighing less than 15 kilograms is safe. Data from well-designed clinical trials are needed to provide further assurance.

Taking the strain out of onchocerciasis? A reanalysis of blindness and transmission data does not support the existence of a savannah blinding strain of onchocerciasis in West Africa

Onchocerciasis (also known as 'river blindness'), is a neglected tropical disease (NTD) caused by the (Simulium-transmitted) filarial nematode Onchocerca volvulus. The occurrence of 'blinding' (savannah) and non-blinding (forest) parasite strains and the existence of corresponding, locally adapted Onchocerca-Simulium complexes were postulated to explain greater blindness prevalence in savannah than in forest foci. As a result, the World Health Organization (WHO) Onchocerciasis Control Programme in West Africa (OCP) focused anti-vectorial and anti-parasitic interventions in savannah endemic areas. In this paper, village-level data on blindness prevalence, microfilarial prevalence, and transmission intensity (measured by the annual transmission potential, the number of infective, L3, larvae per person per year) were extracted from 16 West-Central Africa-based publications, and analysed according to habitat (forest, forest-savannah mosaic, savannah) to test the dichotomous strain hypothesis in relation to blindness. When adjusting for sample size, there were no statistically significant differences in blindness prevalence between the habitats (one-way ANOVA, P=0.68, mean prevalence for forest=1.76±0.37 (SE); mosaic=1.49±0.38; savannah=1.89±0.26). The well-known relationship between blindness prevalence and annual transmission potential for savannah habitats was confirmed and shown to hold for (but not to be statistically different from) forest foci (excluding data from southern Côte d'Ivoire, in which blindness prevalence was significantly lower than in other West African forest communities, but which had been the focus of studies leading to the strain-blindness hypothesis that was accepted by OCP planners). We conclude that the evidence for a savannah blinding onchocerciasis strain in simple contrast with a non-blinding forest strain is equivocal. A re-appraisal of the strain hypothesis to explain patterns of ocular disease is needed to improve understanding of onchocerciasis epidemiology and disease burden estimates in the light of the WHO 2030 goals for onchocerciasis.

Structural Uncertainty in Onchocerciasis Transmission Models Influences the Estimation of Elimination Thresholds and Selection of Age Groups for Seromonitoring

BACKGROUND

The World Health Organization recommends monitoring Onchocerca volvulus Ov16 serology in children aged <10 years for stopping mass ivermectin administration. Transmission models can help to identify the most informative age groups for serological monitoring and investigate the discriminatory power of serology-based elimination thresholds. Model predictions depend on assumed age-exposure patterns and transmission efficiency at low infection levels.

METHODS

The individual-based transmission model, EPIONCHO-IBM, was used to assess (1) the most informative age groups for serological monitoring using receiver operating characteristic curves for different elimination thresholds under various age-dependent exposure assumptions, including those of ONCHOSIM (another widely used model), and (2) the influence of within-human density-dependent parasite establishment (included in EPIONCHO-IBM but not ONCHOSIM) on positive predictive values for different serological thresholds.

RESULTS

When assuming EPIONCHO-IBM exposure patterns, children aged <10 years are the most informative for seromonitoring; when assuming ONCHOSIM exposure patterns, 5-14 year olds are the most informative (as published elsewhere). Omitting density-dependent parasite establishment results in more lenient seroprevalence thresholds, even for higher baseline infection prevalence and shorter treatment durations.

CONCLUSIONS

Selecting appropriate seromonitoring age groups depends critically on age-dependent exposure patterns. The role of density dependence on elimination thresholds largely explains differing EPIONCHO-IBM and ONCHOSIM elimination predictions.

Designing antifilarial drug trials using clinical trial simulators

Lymphatic filariasis and onchocerciasis are neglected tropical diseases (NTDs) targeted for elimination by mass (antifilarial) drug administration. These drugs are predominantly active against the microfilarial progeny of adult worms. New drugs or combinations are needed to improve patient therapy and to enhance the effectiveness of interventions in persistent hotspots of transmission. Several therapies and regimens are currently in (pre-)clinical testing. Clinical trial simulators (CTSs) project patient outcomes to inform the design of clinical trials but have not been widely applied to NTDs, where their resource-saving payoffs could be highly beneficial. We demonstrate the utility of CTSs using our individual-based onchocerciasis transmission model (EPIONCHO-IBM) that projects trial outcomes of a hypothetical macrofilaricidal drug. We identify key design decisions that influence the power of clinical trials, including participant eligibility criteria and post-treatment follow-up times for measuring infection indicators. We discuss how CTSs help to inform target product profiles.

Drugs for filariases are under development and clinical trial simulators could help to inform the design of clinical trials. Here, Walker et al. use an individual-based onchocerciasis transmission model to project trial outcomes of a hypothetical macrofilaricidal drug, resolving key design choices.

Data-driven modelling and spatial complexity supports heterogeneity-based integrative management for eliminating Simulium neavei-transmitted river blindness

Concern is emerging regarding the challenges posed by spatial complexity for modelling and managing the area-wide elimination of parasitic infections. While this has led to calls for applying heterogeneity-based approaches for addressing this complexity, questions related to spatial scale, the discovery of locally-relevant models, and its interaction with options for interrupting parasite transmission remain to be resolved. We used a data-driven modelling framework applied to infection data gathered from different monitoring sites to investigate these questions in the context of understanding the transmission dynamics and efforts to eliminate Simulium neavei- transmitted onchocerciasis, a macroparasitic disease that causes river blindness in Western Uganda and other regions of Africa. We demonstrate that our Bayesian-based data-model assimilation technique is able to discover onchocerciasis models that reflect local transmission conditions reliably. Key management variables such as infection breakpoints and required durations of drug interventions for achieving elimination varied spatially due to site-specific parameter constraining; however, this spatial effect was found to operate at the larger focus level, although intriguingly including vector control overcame this variability. These results show that data-driven modelling based on spatial datasets and model-data fusing methodologies will be critical to identifying both the scale-dependent models and heterogeneity-based options required for supporting the successful elimination of S. neavei-borne onchocerciasis.

Modelling exposure heterogeneity and density dependence in onchocerciasis using a novel individual-based transmission model, EPIONCHO-IBM: Implications for elimination and data needs

BACKGROUND

Density dependence in helminth establishment and heterogeneity in exposure to infection are known to drive resilience to interventions based on mass drug administration (MDA). However, the interaction between these processes is poorly understood. We developed a novel individual-based model for onchocerciasis transmission, EPIONCHO-IBM, which accounts for both processes. We fit the model to pre-intervention epidemiological data and explore parasite dynamics during MDA with ivermectin.

METHODOLOGY/PRINCIPAL FINDINGS

Density dependence and heterogeneity in exposure to blackfly (vector) bites were estimated by fitting the model to matched pre-intervention microfilarial prevalence, microfilarial intensity and vector biting rate data from savannah areas of Cameroon and Côte d'Ivoire/Burkina Faso using Latin hypercube sampling. Transmission dynamics during 25 years of annual and biannual ivermectin MDA were investigated. Density dependence in parasite establishment within humans was estimated for different levels of (fixed) exposure heterogeneity to understand how parametric uncertainty may influence treatment dynamics. Stronger overdispersion in exposure to blackfly bites results in the estimation of stronger density-dependent parasite establishment within humans, consequently increasing resilience to MDA. For all levels of exposure heterogeneity tested, the model predicts a departure from the functional forms for density dependence assumed in the deterministic version of the model.

CONCLUSIONS/SIGNIFICANCE

This is the first, stochastic model of onchocerciasis, that accounts for and estimates density-dependent parasite establishment in humans alongside exposure heterogeneity. Capturing the interaction between these processes is fundamental to our understanding of resilience to MDA interventions. Given that uncertainty in these processes results in very different treatment dynamics, collecting data on exposure heterogeneity would be essential for improving model predictions during MDA. We discuss possible ways in which such data may be collected as well as the importance of better understanding the effects of immunological responses on establishing parasites prior to and during ivermectin treatment.

The World Health Organization 2030 goals for onchocerciasis: Insights and perspectives from mathematical modelling: NTD Modelling Consortium Onchocerciasis Group

The World Health Organization (WHO) has embarked on a consultation process to refine the 2030 goals for priority neglected tropical diseases (NTDs), onchocerciasis among them. Current goals include elimination of transmission (EOT) by 2020 in Latin America, Yemen and selected African countries. The new goals propose that, by 2030, EOT be verified in 10 countries; mass drug administration (MDA) with ivermectin be stopped in at least one focus in 34 countries; and that the proportion of the population no longer in need of MDA be equal or greater than 25%, 50%, 75% and 100% in at least 16, 14, 12, and 10 countries, respectively. The NTD Modelling Consortium onchocerciasis teams have used EPIONCHO and ONCHOSIM to provide modelling insights into these goals. EOT appears feasible in low-moderate endemic areas with long-term MDA at high coverage (≥75%), but uncertain in areas of higher endemicity, poor coverage and adherence, and where MDA has not yet, or only recently, started. Countries will have different proportions of their endemic areas classified according to these categories, and this distribution of pre-intervention prevalence and MDA duration and programmatic success will determine the feasibility of achieving the proposed MDA cessation goals. Highly endemic areas would benefit from switching to biannual or quarterly MDA and implementing vector control where possible (determining optimal frequency and duration of anti-vectorial interventions requires more research). Areas without loiasis that have not yet initiated MDA should implement biannual (preferably with moxidectin) or quarterly MDA from the start. Areas with loiasis not previously treated would benefit from implementing test-and(not)-treat-based interventions, vector control, and anti- Wolbachia therapies, but their success will depend on the levels of screening and coverage achieved and sustained. The diagnostic performance of IgG4 Ov16 serology for assessing EOT is currently uncertain. Verification of EOT requires novel diagnostics at the individual- and population-levels.

