The elimination of the onchocerciasis vector from the island of Bioko as a result of larviciding by the WHO African Programme for Onchocerciasis Control

Approaching onchocerciasis elimination in Equatorial Guinea: Near zero transmission and public health implication

BACKGROUND

Onchocerciasis and lymphatic filariasis (LF) are endemic in Equatorial Guinea with notable variations in disease incidence between island and mainland regions. Historically, efforts to control and map these diseases were concentrated in Bioko Island, where loiasis is absent, allowing for targeted onchocerciasis interruption strategies. With the cessation of onchocerciasis transmission on Bioko and no reported cases on Annobon island, assessing the transmission status in the previously unaddressed mainland region has become imperative. Mapping efforts in mainland Equatorial Guinea have proven low to moderate level of transmission for LF and onchocerciasis, although the results so far have not been very conclusive. The current study aims to update the prevalence estimates for onchocerciasis and LF in mainland Equatorial Guinea using various diagnostic techniques.

METHODS

This is the first cross-sectional study carried out to estimate the prevalence of onchocerciasis and LF in the mainland area of Equatorial Guinea, from September to December 2019, based on the combination of skin snip biopsies, thick blood smears, laboratory serological tests (ELISA tests for the detection of IgG4 antibodies against Onchocerca volvulus recombinant antigen Ov16 and Wuchereria bancrofti recombinant antigen Wb123) and molecular laboratory tests. Frequencies and prevalence rates, along with 95% confidence intervals for interval estimation of a binomial proportion, were computed.

RESULTS

The overall onchocerciasis seroprevalence calculated for the study was 0.3% (95% CI: 0.1 to 0.5%). Microscopic examination of skin biopsies from the eight individuals seropositive for Ov16, out of the 3951 individuals initially tested, revealed no O. volvulus microfilariae. However, DNA extracted from one skin snip was successfully amplified, with subsequent sequencing confirming the presence of O. volvulus. Among the 3951 individuals, 182 were found to have anti-Wb123 antibodies, suggesting exposure to W. bancrofti, with an estimated seroprevalence of 4.6% (95% CI: 4.0 to 5.3%). Microscopy and Filaria-real time-PCR (F-RT-PCR) analysis for W. bancrofti were negative across all samples.

CONCLUSIONS

The findings indicate that onchocerciasis may no longer constitutes a public health problem in Equatorial Guinea, positioning the country on the verge of achieving elimination. Additionally, the mapped prevalence of LF will facilitate the formulation of national strategies aimed at eradicating filariases countrywide.

Black flies (Diptera: Simuliidae) in the Galapagos Islands: Native or adventive?

Invasive species are a threat to ecosystems worldwide, but determining if a species is adventive or native is not always straightforward. The black flies that inhabit the Galapagos Islands, long known as Simulium ochraceum, are blood-feeding pests of humans and livestock. They first came to the attention of residents in 1989, suggesting a recent arrival. Earlier colonization, however, has been suggested, based largely on polymorphic genetic loci. To address questions of origin, provenance, and length of residency, we conducted a macrogenomic analysis of the polytene chromosomes of the S. ochraceum complex from seven sites in the Galapagos Islands and 30 sites in mainland Ecuador, Central America, and the Caribbean. Among 500 analyzed larvae, we discovered 88 chromosomal rearrangements representing 13 cytoforms, at least seven of which are probably full species. All evidence points to a single, cohesive cytoform with full species status in the Galapagos, conspecific with mainland populations, and widely distributed in the Neotropical Region. It has an identical, nearly monomorphic banding sequence with 10 novel fixed inversions and a subtle but unique Y-linked chromosomal rearrangement across all populations sampled in the Galapagos, the mainland, and the Caribbean. We recalled the name Simulium antillarum from synonymy with S. ochraceum and applied it to the Galapagos black flies, and we established that wolcotti is a junior synonym of antillarum. The time(s) and mode(s) of arrival of S. antillarum in the Galapagos remain uncertain, although the wide geographic distribution, including islands in the Caribbean, suggests that the species is an adept colonizer. Regardless of how long it has been in the archipelago, S. antillarum might have assumed a functional role in the streams of San Cristobal, but otherwise has had a detrimental effect on humans and livestock and potentially on the unique birds and mammals of the Galapagos Islands.