Prevalence of onchocerciasis and associated clinical manifestations in selected hypoendemic communities in Ghana following long-term administration of ivermectin

Background

Onchocerciasis is a neglected tropical disease which is still of immense major public health concern in several areas of Africa and the Americas. The disease manifests either as ocular or as dermal onchocerciasis with several symptoms including itching, nodules, skin thickening, visual impairment and blindness. Ivermectin has been an efficient microfilaricide against the causative agent of the disease (Onchocerca volvulus) but reports from some areas in Africa suggest the development of resistance to this drug. The aim of this study was to determine the prevalence of onchocerciasis and associated clinical conditions frequently associated with the disease in three endemic communities in Ghana which have been subjected to 18 to 20 rounds of mass drug administration of ivermectin. This was to help determine whether or not onchocerciasis persists in these communities.

Methods

A cross-sectional study design was adopted. Three communities (Tanfiano, Senya and Kokompe) in the Nkoranza North District of Ghana where mass drug administration of ivermectin had been ongoing for more than two decades were selected for the study. The population was randomly sampled and 114 participants recruited for the study based on the eligibility criteria. The study participants were examined for the presence of parasites and clinical manifestations of onchocerciasis following established protocols.

Results

The study showed that the prevalence of microfilaria in the Tanfiano, Senya, Kokompe communities were 13.2, 2.4, and 2.9%, with nodule prevalence being 5.3, 4.9 and 14.3% respectively. Females in the study communities had a higher prevalence of microfilaria carriers (5.17%) relative to males (2.44%), but this difference was not statistically significant (p = 0.2800, unpaired t test). The most frequent clinical manifestation observed in this study among all participants was dermatitis (25.4%), followed by visual impairment & nodules (7.9% each) and then by blindness (4.4%).

Conclusion

The study showed that despite several years of mass drug administration with ivermectin, infection with onchocerciasis and the commonly associated clinical manifestations of the disease still persist in the study communities. This calls for a greater urgency for research and development aimed at discovering new or repurposed anti-filarial agents which will augment ivermectin if global onchocerciasis eradication targets are to be achieved.

On-going transmission of human onchocerciasis in the Massangam health district in the West Region of Cameroon: Better understanding transmission dynamics to inform changes in programmatic interventions

BACKGROUND

Massangam health district (HD), in the West Region of Cameroon, has received ivermectin mass drug administration (MDA) for 20 years, however there is evidence of continued high transmission of Onchocerca volvulus. In order to better understand the transmission dynamics in the HD and inform intervention strategies there is a need to delineate the boundaries of the suspected area of high transmission within the wider transmission zone.

METHODOLOGY/PRINCIPAL FINDINGS

Parasitological and entomological surveys were conducted to map out the breeding sites of Simulium damnosum and evaluate the prevalence of onchocerciasis in neighbouring communities, including Makouopsap sentinel community. Potential rapids were prospected for identification of S. damnosum larvae and black flies collected to determine infectivity rates. Adults were assessed for the presence of O. volvulus microfilariae through a skin snip biopsy and examined for the presence of nodules. Anti Ov-16 antibodies were tested for in children. Four perennial breeding sites were identified on the Rivers Mbam and Nja. Large number of flies were collected along the River Mbam, especially in the rainy season, with up to 955 flies per day, suggesting this river is a perennial source of black flies. A total of 0.8% of parous flies were infective across the study area. Parasitological studies provided evidence of high rates of infection in the sentinel community and three neighbouring communities, with 37.1% of adults microfilariae positive in Makouopsap. High Ov-16 seropositivity in children also provided evidence of recent on-going transmission. In comparison, communities sampled further away from the sentinel community and neighbouring breeding sites were much closer to reaching onchocerciasis elimination targets.

CONCLUSIONS/SIGNIFICANCE

This study provides evidence of a particular geographic area of high transmission in an approximate 12 km range around the sentinel community of Makouopsap and the neighbouring breeding sites on the River Nja. To eliminate onchocerciasis by 2025, there is a need to explore alternative intervention strategies in this area of high transmission.

Substantiating freedom from parasitic infection by combining transmission model predictions with disease surveys

Stopping interventions is a critical decision for parasite elimination programmes. Quantifying the probability that elimination has occurred due to interventions can be facilitated by combining infection status information from parasitological surveys with extinction thresholds predicted by parasite transmission models. Here we demonstrate how the integrated use of these two pieces of information derived from infection monitoring data can be used to develop an analytic framework for guiding the making of defensible decisions to stop interventions. We present a computational tool to perform these probability calculations and demonstrate its practical utility for supporting intervention cessation decisions by applying the framework to infection data from programmes aiming to eliminate onchocerciasis and lymphatic filariasis in Uganda and Nigeria, respectively. We highlight a possible method for validating the results in the field, and discuss further refinements and extensions required to deploy this predictive tool for guiding decision making by programme managers.

The decision when to stop an intervention is a critical component of parasite elimination programmes, but reliance on surveillance data alone can be inaccurate. Here, Michael et al. combine parasite transmission model predictions with disease survey data to more reliably determine when interventions can be stopped.

Modelling the elimination of river blindness using long-term epidemiological and programmatic data from Mali and Senegal

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Onchocerciasis is earmarked for elimination in some African countries by 2020/2025.

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15+ years of ivermectin treatment drove infection prevalence to zero in areas of Mali & Senegal.

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Data-driven model projections are used to evaluate the risk of infection resurgence.

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Latent infections can initiate slow resurgence in communities with high transmission propensity.

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Highly sensitive and long-term surveillance will be necessary to verify elimination.

The onchocerciasis transmission models EPIONCHO and ONCHOSIM have been independently developed and used to explore the feasibility of eliminating onchocerciasis from Africa with mass (annual or biannual) distribution of ivermectin within the timeframes proposed by the World Health Organization (WHO) and endorsed by the 2012 London Declaration on Neglected Tropical Diseases (i.e. by 2020/2025). Based on the findings of our previous model comparison, we implemented technical refinements and tested the projections of EPIONCHO and ONCHOSIM against long-term epidemiological data from two West African transmission foci in Mali and Senegal where the observed prevalence of infection was brought to zero circa 2007–2009 after 15–17 years of mass ivermectin treatment. We simulated these interventions using programmatic information on the frequency and coverage of mass treatments and trained the model projections using longitudinal parasitological data from 27 communities, evaluating the projected outcome of elimination (local parasite extinction) or resurgence. We found that EPIONCHO and ONCHOSIM captured adequately the epidemiological trends during mass treatment but that resurgence, while never predicted by ONCHOSIM, was predicted by EPIONCHO in some communities with the highest (inferred) vector biting rates and associated pre-intervention endemicities. Resurgence can be extremely protracted such that low (microfilarial) prevalence between 1% and 5% can be maintained for 3–5 years before manifesting more prominently. We highlight that post-treatment and post-elimination surveillance protocols must be implemented for long enough and with high enough sensitivity to detect possible residual latent infections potentially indicative of resurgence. We also discuss uncertainty and differences between EPIONCHO and ONCHOSIM projections, the potential importance of vector control in high-transmission settings as a complementary intervention strategy, and the short remaining timeline for African countries to be ready to stop treatment safely and begin surveillance in order to meet the impending 2020/2025 elimination targets.

How Can Onchocerciasis Elimination in Africa Be Accelerated? Modeling the Impact of Increased Ivermectin Treatment Frequency and Complementary Vector Control

Background

Great strides have been made toward onchocerciasis elimination by mass drug administration (MDA) of ivermectin. Focusing on MDA-eligible areas, we investigated where the elimination goal can be achieved by 2025 by continuation of current practice (annual MDA with ivermectin) and where intensification or additional vector control is required. We did not consider areas hypoendemic for onchocerciasis with loiasis coendemicity where MDA is contraindicated.

Methods

We used 2 previously published mathematical models, ONCHOSIM and EPIONCHO, to simulate future trends in microfilarial prevalence for 80 different settings (defined by precontrol endemicity and past MDA frequency and coverage) under different future treatment scenarios (annual, biannual, or quarterly MDA with different treatment coverage through 2025, with or without vector control strategies), assessing for each strategy whether it eventually leads to elimination.

Results

Areas with 40%-50% precontrol microfilarial prevalence and ≥10 years of annual MDA may achieve elimination with a further 7 years of annual MDA, if not achieved already, according to both models. For most areas with 70%-80% precontrol prevalence, ONCHOSIM predicts that either annual or biannual MDA is sufficient to achieve elimination by 2025, whereas EPIONCHO predicts that elimination will not be achieved even with complementary vector control.

Conclusions

Whether elimination will be reached by 2025 depends on precontrol endemicity, control history, and strategies chosen from now until 2025. Biannual or quarterly MDA will accelerate progress toward elimination but cannot guarantee it by 2025 in high-endemicity areas. Long-term concomitant MDA and vector control for high-endemicity areas might be useful.

Seroepidemiology of helminths and the association with severe malaria among infants and young children in Tanzania

The disease burden of Wuchereria bancrofti and Plasmodium falciparum malaria is high, particularly in Africa, and co-infection is common. However, the effects of filarial infection on the risk of severe malaria are unknown. We used the remaining serum samples from a large cohort study in Muheza, Tanzania to describe vector-borne filarial sero-reactivity among young children and to identify associations between exposure to filarial parasites and subsequent severe malaria infections. We identified positive filarial antibody responses (as well as positive antibody responses to Strongyloides stercoralis) among infants as young as six months. In addition, we found a significant association between filarial seropositivity at six months of age and subsequent severe malaria. Specifically, infants who developed severe malaria by one year of age were 3.9 times more likely (OR = 3.9, 95% CI: 1.2, 13.0) to have been seropositive for filarial antigen at six months of age compared with infants who did not develop severe malaria.

Factors affecting onchocerciasis transmission: lessons for infection control

INTRODUCTION

Onchocerca volvulus infects in excess of 15 million people. The vectors are Simulium blackflies, varieties of which differ in their ecologies, behavior and vectorial abilities. Control of the vectors and mass administrations of ivermectin have succeeded in reducing prevalences with elimination achieved in some foci, particularly in Central and southern America. In Africa, progress towards elimination has been less successful. Areas covered: Even with community directed treatment with ivermectin (CDTI), control has been difficult in African areas with initial prevalences in excess of 55%, especially if only annual treatments are dispensed. This is partly attributable to insufficient coverage, but the appearance of incipiently resistant non-responding parasites and lack of attention to vector biology in modeling and planning outcomes of intervention programmes have also played their parts, with recrudescence now appearing in some treated areas. Expert commentary: The biology of onchocerciasis is complex involving different vectors with differing abilities to transmit parasites, diverse pathologies related to geographical and parasite variations and endosymbionts in both parasite and vector. Modeling to predict epidemiological and control outcomes is addressing this complexity but more attention needs to be given to the vectors' roles to further understanding of where and when control measures will succeed.

Factors Associated with Ivermectin Non-Compliance and Its Potential Role in Sustaining Onchocerca volvulus Transmission in the West Region of Cameroon

Background

Community Directed Treatment with ivermectin is the cornerstone of current efforts to eliminate onchocerciasis. However recent studies suggest there are foci where long-term annual distribution of the drug alone has failed to ensure elimination thresholds are reached. It is important to achieve high levels of compliance in order to obtain elimination targets. An epidemiological and entomological evaluation conducted in the western region of Cameroon in 2011 revealed that two health districts remained with a high prevalence of infection, despite long-term distribution of ivermectin since 1996. This paper explores potential factors that may have contributed to the non-interruption of transmission, focusing on ivermectin treatment compliance and the importance of systematic non-compliance within the population.

Methodology/Principal findings

A mixed methods approach was used, including a population-based survey to assess treatment compliance and factors associated and qualitative assessments including focus group discussions and in-depth interviews with key programme stakeholders and drug distributors. Compliance was reported at 71.2% (95%CI: 61.7–79.2%;n = 853/1198). The key factors related to compliance in the most recent round related to either programmatic and delivery issues, primarily absenteeism at the time of the campaign or alternatively individual determinants. An individual’s experience of side effects in the past was strongly associated with non-compliance to ivermectin. Other factors included ethnicity, how long lived in the village and age. There was a high percentage of reported systematic non-compliance at 7.4% (95% CI: 4.3–12.3%; n = 86/1165), higher amongst females. This group may be important in facilitating the sustainment of on-going transmission.

Conclusions/Significance

Efforts to reduce the number of systematic non-compliers and non-compliance in certain groups may be important in ensuring the interruption of transmission in the study area. However, in areas with high pre-control force of transmission, as in these districts, annual distribution with ivermectin, even if sustaining high levels of compliance, may still be inadequate to achieve elimination. Further studies are required to better understand the transmission dynamics and focus of on-going transmission in the study districts.

Community Directed Treatment with ivermectin is the cornerstone of current efforts to eliminate onchocerciasis. Ivermectin distribution alone has been shown to be able to interrupt transmission but there are foci where long-term distribution of the drug alone has failed to ensure elimination thresholds are reached. Two health districts in the western region of Cameroon remain with high prevalence of infection despite annual distribution of ivermectin since 1996. The study aims to explore factors related to non-compliance in two health districts in the west region of Cameroon. Nearly 30% of the population did not take ivermectin during the most recent round of mass drug administration and there was a significant proportion of the population that had reportedly never taken the drug. The key factors related to drug compliance in the most recent round, related to either programmatic and delivery issues, primarily absenteeism at the time of the campaign, or alternatively individual determinants, such as side effects associated with the drug, ethnicity, age and years lived in the village. Efforts to reduce the number of systematic non-compliers and non-compliance in certain groups are likely to be important in ensuring the interruption of transmission in the study area.

Heterogeneous dynamics, robustness/fragility trade-offs, and the eradication of the macroparasitic disease, lymphatic filariasis

BACKGROUND

The current WHO-led initiative to eradicate the macroparasitic disease, lymphatic filariasis (LF), based on single-dose annual mass drug administration (MDA) represents one of the largest health programs devised to reduce the burden of tropical diseases. However, despite the advances made in instituting large-scale MDA programs in affected countries, a challenge to meeting the goal of global eradication is the heterogeneous transmission of LF across endemic regions, and the impact that such complexity may have on the effort required to interrupt transmission in all socioecological settings.

METHODS

Here, we apply a Bayesian computer simulation procedure to fit transmission models of LF to field data assembled from 18 sites across the major LF endemic regions of Africa, Asia and Papua New Guinea, reflecting different ecological and vector characteristics, to investigate the impacts and implications of transmission heterogeneity and complexity on filarial infection dynamics, system robustness and control.

RESULTS

We find firstly that LF elimination thresholds varied significantly between the 18 study communities owing to site variations in transmission and initial ecological parameters. We highlight how this variation in thresholds lead to the need for applying variable durations of interventions across endemic communities for achieving LF elimination; however, a major new result is the finding that filarial population responses to interventions ultimately reflect outcomes of interplays between dynamics and the biological architectures and processes that generate robustness/fragility trade-offs in parasite transmission. Intervention simulations carried out in this study further show how understanding these factors is also key to the design of options that would effectively eliminate LF from all settings. In this regard, we find how including vector control into MDA programs may not only offer a countermeasure that will reliably increase system fragility globally across all settings and hence provide a control option robust to differential locality-specific transmission dynamics, but by simultaneously reducing transmission regime variability also permit more reliable macroscopic predictions of intervention effects.

CONCLUSIONS

Our results imply that a new approach, combining adaptive modelling of parasite transmission with the use of biological robustness as a design principle, is required if we are to both enhance understanding of complex parasitic infections and delineate options to facilitate their elimination effectively.

Evidence of suppression of onchocerciasis transmission in the Venezuelan Amazonian focus

BACKGROUND

The World Health Organization (WHO) has set goals for onchocerciasis elimination in Latin America by 2015. Most of the six previously endemic countries are attaining this goal by implementing twice a year (and in some foci, quarterly) mass ivermectin (Mectizan®) distribution. Elimination of transmission has been verified in Colombia, Ecuador and Mexico. Challenges remain in the Amazonian focus straddling Venezuela and Brazil, where the disease affects the hard-to-reach Yanomami indigenous population. We provide evidence of suppression of Onchocerca volvulus transmission by Simulium guianense s.l. in 16 previously hyperendemic Yanomami communities in southern Venezuela after 15 years of 6-monthly and 5 years of 3-monthly mass ivermectin treatment.

METHODS

Baseline and monitoring and evaluation parasitological, ophthalmological, entomological and serological surveys were conducted in selected sentinel and extra-sentinel communities of the focus throughout the implementation of the programme.