The preparatory phase for ground larviciding implementation for chocerciasis control in the Meme River Basin in South West Cameroon: the COUNTDOWN Consortium alternative strategy implementation trial

BACKGROUND

Onchocerciasis control using ivermectin alone has been achieved in some endemic savannah zones of Africa. In the forest regions, the co-endemicity with Loa loa has led to severe adverse events (SAEs) resulting in poor adherence of community members to ivermectin mass drug administration (MDA). This may jeopardize achieving the interruption of transmission of onchocerciasis. Therefore, to accelerate the elimination of onchocerciasis in L. loa co-endemic zones, alternative treatment strategies (ATS) including ground larviciding may be necessary. This study aimed at identifying Simulium breeding sites, cytospecies, transmission profile, susceptibility of Simulium larvae to insecticide (temephos) and identification of some non-target aquatic fauna prior to the implementation of the COUNTDOWN consortium ground larviciding alternative strategy in the Meme River Basin in South West Cameroon.

METHODS

A topographic map and entomological survey were used to determine breeding sites. Larvae and adults were identified using standard identification keys. Susceptibility tests were carried out on collected larvae by exposing them to decreasing concentrations of temephos and assessing survival rates while the cytospecies were identified using cytotaxonomy. Various entomological indicators were assessed from dissected flies. Fishing was used as proxy to traps to assess some aquatic fauna at different sites.

RESULTS

Twenty-two breeding sites were prospected in the Meme River Basin with eight productive for larvae. A concentration of 0.5-0.1 mg/l temephos induced 100% larval mortality. As the concentration of temephos decreased from 0.05 to 0.0025 mg/l, mortality of larvae also decreased from 98.7 to 12%. Nine cytospecies were observed in the Meme River Basin; 13,633 flies were collected and 4033 dissected. A total of 1455 flies were parous (36.1%), 224 flies were infected (5.5%), and 64 were infective (1.6%). Aquatic fauna observed included Cyprinus spp., Clarias spp., crabs, tadpoles, beetles and larvae of damsel fly.

CONCLUSIONS

Onchocerciasis is being actively transmitted within the Meme River Basin. Simulium larvae are susceptible to temephos, and nine cytospecies are present. Non-target fauna observed included fishes, frogs, crabs and insects. Besides treatment with ivermectin, vector control through ground larviciding may be a complementary strategy to accelerate onchocerciasis elimination in the study area.

Capture of high numbers of Simulium vectors can be achieved with Host Decoy Traps to support data acquisition in the onchocerciasis elimination endgame

Onchocerciasis elimination is within reach in many countries but requires enhanced surveillance of the Simulium vectors of Onchocerca volvulus. Collection of sufficient numbers of adult Simulium to detect infective O. volvulus larvae is hindered by limited sampling tools for these flies. Here, we tested for the first time the Host Decoy Trap (HDT), an exposure free method previously developed for Anopheles vectors of malaria parasites, as a potential sampling tool for adult Simulium. In three replicates of a randomized Latin square experimental design, the HDT was compared to Human Landing Catches (HLC) and the Esperanza Window Trap (EWT). A total of 8,531 adult S. damnosum sensu lato blackflies (S. squamosum group) were found in catches from the three different trapping methods. The HDT (mean catch 533 ± 111) caught significantly more S. squamosum than the EWT (mean catch 9.1 ± 2.2), a nearly 60-fold difference. There was no significant difference between the HLC (mean catch 385.6 ± 80.9) and the HDT. Larvae indistinguishable from those of O. volvulus were dissected from 2.86% of HDT samples (n = 70) and 0.35% of HLC samples (n = 285); a single infective third-stage larvae (L3) was  found during dissection of a sample from the HDT. Owing to its very high capture rate, which was comparable to the HLC and significantly greater than EWT, alongside the presence of infected flies in its catch, the HDT represents a potentially valuable new tool for blackfly collection in elimination settings, where thousands of flies are needed for parasite screening.