RESULTS

From 2010 to 2012-2015, clinico-parasitological surveys indicate a substantial decrease in skin microfilarial prevalence and intensity of infection; accompanied by no evidence (or very low prevalence and intensity) of ocular microfilariae in the examined population. Of a total of 51,341 S. guianense flies tested by PCR none had L3 infection (heads only). Prevalence of infective flies and seasonal transmission potentials in 2012-2013 were, respectively, under 1% and 20 L3/person/transmission season. Serology in children aged 1-10 years demonstrated that although 26 out of 396 (7%) individuals still had Ov-16 antibodies, only 4/218 (2%) seropositives were aged 1-5 years.

CONCLUSIONS

We report evidence of recent transmission and morbidity suppression in some communities of the focus representing 75% of the Yanomami population and 70% of all known communities. We conclude that onchocerciasis transmission could be feasibly interrupted in the Venezuelan Amazonian focus.

River Blindness: Mathematical Models for Control and Elimination

Human onchocerciasis (river blindness) is one of the few neglected tropical diseases (NTDs) whose control strategies have been informed by mathematical modelling. With the change in focus from elimination of the disease burden to elimination of Onchocerca volvulus, much remains to be done to refine, calibrate and validate existing models. Under the impetus of the NTD Modelling Consortium, the teams that developed EPIONCHO and ONCHOSIM have joined forces to compare and improve these frameworks to better assist ongoing elimination efforts. We review their current versions and describe how they are being used to address two key questions: (1) where can onchocerciasis be eliminated with current intervention strategies by 2020/2025? and (2) what alternative/complementary strategies could help to accelerate elimination where (1) cannot be achieved? The control and elimination of onchocerciasis from the African continent is at a crucial crossroad. The African Programme for Onchocerciasis Control closed at the end of 2015, and although a new platform for support and integration of NTD control has been launched, the disease will have to compete with a myriad of other national health priorities at a pivotal time in the road to elimination. However, never before had onchocerciasis control a better arsenal of intervention strategies as well as diagnostics. It is, therefore, timely to present two models of different geneses and modelling traditions as they come together to produce robust decision-support tools. We start by describing the structural and parametric assumptions of EPIONCHO and ONCHOSIM; we continue by summarizing the modelling of current treatment strategies with annual (or biannual) mass ivermectin distribution and introduce a number of alternative strategies, including other microfilaricidal therapies (such as moxidectin), macrofilaricidal (anti-wolbachial) treatments, focal vector control and the possibility of an onchocerciasis vaccine. We conclude by discussing challenges, opportunities and future directions.

Required duration of mass ivermectin treatment for onchocerciasis elimination in Africa: a comparative modelling analysis

Background

The World Health Organization (WHO) has set ambitious targets for the elimination of onchocerciasis by 2020–2025 through mass ivermectin treatment. Two different mathematical models have assessed the feasibility of reaching this goal for different settings and treatment scenarios, namely the individual-based microsimulation model ONCHOSIM and the population-based deterministic model EPIONCHO. In this study, we harmonize some crucial assumptions and compare model predictions on common outputs.

Methods

Using a range of initial endemicity levels and treatment scenarios, we compared the models with respect to the following outcomes: 1) model-predicted trends in microfilarial (mf) prevalence and mean mf intensity during 25 years of (annual or biannual) mass ivermectin treatment; 2) treatment duration needed to bring mf prevalence below a provisional operational threshold for treatment interruption (pOTTIS, i.e. 1.4 %), and 3) treatment duration needed to drive the parasite population to local elimination, even in the absence of further interventions. Local elimination was judged by stochastic fade-out in ONCHOSIM and by reaching transmission breakpoints in EPIONCHO.

Results

ONCHOSIM and EPIONCHO both predicted that in mesoendemic areas the pOTTIS can be reached with annual treatment, but that this strategy may be insufficient in very highly hyperendemic areas or would require prolonged continuation of treatment. For the lower endemicity levels explored, ONCHOSIM predicted that the time needed to reach the pOTTIS is longer than that needed to drive the parasite population to elimination, whereas for the higher endemicity levels the opposite was true. In EPIONCHO, the pOTTIS was reached consistently sooner than the breakpoint.

Conclusions

The operational thresholds proposed by APOC may have to be adjusted to adequately reflect differences in pre-control endemicities. Further comparative modelling work will be conducted to better understand the main causes of differences in model-predicted trends. This is a pre-requisite for guiding elimination programmes in Africa and refining operational criteria for stopping mass treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1159-9) contains supplementary material, which is available to authorized users.

Human Onchocerciasis: Modelling the Potential Long-term Consequences of a Vaccination Programme

Background

Currently, the predominant onchocerciasis control strategy in Africa is annual mass drug administration (MDA) with ivermectin. However, there is a consensus among the global health community, supported by mathematical modelling, that onchocerciasis in Africa will not be eliminated within proposed time frameworks in all endemic foci with only annual MDA, and novel and alternative strategies are urgently needed. Furthermore, use of MDA with ivermectin is already compromised in large areas of central Africa co-endemic with Loa loa, and there are areas where suboptimal or atypical responses to ivermectin have been documented. An onchocerciasis vaccine would be highly advantageous in these areas.

Methodology/Principal Findings

We used a previously developed onchocerciasis transmission model (EPIONCHO) to investigate the impact of vaccination in areas where loiasis and onchocerciasis are co-endemic and ivermectin is contraindicated. We also explore the potential influence of a vaccination programme on infection resurgence in areas where local elimination has been successfully achieved. Based on the age range included in the Expanded Programme on Immunization (EPI), the vaccine was assumed to target 1 to 5 year olds. Our modelling results indicate that the deployment of an onchocerciasis vaccine would have a beneficial impact in onchocerciasis–loiasis co-endemic areas, markedly reducing microfilarial load in the young (under 20 yr) age groups.

Conclusions/Significance

An onchocerciasis prophylactic vaccine would reduce the onchocerciasis disease burden in populations where ivermectin cannot be administered safely. Moreover, a vaccine could substantially decrease the chance of re-emergence of Onchocerca volvulus infection in areas where it is deemed that MDA with ivermectin can be stopped. Therefore, a vaccine would protect the substantial investments made by present and past onchocerciasis control programmes, decreasing the chance of disease recrudescence and offering an important additional tool to mitigate the potentially devastating impact of emerging ivermectin resistance.

Novel and alternative strategies are required to meet the demanding control and elimination (of infection) goals for human onchocerciasis (river blindness) in Africa. Due to the overlapping distribution of onchocerciasis and loiasis (African eye worm) in forested areas of central Africa, millions of people living in such areas are not well served by current interventions because they cannot safely receive the antiparasitic drug ivermectin that is distributed en masse to treat onchocerciasis elsewhere in Africa. The Onchocerciasis Vaccine for Africa—TOVA—Initiative has been established to develop and trial an onchocerciasis vaccine. We model the potential impact of a hypothetical childhood vaccination programme rolled out in areas where co-endemicity of onchocerciasis and African eye worm makes mass distribution of ivermectin difficult and potentially unsafe for treating, controlling and eliminating river blindness. We find that, 15 years into the programme, a vaccine would substantially reduce infection levels in children and young adults, protecting them from the morbidity and mortality associated with onchocerciasis. Most benefit would be reaped from a long-lived vaccine, even if only partially protective. We also discuss how a vaccine could substantially reduce the risk of re-emergence of onchocerciasis in areas freed from infection after years of successful intervention.

Onchocerciasis transmission in Ghana: persistence under different control strategies and the role of the simuliid vectors

BACKGROUND

The World Health Organization (WHO) aims at eliminating onchocerciasis by 2020 in selected African countries. Current control focuses on community-directed treatment with ivermectin (CDTI). In Ghana, persistent transmission has been reported despite long-term control. We present spatial and temporal patterns of onchocerciasis transmission in relation to ivermectin treatment history.

METHODOLOGY/PRINCIPAL FINDINGS

Host-seeking and ovipositing blackflies were collected from seven villages in four regions of Ghana with 3-24 years of CDTI at the time of sampling. A total of 16,443 flies was analysed for infection; 5,812 (35.3%) were dissected for parity (26.9% parous). Heads and thoraces of 12,196 flies were dissected for Onchocerca spp. and DNA from 11,122 abdomens was amplified using Onchocerca primers. A total of 463 larvae (0.03 larvae/fly) from 97 (0.6%) infected and 62 (0.4%) infective flies was recorded; 258 abdomens (2.3%) were positive for Onchocerca DNA. Infections (all were O. volvulus) were more likely to be detected in ovipositing flies. Transmission occurred, mostly in the wet season, at Gyankobaa and Bosomase, with transmission potentials of, respectively, 86 and 422 L3/person/month after 3 and 6 years of CDTI. The numbers of L3/1,000 parous flies at these villages were over 100 times the WHO threshold of one L3/1,000 for transmission control. Vector species influenced transmission parameters. At Asubende, the number of L3/1,000 ovipositing flies (1.4, 95% CI = 0-4) also just exceeded the threshold despite extensive vector control and 24 years of ivermectin distribution, but there were no infective larvae in host-seeking flies.

CONCLUSIONS/SIGNIFICANCE

Despite repeated ivermectin treatment, evidence of O. volvulus transmission was documented in all seven villages and above the WHO threshold in two. Vector species influences transmission through biting and parous rates and vector competence, and should be included in transmission models. Oviposition traps could augment vector collector methods for monitoring and surveillance.

The potential impact of moxidectin on onchocerciasis elimination in Africa: an economic evaluation based on the Phase II clinical trial data

BACKGROUND

Spurred by success in several foci, onchocerciasis control policy in Africa has shifted from morbidity control to elimination of infection. Clinical trials have demonstrated that moxidectin is substantially more efficacious than ivermectin in effecting sustained reductions in skin microfilarial load and, therefore, may accelerate progress towards elimination. We compare the potential cost-effectiveness of annual moxidectin with annual and biannual ivermectin treatment.

METHODS

Data from the first clinical study of moxidectin were used to parameterise the onchocerciasis transmission model EPIONCHO to investigate, for different epidemiological and programmatic scenarios in African savannah settings, the number of years and in-country costs necessary to reach the operational thresholds for cessation of treatment, comparing annual and biannual ivermectin with annual moxidectin treatment.

RESULTS

Annual moxidectin and biannual ivermectin treatment would achieve similar reductions in programme duration relative to annual ivermectin treatment. Unlike biannual ivermectin treatment, annual moxidectin treatment would not incur a considerable increase in programmatic costs and, therefore, would generate sizeable in-country cost savings (assuming the drug is donated). Furthermore, the impact of moxidectin, unlike ivermectin, was not substantively influenced by the timing of treatment relative to seasonal patterns of transmission.

CONCLUSIONS

Moxidectin is a promising new drug for the control and elimination of onchocerciasis. It has high programmatic value particularly when resource limitation prevents a biannual treatment strategy, or optimal timing of treatment relative to peak transmission season is not feasible.

Onchocerciasis transmission in Ghana: biting and parous rates of host-seeking sibling species of the Simulium damnosum complex

Background

Ghana is renowned for its sibling species diversity of the Simulium damnosum complex, vectors of Onchocerca volvulus. Detailed entomological knowledge becomes a priority as onchocerciasis control policy has shifted from morbidity reduction to elimination of infection. To date, understanding of transmission dynamics of O. volvulus has been mainly based on S. damnosum sensu stricto (s.s.) data. We aim to elucidate bionomic features of vector species of importance for onchocerciasis elimination efforts.

Methods

We collected S. damnosum sensu lato from seven villages in four Ghanaian regions between 2009 and 2011, using standard vector collection, and human- and cattle-baited tents. Taxa were identified using morphological and molecular techniques. Monthly biting rates (MBR), parous rates and monthly parous biting rates (MPBR) are reported by locality, season, trapping method and hour of collection for each species.

Results

S. damnosum s.s./S. sirbanum were collected at Asubende and Agborlekame, both savannah villages. A range of species was caught in the Volta region (forest-savannah mosaic) and Gyankobaa (forest), with S. squamosum or S. sanctipauli being the predominant species, respectively. In Bosomase (southern forest region) only S. sanctipauli was collected in the 2009 wet season, but in the 2010 dry season S. yahense was also caught. MBRs ranged from 714 bites/person/month at Agborlekame (100% S. damnosum s.s./S. sirbanum) to 8,586 bites/person/month at Pillar 83/Djodji (98.5% S. squamosum). MBRs were higher in the wet season. In contrast, parous rates were higher in the dry season (41.8% vs. 18.4%), resulting in higher MPBRs in the dry season. Daily host-seeking activity of S. damnosum s.s./S. sirbanum was bimodal, whilst S. squamosum and S. sanctipauli had unimodal afternoon peaks.

Conclusions

The bionomic differences between sibling species of the S. damnosum complex need to be taken into account when designing entomological monitoring protocols for interventions and parameterising mathematical models for onchocerciasis control and elimination.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0511-9) contains supplementary material, which is available to authorized users.

Reaching the london declaration on neglected tropical diseases goals for onchocerciasis: an economic evaluation of increasing the frequency of ivermectin treatment in Africa

Although switching from annual to biannual ivermectin treatment yields small additional health benefits, in the context of elimination goals its benefit is pronounced, increasing the feasibility of and shortening the time frames for reaching proposed operational thresholds for stopping treatment.

Background. Recently, there has been a shift in onchocerciasis control policy, changing from prevention of morbidity toward elimination of infection. Switching from annual to biannual ivermectin distribution may accelerate progress toward the elimination goals. However, the settings where this strategy would be cost effective in Africa have not been described.

Methods. An onchocerciasis transmission framework (EpiOncho) was coupled to a disease model in order to explore the impact on disability-adjusted life years averted, program cost, and program duration of biannual ivermectin treatment in different epidemiological and programmatic scenarios in African savannah.

Results. While biannual treatment yields only small additional health gains, its benefit is pronounced in the context of the elimination goals, shortening the time frames for and increasing the feasibility of reaching the proposed operational thresholds for stopping treatment. In settings with high precontrol endemicity (and/or poor coverage and compliance), it may not be possible to reach such thresholds even within 50 years of annual ivermectin, requiring adoption of biannual treatment. Our projections highlight the crucial role played by coverage and compliance in achieving the elimination goals.

Conclusions. Biannual ivermectin treatment improves the chances of reaching the 2020/2025 elimination goals, potentially generating programmatic cost savings in settings with high precontrol endemicity. However, its benefit and cost are highly sensitive to levels of systematic noncompliance and, in many settings, it will lead to an increase in costs. Furthermore, it may not always be feasible to implement biannual treatment, particularly in hard-to-reach populations. This highlights the continued need for a macrofilaricide.

Estimated effects of projected climate change on the basic reproductive number of the Lyme disease vector Ixodes scapularis

BACKGROUND

The extent to which climate change may affect human health by increasing risk from vector-borne diseases has been under considerable debate.

OBJECTIVES

We quantified potential effects of future climate change on the basic reproduction number (R0) of the tick vector of Lyme disease, Ixodes scapularis, and explored their importance for Lyme disease risk, and for vector-borne diseases in general.

METHODS

We applied observed temperature data for North America and projected temperatures using regional climate models to drive an I. scapularis population model to hindcast recent, and project future, effects of climate warming on R0. Modeled R0 increases were compared with R0 ranges for pathogens and parasites associated with variations in key ecological and epidemiological factors (obtained by literature review) to assess their epidemiological importance.

RESULTS

R0 for I. scapularis in North America increased during the years 1971-2010 in spatio-temporal patterns consistent with observations. Increased temperatures due to projected climate change increased R0 by factors (2-5 times in Canada and 1.5-2 times in the United States), comparable to observed ranges of R0 for pathogens and parasites due to variations in strains, geographic locations, epidemics, host and vector densities, and control efforts.

CONCLUSIONS

Climate warming may have co-driven the emergence of Lyme disease in northeastern North America, and in the future may drive substantial disease spread into new geographic regions and increase tick-borne disease risk where climate is currently suitable. Our findings highlight the potential for climate change to have profound effects on vectors and vector-borne diseases, and the need to refocus efforts to understand these effects.

Modelling the impact of ivermectin on River Blindness and its burden of morbidity and mortality in African Savannah: EpiOncho projections

Background

The African Programme for Onchocerciasis Control (APOC) has refocused its goals on the elimination of infection where possible, seemingly achievable by 15–17 years of annual mass distribution of ivermectin in some African foci. Previously, APOC had focused on the elimination of onchocerciasis as a public health problem. Timeframes have been set by the World Health Organization, the London Declaration on Neglected Tropical Diseases and the World Bank to achieve these goals by 2020–2025.

Methods

A novel mathematical model of the dynamics of onchocercal disease is presented which links documented associations between Onchocerca volvulus infection and the prevalence and incidence of morbidity and mortality to model outputs from our host age- and sex-structured onchocerciasis transmission framework (EpiOncho). The model is calibrated for African savannah settings, and used to assess the impact of long-term annual mass administration of ivermectin on infection and ocular and skin disease and to explore how this depends on epidemiological and programmatic variables.

Results

Current onchocerciasis disease projections, which do not account for excess mortality of sighted individuals with heavy microfilarial loads, underestimate disease burden. Long-term annual ivermectin treatment is highly effective at reducing both the morbidity and mortality associated with onchocerciasis, and this result is not greatly influenced by treatment coverage and compliance. By contrast, impact on microfilarial prevalence and intensity is highly dependent on baseline endemicity, treatment coverage and systematic non-compliance.

Conclusions

The goals of eliminating morbidity and infection with ivermectin alone are distinctly influenced by epidemiological and programmatic factors. Whilst the former goal is most certainly achievable, reaching the latter will strongly depend on initial endemicity (the higher the endemicity, the greater the magnitude of inter-treatment transmission), advising caution when generalising the applicability of successful elimination outcomes to other areas. The proportion of systematic non-compliers will become far more influential in terms of overall success in achieving elimination goals.