The burden of skin disease and eye disease due to onchocerciasis in countries formerly under the African Programme for Onchocerciasis Control mandate for 1990, 2020, and 2030

BACKGROUND

Onchocerciasis ("river blindness") can cause severe morbidity, including vision loss and various skin manifestations, and is targeted for elimination using ivermectin mass drug administration (MDA). We calculated the number of people with Onchocerca volvulus infection and onchocercal skin and eye disease as well as disability-adjusted life years (DALYs) lost from 1990 through to 2030 in areas formerly covered by the African Programme for Onchocerciasis Control.

METHODS

Per MDA implementation unit, we collated data on the pre-control distribution of microfilariae (mf) prevalence and the history of control. Next, we predicted trends in infection and morbidity over time using the ONCHOSIM simulation model. DALY estimates were calculated using disability weights from the Global Burden of Disease Study.

RESULTS

In 1990, prior to MDA implementation, the total population at risk was 79.8 million with 26.0 million (32.5%) mf-positive individuals, of whom 17.5 million (21.9%) had some form of onchocercal skin or eye disease (2.5 million DALYs lost). By 2030, the total population was predicted to increase to 236.1 million, while the number of mf-positive cases (about 6.8 million, 2.9%), people with skin or eye morbidity (4.2 million, 1.8%), and DALYs lost (0.7 million) were predicted to decline.

CONCLUSIONS

MDA has had a remarkable impact on the onchocerciasis burden in countries previously under the APOC mandate. In the few countries where we predict continued transmission between now and 2030, intensified MDA could be combined with local vector control efforts, or the introduction of new drugs for mopping up residual cases of infection and morbidity.

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.

Onchocerciasis (river blindness) - more than a century of research and control

This review summarises more than a century of research on onchocerciasis, also known as river blindness, and its control. River blindness is an infection caused by the tissue filaria Onchocerca volvulus affecting the skin, subcutaneous tissue and eyes and leading to blindness in a minority of infected persons. The parasite is transmitted by its intermediate hosts Simulium spp. which breed in rivers. Featured are history and milestones in onchocerciasis research and control, state-of-the-art data on the parasite, its endobacteria Wolbachia, on the vectors, previous and current prevalence of the infection, its diagnostics, the interaction between the parasite and its host, immune responses and the pathology of onchocerciasis. Detailed information is documented on the time course of control programmes in the afflicted countries in Africa and the Americas, a long road from previous programmes to current successes in control of the transmission of this infectious disease. By development, adjustment and optimization of the control measures, transmission by the vector has been interrupted in foci of countries in the Americas, in Uganda, in Sudan and elsewhere, followed by onchocerciasis eliminations. The current state and future perspectives for control, elimination and eradication within the next 20-30 years are described and discussed. This review contributes to a deeper comprehension of this disease by a tissue-dwelling filaria and it will be helpful in efforts to control and eliminate other filarial infections.

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.

The Mbam drainage system and onchocerciasis transmission post ivermectin mass drug administration (MDA) campaign, Cameroon

Background

The impact of large scale Mass Drug Adminstration (MDA) of ivermectin on active onchocerciasis transmission by Simulium damnosum, which transmits the parasite O. volvulus is of great importance for onchocerciasis control programmes. We investigated in the Mbam river system area, the impact of MDA of ivermectin on entomological indices and also verify if there are river system factors that could have favoured the transmission of onchocerciasis in this area and contribute to the persistence of disease. We compared three independent techniques to detect Onchocerca larvae in blackflies and also analyzed the river system within 9 months post-MDA of ivermectin.

Method

Simulium flies were captured before and after 1, 3, 6 and 9months of ivermectin-MDA. The biting rate was determined and 41% of the flies dissected while the rest were grouped into pools of 100 flies for DNA extraction. The extracted DNA was then subjected to O-150 LAMP and real-time PCR for the detection of infection by Onchocerca species using pool screening. The river system was analysed and the water discharge compared between rainy and dry seasons.