Uncertainty surrounding projections of the long-term impact of ivermectin treatment on human onchocerciasis

Background

Recent studies in Mali, Nigeria, and Senegal have indicated that annual (or biannual) ivermectin distribution may lead to local elimination of human onchocerciasis in certain African foci. Modelling-based projections have been used to estimate the required duration of ivermectin distribution to reach elimination. A crucial assumption has been that microfilarial production by Onchocerca volvulus is reduced irreversibly by 30–35% with each (annual) ivermectin round. However, other modelling-based analyses suggest that ivermectin may not have such a cumulative effect. Uncertainty in this (biological) and other (programmatic) assumptions would affect projected outcomes of long-term ivermectin treatment.

Methodology/Principal Findings

We modify a deterministic age- and sex-structured onchocerciasis transmission model, parameterised for savannah O. volvulus–Simulium damnosum, to explore the impact of assumptions regarding the effect of ivermectin on worm fertility and the patterns of treatment coverage compliance, and frequency on projections of parasitological outcomes due to long-term, mass ivermectin administration in hyperendemic areas. The projected impact of ivermectin distribution on onchocerciasis and the benefits of switching from annual to biannual distribution are strongly dependent on assumptions regarding the drug's effect on worm fertility and on treatment compliance. If ivermectin does not have a cumulative impact on microfilarial production, elimination of onchocerciasis in hyperendemic areas may not be feasible with annual ivermectin distribution.

Conclusions/Significance

There is substantial (biological and programmatic) uncertainty surrounding modelling projections of onchocerciasis elimination. These uncertainties need to be acknowledged for mathematical models to inform control policy reliably. Further research is needed to elucidate the effect of ivermectin on O. volvulus reproductive biology and quantify the patterns of coverage and compliance in treated communities.

Studies in Mali, Nigeria, and Senegal suggest that, in some settings, it is possible to eliminate onchocerciasis after 15–17 years of ivermectin distribution. Computer models have been used to estimate the required duration of ivermectin distribution to reach elimination. Some models assume that annual ivermectin treatment reduces the fertility of the causing parasite, Onchocerca volvulus, by 30–35% each time the drug is taken. Other analyses suggest that ivermectin may not have such an effect. We explore how assumptions regarding: a) treatment effects on microfilarial production by female worms (fertility), b) proportion of people who receive the drug (coverage), c) proportion of people who adhere to treatment (compliance), and d) whether people are treated once or twice per year (frequency) affect temporal projections of infection load and prevalence in highly endemic African savannah settings. We find that if treatment does not affect parasite fertility cumulatively, elimination of onchocerciasis in highly endemic areas of Africa may not be feasible with annual ivermectin distribution alone. If two areas have equal coverage but dissimilar compliance, they may experience very different infection load, prevalence and persistence trends. Projections such as these are crucial to help onchocerciasis control programmes to plan elimination strategies effectively.

Onchocerciasis: the pre-control association between prevalence of palpable nodules and skin microfilariae

BACKGROUND

The prospect of eliminating onchocerciasis from Africa by mass treatment with ivermectin has been rejuvenated following recent successes in foci in Mali, Nigeria and Senegal. Elimination prospects depend strongly on local transmission conditions and therefore on pre-control infection levels. Pre-control infection levels in Africa have been mapped largely by means of nodule palpation of adult males, a relatively crude method for detecting infection. We investigated how informative pre-control nodule prevalence data are for estimating the pre-control prevalence of microfilariae (mf) in the skin and discuss implications for assessing elimination prospects.

METHODS AND FINDINGS

We analyzed published data on pre-control nodule prevalence in males aged ≥ 20 years and mf prevalence in the population aged ≥ 5 years from 148 African villages. A meta-analysis was performed by means of Bayesian hierarchical multivariate logistic regression, accounting for measurement error in mf and nodule prevalence, bioclimatic zones, and other geographical variation. There was a strong positive correlation between nodule prevalence in adult males and mf prevalence in the general population. In the forest-savanna mosaic area, the pattern in nodule and mf prevalence differed significantly from that in the savanna or forest areas.

SIGNIFICANCE

We provide a tool to convert pre-control nodule prevalence in adult males to mf prevalence in the general population, allowing historical data to be interpreted in terms of elimination prospects and disease burden of onchocerciasis. Furthermore, we identified significant geographical variation in mf prevalence and nodule prevalence patterns warranting further investigation of geographical differences in transmission patterns of onchocerciasis.

Time series analysis of onchocerciasis data from Mexico: a trend towards elimination

BACKGROUND

In Latin America, there are 13 geographically isolated endemic foci distributed among Mexico, Guatemala, Colombia, Venezuela, Brazil and Ecuador. The communities of the three endemic foci found within Mexico have been receiving ivermectin treatment since 1989. In this study, we predicted the trend of occurrence of cases in Mexico by applying time series analysis to monthly onchocerciasis data reported by the Mexican Secretariat of Health between 1988 and 2011 using the software R.

RESULTS

A total of 15,584 cases were reported in Mexico from 1988 to 2011. The data of onchocerciasis cases are mainly from the main endemic foci of Chiapas and Oaxaca. The last case in Oaxaca was reported in 1998, but new cases were reported in the Chiapas foci up to 2011. Time series analysis performed for the foci in Mexico showed a decreasing trend of the disease over time. The best-fitted models with the smallest Akaike Information Criterion (AIC) were Auto-Regressive Integrated Moving Average (ARIMA) models, which were used to predict the tendency of onchocerciasis cases for two years ahead. According to the ARIMA models predictions, the cases in very low number (below 1) are expected for the disease between 2012 and 2013 in Chiapas, the last endemic region in Mexico.

CONCLUSION

The endemic regions of Mexico evolved from high onchocerciasis-endemic states to the interruption of transmission due to the strategies followed by the MSH, based on treatment with ivermectin. The extremely low level of expected cases as predicted by ARIMA models for the next two years suggest that the onchocerciasis is being eliminated in Mexico. To our knowledge, it is the first study utilizing time series for predicting case dynamics of onchocerciasis, which could be used as a benchmark during monitoring and post-treatment surveillance.

Density-dependent mortality of the human host in onchocerciasis: relationships between microfilarial load and excess mortality

Background

The parasite Onchocerca volvulus has, until recently, been regarded as the cause of a chronic yet non-fatal condition. Recent analyses, however, have indicated that in addition to blindness, the parasite can also be directly associated with human mortality. Such analyses also suggested that the relationship between microfilarial load and excess mortality might be non-linear. Determining the functional form of such relationship would contribute to quantify the population impact of mass microfilaricidal treatment.

Methodology/Principal Findings

Data from the Onchocerciasis Control Programme in West Africa (OCP) collected from 1974 through 2001 were used to determine functional relationships between microfilarial load and excess mortality of the human host. The goodness-of-fit of three candidate functional forms (a (log-) linear model and two saturating functions) were explored and a saturating (log-) sigmoid function was deemed to be statistically the best fit. The excess mortality associated with microfilarial load was also found to be greater in younger hosts. The attributable mortality risk due to onchocerciasis was estimated to be 5.9%.

Conclusions/Significance

Incorporation of this non-linear functional relationship between microfilarial load and excess mortality into mathematical models for the transmission and control of onchocerciasis will have important implications for our understanding of the population biology of O. volvulus, its impact on human populations, the global burden of disease due to onchocerciasis, and the projected benefits of control programmes in both human and economic terms.

Human onchocerciasis (River Blindness) is a parasitic disease leading to visual impairment including blindness. Blindness may lead to premature death, but infection with the parasite itself (Onchocerca volvulus) may also cause excess mortality in sighted individuals. The excess risk of mortality may not be directly (linearly) proportional to the intensity of infection (a measure of how many parasites an individual harbours). We analyze cohort data from the Onchocerciasis Control Programme in West Africa, collected between 1974 and 2001, by fitting a suite of quantitative models (including a ‘null’ model of no relationship between infection intensity and mortality, a (log-) linear function, and two plateauing curves), and choosing the one that is the most statistically adequate. The risk of human mortality initially increases with parasite density but saturates at high densities (following an S-shape curve), and such risk is greater in younger individuals for a given infection intensity. Our results have important repercussions for programmes aiming to control onchocerciasis (in terms of how the benefits of the programme are calculated), for measuring the burden of disease and mortality caused by the infection, and for a better understanding of the processes that govern the density of parasite populations among human hosts.