Principal findings

We used human landing collection method (previously called human bait) to collect 22,274 adult female Simulium flies from Mbam River System. Of this number, 9,134 were dissected while 129 pools constituted for molecular screening. Overall biting and parous rates of 1113 flies/man/day and 24.7%, respectively, were observed. All diagnostic techniques detected similar rates of O. volvulus infection (P = 0.9252) and infectivity (P = 0.4825) at all monitoring time points. Onchocerca ochengi larvae were only detected in 2 of the 129 pools. Analysis of the river drainage revealed two hydroelectric dams constructed on the tributaries of the Mbam river were the key contributing factor to the high-water discharge during both rainy and dry seasons.

Conclusion

Results from fly dissection (Microscopy), real-time PCR and LAMP revealed the same trends pre- and post-MDA. The infection rate with animal Onchocerca sp was exceptionally low. The dense river system generate important breeding sites that govern the abundance of Simulium during both dry and rainy seasons.

The presence of parasite strains that respond sub-optimally to an approved drug, favourable breeding sites for the vector and infected individual in an area, will surely provide conditions for continuous and persistent transmission of a disease despite a long-term control intervention. We investigated the impact of ivermectin on entomological indices within 9 months following a large-scale MDA in the Mbam river drainage. The river system factors that could have favoured abundance vector breeding and contribute to the persistence of disease transmission were also examined within the study period. We observed vector abundance and high entomological indices throughout the study period following ivermectin MDA. We also observed high water discharge along the main river of the drainage basin in both the rainy and dry seasons and this is due to the presence of two dams constructed upstream at Bamendjing and Mape to regularize the course of river Sanaga in view of generating hydroelectric power at Edea. Factors favouring continuous and persistent disease transmission are present in this drainage basin despite over 20 years of annual IVM-MDA. There is need for alternative control strategy in order to accelerate the fight against onchocerciasis in the area.

Onchocerciasis Elimination: Progress and Challenges

Onchocerciasis is a parasitic infection caused by the filarial nematode Onchocerca volvulus and transmitted through the bites of black flies of the genus Similium that breed in rivers and streams. The impact of mass treatment with ivermectin and supplemented by vector control in some countries has changed the global scene of onchocerciasis. There has been reported progress made in elimination of onchocerciasis in central and southern American countries and in some localities in Africa. The target for elimination in the Americas has been set at 2022 while for 12 countries in Africa this is expected in 2030. This review was conducted to examine the current status of onchocerciasis elimination at the global level and report on progress made. Literature searches were made through PubMed, articles in English or English abstracts, reports and any other relevant articles related to the subject. The global burden of onchocerciasis is progressively reducing and is no longer a public health problem in some regions. However, programs are challenged with a range of issues: cross-border transmission, diagnostic tools, Loa loa co-endemicity, limited workforce in entomology and maintaining enthusiasm among community drug distributors. More concerted effort using appropriate tools is required to overcome the challenges.

Implementation of test-and-treat with doxycycline and temephos ground larviciding as alternative strategies for accelerating onchocerciasis elimination in an area of loiasis co-endemicity: the COUNTDOWN consortium multi-disciplinary study protocol

Background

Onchocerciasis is a priority neglected tropical disease targeted for elimination by 2025. The standard strategy to combat onchocerciasis is annual Community-Directed Treatment with ivermectin (CDTi). Yet, high prevalence rates and transmission persist following > 12 rounds in South-West Cameroon. Challenges include programme coverage, adherence to, and acceptability of ivermectin in an area of Loa loa co-endemicity. Loiasis patients harbouring heavy infections are at risk of potentially fatal serious adverse events following CDTi. Alternative strategies are therefore needed to achieve onchocerciasis elimination where CDTi effectiveness is suboptimal.