Assessment and monitoring of onchocerciasis in Latin America

Onchocerciasis has historically been one of the leading causes of infectious blindness worldwide. It is endemic to tropical regions both in Africa and Latin America and in the Yemen. In Latin America, it is found in 13 foci located in 6 different countries. The epidemiologically most important focus of onchocerciasis in the Americas is located in a region spanning the border between Guatemala and Mexico. However, the Amazonian focus straddling the border of Venezuela and Brazil is larger in overall area because the Yanomami populations are scattered over a very large geographical region. Onchocerciasis is caused by infection with the filarial parasite Onchocerca volvulus. The infection is spread through the bites of an insect vector, black flies of the genus Simulium. In Africa, the major vectors are members of the S. damnosum complex, while numerous species serve as vectors of the parasite in Latin America. Latin America has had a long history of attempts to control onchocerciasis, stretching back almost 100 years. The earliest programmes used a strategy of surgical removal of the adult parasites from affected individuals. However, because many of the adult parasites lodge in undetectable and inaccessible areas of the body, the overall effect of this strategy on the prevalence of infection was relatively minor. In 1988, a new drug, ivermectin, was introduced that effectively killed the larval stage (microfilaria) of the parasite in infected humans. As the microfilaria is both the stage that is transmitted by the vector fly and the cause of most of the pathologies associated with the infection, ivermectin opened up a new strategy for the control of onchocerciasis. Concurrent with the use of ivermectin for the treatment of onchocerciasis, a number of sensitive new diagnostic tools were developed (both serological and nucleic acid based) that provided the efficiency, sensitivity and specificity necessary to monitor the decline and eventual elimination of onchocerciasis as a result of successful control. As a result of these advances, a strategy for the elimination of onchocerciasis was developed, based upon mass distribution of ivermectin to afflicted communities for periods lasting long enough to ensure that the parasite population was placed on the road to local elimination. This strategy has been applied for the past decade to the foci in Latin America by a programme overseen by the Onchocerciasis Elimination Program for the Americas (OEPA). The efforts spearheaded by OEPA have been very successful, eliminating ocular disease caused by O. volvulus, and eliminating and interrupting transmission of the parasite in 8 of the 13 foci in the region. As onchocerciasis approaches elimination in Latin America, several questions still need to be addressed. These include defining an acceptable upper limit for transmission in areas in which transmission is thought to have been suppressed (e.g. what is the maximum value for the upper bound of the 95% confidence interval for transmission rates in areas where transmission is no longer detectable), how to develop strategies for conducting surveillance for recrudescence of infection in areas in which transmission is thought to be interrupted and how to address the problem in areas where the mass distribution of ivermectin seems to be unable to completely eliminate the infection.

Elimination of human onchocerciasis: history of progress and current feasibility using ivermectin (Mectizan(®)) monotherapy

We review and analyze approaches over a 65 year period that have proven successful for onchocerciasis control in several different epidemiological settings. These include vector control with the goal of transmission interruption versus the use of mass drug administration using ivermectin (Mectizan(®)) monotherapy. Ivermectin has proven exceedingly effective because it is highly efficacious against Onchocerca volvulus microfilariae, the etiological agent of onchocercal skin and ocular disease and the infective stage for the vector. For these reasons, the drug was donated by the Merck Company for regional control programs in Africa and the Americas. Recurrent treatment with ivermectin at semi-annual intervals also impacts adult worms and result in loss of fecundity and increased mortality. Using a strategy of 6-monthly treatments with high coverage rates, the Onchocerciasis Elimination Program for the Americas has interrupted transmission in seven of the thirteen foci in the Americas and is on track to eliminate onchocerciasis in the region by 2015. Treatments given annually or semi-annually for 15-17 years in three hyperendemic onchocerciasis foci in Mali and Senegal also have resulted in a few infections in the human population with transmission levels below thresholds postulated for elimination. Follow-up evaluations did not detect any recrudescence of infection or transmission, suggesting that onchocerciasis elimination could be feasible with Mectizan(®) treatment in some endemic foci in Africa.

Mapping helminth co-infection and co-intensity: geostatistical prediction in ghana

Background

Morbidity due to Schistosoma haematobium and hookworm infections is marked in those with intense co-infections by these parasites. The development of a spatial predictive decision-support tool is crucial for targeting the delivery of integrated mass drug administration (MDA) to those most in need. We investigated the co-distribution of S. haematobium and hookworm infection, plus the spatial overlap of infection intensity of both parasites, in Ghana. The aim was to produce maps to assist the planning and evaluation of national parasitic disease control programs.

Methodology/Principal Findings

A national cross-sectional school-based parasitological survey was conducted in Ghana in 2008, using standardized sampling and parasitological methods. Bayesian geostatistical models were built, including a multinomial regression model for S. haematobium and hookworm mono- and co-infections and zero-inflated Poisson regression models for S. haematobium and hookworm infection intensity as measured by egg counts in urine and stool respectively. The resulting infection intensity maps were overlaid to determine the extent of geographical overlap of S. haematobium and hookworm infection intensity. In Ghana, prevalence of S. haematobium mono-infection was 14.4%, hookworm mono-infection was 3.2%, and S. haematobium and hookworm co-infection was 0.7%. Distance to water bodies was negatively associated with S. haematobium and hookworm co-infections, hookworm mono-infections and S. haematobium infection intensity. Land surface temperature was positively associated with hookworm mono-infections and S. haematobium infection intensity. While high-risk (prevalence >10–20%) of co-infection was predicted in an area around Lake Volta, co-intensity was predicted to be highest in foci within that area.

Conclusions/Significance

Our approach, based on the combination of co-infection and co-intensity maps allows the identification of communities at increased risk of severe morbidity and environmental contamination and provides a platform to evaluate progress of control efforts.

Urinary schistosomiasis and hookworm infections cause considerable morbidity in school age children in West Africa. Severe morbidity is predominantly observed in individuals infected with both parasite types and, in particular, with heavy infections. We investigated for the first time the distribution of S. haematobium and hookworm co-infections and distribution of co-intensity of these parasites in Ghana. Bayesian geostatistical models were developed to generate a national co-infection map and national intensity maps for each parasite, using data on S. haematobium and hookworm prevalence and egg concentration (expressed as eggs per 10 mL of urine for S. haematobium and expressed as eggs per gram of faeces for hookworm), collected during a pre-intervention baseline survey in Ghana, 2008. In contrast with previous findings from the East Africa region, we found that both S. haematobium and hookworm infections are highly focal, resulting in small, localized clusters of co-infection and areas of high co-intensity. Overlaying on a single map the co-infection and the intensity of multiple parasite infections allows identification of areas where parasite environmental contamination and morbidity are at its highest, while providing an evidence base for the assessment of the progress of successive rounds of mass drug administration (MDA) in integrated parasitic disease control programs.

Individual predisposition, household clustering and risk factors for human infection with Ascaris lumbricoides: new epidemiological insights

Background

Much of our current understanding of the epidemiology of Ascaris lumbricoides infections in humans has been acquired by analyzing worm count data. These data are collected by treating infected individuals with anthelmintics so that worms are expelled intact from the gastrointestinal tract. Analysis of such data established that individuals are predisposed to infection with few or many worms and members of the same household tend to harbor similar numbers of worms. These effects, known respectively as individual predisposition and household clustering, are considered characteristic of the epidemiology of ascariasis. The mechanisms behind these phenomena, however, remain unclear. In particular, the impact of heterogeneous individual exposures to infectious stages has not been thoroughly explored.

Methodology/Principal Findings

Bayesian methods were used to fit a three-level hierarchical statistical model to A. lumbricoides worm counts derived from a three-round chemo-expulsion study carried out in Dhaka, Bangladesh. The effects of individual predisposition, household clustering and household covariates of the numbers of worms per host (worm burden) were considered simultaneously. Individual predisposition was found to be of limited epidemiological significance once household clustering had been accounted for. The degree of intra-household variability among worm burdens was found to be reduced by approximately 58% when household covariates were included in the model. Covariates relating to decreased affluence and quality of housing construction were associated with a statistically significant increase in worm burden.

Conclusions/Significance

Heterogeneities in the exposure of individuals to infectious eggs have an important role in the epidemiology of A. lumbricoides infection. The household covariates identified as being associated with worm burden provide valuable insights into the source of these heterogeneities although above all emphasize and reiterate that infection with A. lumbricoides is inextricably associated with acute poverty.

Numerous analyses have found that people infected with roundworm (Ascaris lumbricoides) are predisposed to harbor either many or few worms. Members of the same household also tend to harbor similar numbers of worms. These phenomena are called individual predisposition and household clustering respectively. In this article, we use Bayesian methods to fit a statistical model to worm count data collected from a cohort of participants at baseline and after two rounds of re-infection following curative treatment. We show that individual predisposition is extremely weak once the clustering effect of the household has been accounted for. This suggests that predisposition is of limited importance to the epidemiology of roundworm infection. Further, we show that over half of the variability in average worm counts among households is explained by household risk factors. This implies that exposures to infectious roundworm eggs shared by household members are important determinants of household clustering. We argue that these results support the hypothesis proposed in the literature that the household is a key focus of roundworm transmission.

The applications of model-based geostatistics in helminth epidemiology and control

Funding agencies are dedicating substantial resources to tackle helminth infections. Reliable maps of the distribution of helminth infection can assist these efforts by targeting control resources to areas of greatest need. The ability to define the distribution of infection at regional, national and subnational levels has been enhanced greatly by the increased availability of good quality survey data and the use of model-based geostatistics (MBG), enabling spatial prediction in unsampled locations. A major advantage of MBG risk mapping approaches is that they provide a flexible statistical platform for handling and representing different sources of uncertainty, providing plausible and robust information on the spatial distribution of infections to inform the design and implementation of control programmes. Focussing on schistosomiasis and soil-transmitted helminthiasis, with additional examples for lymphatic filariasis and onchocerciasis, we review the progress made to date with the application of MBG tools in large-scale, real-world control programmes and propose a general framework for their application to inform integrative spatial planning of helminth disease control programmes.

Lymphatic filariasis and onchocerciasis

Lymphatic filariasis and onchocerciasis are parasitic helminth diseases that constitute a serious public health issue in tropical regions. The filarial nematodes that cause these diseases are transmitted by blood-feeding insects and produce chronic and long-term infection through suppression of host immunity. Disease pathogenesis is linked to host inflammation invoked by the death of the parasite, causing hydrocoele, lymphoedema, and elephantiasis in lymphatic filariasis, and skin disease and blindness in onchocerciasis. Most filarial species that infect people co-exist in mutualistic symbiosis with Wolbachia bacteria, which are essential for growth, development, and survival of their nematode hosts. These endosymbionts contribute to inflammatory disease pathogenesis and are a target for doxycycline therapy, which delivers macrofilaricidal activity, improves pathological outcomes, and is effective as monotherapy. Drugs to treat filariasis include diethylcarbamazine, ivermectin, and albendazole, which are used mostly in combination to reduce microfilariae in blood (lymphatic filariasis) and skin (onchocerciasis). Global programmes for control and elimination have been developed to provide sustained delivery of drugs to affected communities to interrupt transmission of disease and ultimately eliminate this burden on public health.

Identifying sub-optimal responses to ivermectin in the treatment of River Blindness

Identification of drug resistance before it becomes a public health concern requires a clear distinction between what constitutes a normal and a suboptimal treatment response. A novel method of analyzing drug efficacy studies in human helminthiases is proposed and used to investigate recent claims of atypical responses to ivermectin in the treatment of River Blindness. The variability in the rate at which Onchocerca volvulus microfilariae repopulate host's skin following ivermectin treatment is quantified using an individual-based onchocerciasis mathematical model. The model estimates a single skin repopulation rate for every host sampled, allowing reports of suboptimal responses to be statistically compared with responses from populations with no prior exposure to ivermectin. Statistically faster rates of skin repopulation were observed in 3 Ghanaian villages (treated 12-17 times), despite the wide variability in repopulation rates observed in ivermectin-naïve populations. Another village previously thought to have high rates of skin repopulation was shown to be indistinguishable from the normal treatment response. The model is used to generate testable hypotheses to identify whether atypical rates of skin repopulation by microfilariae could result from low treatment coverage alone or provide evidence of decreased ivermectin efficacy. Further work linking phenotypic poor responses to treatment with parasite molecular genetics markers will be required to confirm drug resistance. Limitations of the skin-snipping method for estimating parasite load indicates that changes in the distribution of microfilarial repopulation rates, rather than their absolute values, maybe a more sensitive indicator of emerging ivermectin resistance.

The role of human movement in the transmission of vector-borne pathogens

Background

Human movement is a key behavioral factor in many vector-borne disease systems because it influences exposure to vectors and thus the transmission of pathogens. Human movement transcends spatial and temporal scales with different influences on disease dynamics. Here we develop a conceptual model to evaluate the importance of variation in exposure due to individual human movements for pathogen transmission, focusing on mosquito-borne dengue virus.

Methodology and Principal Findings

We develop a model showing that the relevance of human movement at a particular scale depends on vector behavior. Focusing on the day-biting Aedes aegypti, we illustrate how vector biting behavior combined with fine-scale movements of individual humans engaged in their regular daily routine can influence transmission. Using a simple example, we estimate a transmission rate (R₀) of 1.3 when exposure is assumed to occur only in the home versus 3.75 when exposure at multiple locations—e.g., market, friend's—due to movement is considered. Movement also influences for which sites and individuals risk is greatest. For the example considered, intriguingly, our model predicts little correspondence between vector abundance in a site and estimated R₀ for that site when movement is considered. This illustrates the importance of human movement for understanding and predicting the dynamics of a disease like dengue. To encourage investigation of human movement and disease, we review methods currently available to study human movement and, based on our experience studying dengue in Peru, discuss several important questions to address when designing a study.

Conclusions/Significance

Human movement is a critical, understudied behavioral component underlying the transmission dynamics of many vector-borne pathogens. Understanding movement will facilitate identification of key individuals and sites in the transmission of pathogens such as dengue, which then may provide targets for surveillance, intervention, and improved disease prevention.

Vector-borne diseases constitute a largely neglected and enormous burden on public health in many resource-challenged environments, demanding efficient control strategies that could be developed through improved understanding of pathogen transmission. Human movement—which determines exposure to vectors—is a key behavioral component of vector-borne disease epidemiology that is poorly understood. We develop a conceptual framework to organize past studies by the scale of movement and then examine movements at fine-scale—i.e., people going through their regular, daily routine—that determine exposure to insect vectors for their role in the dynamics of pathogen transmission. We develop a model to quantify risk of vector contact across locations people visit, with emphasis on mosquito-borne dengue virus in the Amazonian city of Iquitos, Peru. An example scenario illustrates how movement generates variation in exposure risk across individuals, how transmission rates within sites can be increased, and that risk within sites is not solely determined by vector density, as is commonly assumed. Our analysis illustrates the importance of human movement for pathogen transmission, yet little is known—especially for populations most at risk to vector-borne diseases (e.g., dengue, leishmaniasis, etc.). We outline several important considerations for designing epidemiological studies to encourage investigation of individual human movement, based on experience studying dengue.

Onchocerca-Simulium interactions and the population and evolutionary biology of Onchocerca volvulus

Parasite-vector interactions shape the population dynamics of vector-borne infections and contribute to observed epidemiological patterns. Also, parasites and their vectors may co-evolve, giving rise to locally adapted combinations or complexes with the potential to stabilise the infection. Here, we focus on Onchocerca-Simulium interactions with particular reference to the transmission dynamics of human onchocerciasis. A wide range of simuliid species may act as vectors of Onchocerca volvulus, each exerting their own influence over the local epidemiology and the feasibility of controlling/eliminating the infection. Firstly, current understanding of the processes involved in parasite acquisition by, and development within, different Simulium species in West Africa and Latin America will be reviewed. A description of how Onchocerca and Simulium exert reciprocal effects on each other's survival at various stages of the parasite's life cycle within the blackfly, and may have adapted to minimise deleterious effects on fitness and maximise transmission will be given. Second, we describe the interactions in terms of resultant (positive and negative) density-dependent processes that regulate parasite abundance, and discuss their incorporation into mathematical models that provide useful qualitative insight regarding transmission breakpoints. Finally, we examine the interactions' influence upon the evolution of anthelmintic resistance, and conclude that local adaptation of Onchocerca-Simulium complexes will influence the feasibility of eliminating the parasite reservoir in different foci.

The development of an age-structured model for trachoma transmission dynamics, pathogenesis and control

BACKGROUND

Trachoma, the worldwide leading infectious cause of blindness, is due to repeated conjunctival infection with Chlamydia trachomatis. The effects of control interventions on population levels of infection and active disease can be promptly measured, but the effects on severe ocular sequelae require long-term monitoring. We present an age-structured mathematical model of trachoma transmission and disease to predict the impact of interventions on the prevalence of blinding trachoma.

METHODOLOGY/PRINCIPAL FINDINGS

The model is based on the concept of multiple reinfections leading to progressive conjunctival scarring, trichiasis, corneal opacity and blindness. It also includes aspects of trachoma natural history, such as an increasing rate of recovery from infection and a decreasing chlamydial load with subsequent infections that depend upon a (presumed) acquired immunity that clears infection with age more rapidly. Parameters were estimated using maximum likelihood by fitting the model to pre-control infection prevalence data from hypo-, meso- and hyperendemic communities from The Gambia and Tanzania. The model reproduces key features of trachoma epidemiology: 1) the age-profile of infection prevalence, which increases to a peak at very young ages and declines at older ages; 2) a shift in this prevalence peak, toward younger ages in higher force of infection environments; 3) a raised overall profile of infection prevalence with higher force of infection; and 4) a rising profile, with age, of the prevalence of the ensuing severe sequelae (trachomatous scarring, trichiasis), as well as estimates of the number of infections that need to occur before these sequelae appear.

CONCLUSIONS/SIGNIFICANCE

We present a framework that is sufficiently comprehensive to examine the outcomes of the A (antibiotic) component of the SAFE strategy on disease. The suitability of the model for representing population-level patterns of infection and disease sequelae is discussed in view of the individual processes leading to these patterns.