Methods/design

We designed an implementation study to evaluate integrating World Health Organisation-endorsed alternative strategies for the elimination of onchocerciasis, namely test-and-treat with the macrofilaricide, doxycycline (TTd), and ground larviciding for suppression of blackfly vectors with the organophosphate temephos. A community-based controlled before-after intervention study will be conducted among > 2000 participants in 20 intervention (Meme River Basin) and 10 control (Indian River Basin) communities. The primary outcome measure is O. volvulus prevalence at follow-up 18-months post-treatment. The study involves four inter-disciplinary components: parasitology, entomology, applied social sciences and health economics. Onchocerciasis skin infection will be diagnosed by skin biopsy and Loa loa infection will be diagnosed by parasitological examination of finger-prick blood samples. A simultaneous clinical skin disease assessment will be made. Eligible skin-snip-positive individuals will be offered directly-observed treatment for 5 weeks with 100 mg/day doxycycline. Transmission assessments of onchocerciasis in the communities will be collected post-human landing catch of the local biting blackfly vector prior to ground larviciding with temephos every week (0.3 l/m³) until biting rate falls below 5/person/day. Qualitative research, including in-depth interviews and focus-group discussions will be used to assess acceptability and feasibility of the implemented alternative strategies among intervention recipients and providers. Health economics will assess the cost-effectiveness of the implemented interventions.

Conclusions

Using a multidisciplinary approach, we aim to assess the effectiveness of TTd, alone or in combination with ground larviciding, following a single intervention round and scrutinise the acceptability and feasibility of implementing at scale in similar hotspots of onchocerciasis infection, to accelerate onchocerciasis elimination.

Accelerating river blindness elimination by supplementing MDA with a vegetation "slash and clear" vector control strategy: a data-driven modeling analysis

Attention is increasingly focusing on how best to accelerate progress toward meeting the WHO's 2030 goals for neglected tropical diseases (NTDs). For river blindness, a major NTD targeted for elimination, there is a long history of using vector control to suppress transmission, but traditional larvicide-based approaches are limited in their utility. One innovative and sustainable approach, "slash and clear", involves clearing vegetation from breeding areas, and recent field trials indicate that this technique very effectively reduces the biting density of Simulium damnosum s.s. In this study, we use a Bayesian data-driven mathematical modeling approach to investigate the potential impact of this intervention on human onchocerciasis infection. We developed a novel "slash and clear" model describing the effect of the intervention on seasonal black fly biting rates and coupled this with our population dynamics model of Onchocerca volvulus transmission. Our results indicate that supplementing annual drug treatments with "slash and clear" can significantly accelerate the achievement of onchocerciasis elimination. The efficacy of the intervention is not very sensitive to the timing of implementation, and the impact is meaningful even if vegetation is cleared only once per year. As such, this community-driven technique will represent an important option for achieving and sustaining O. volvulus elimination.

Ecological and Societal Services of Aquatic Diptera

More than any other group of macro-organisms, true flies (Diptera) dominate the freshwater environment. Nearly one-third of all flies—roughly 46,000 species—have some developmental connection with an aquatic environment. Their abundance, ubiquity, and diversity of adaptations to the aquatic environment position them as major drivers of ecosystem processes and as sources of products and bioinspiration for the benefit of human society. Larval flies are well represented as ecosystem engineers and keystone species that alter the abiotic and biotic environments through activities such as burrowing, grazing, suspension feeding, and predation. The enormous populations sometimes achieved by aquatic flies can provide the sole or major dietary component for other organisms. Harnessing the services of aquatic Diptera for human benefit depends on the ingenuity of the scientific community. Aquatic flies have played a role as indicators of water quality from the earliest years of bioassessment. They serve as indicators of historical and future ecological and climate change. As predators and herbivores, they can serve as biological control agents. The association of flies with animal carcasses in aquatic environments provides an additional set of tools for forensic science. The extremophilic attributes of numerous species of Diptera offer solutions for human adaptation to harsh terrestrial and extraterrestrial environments. The potential pharmaceutical and industrial applications of the symbiotic microbial community in extremophilic Diptera are better explored than are those of dipteran chemistry. Many flies provide valuable ecological and human services as aquatic immatures, but are also pests and vectors of disease agents as terrestrial adults. The scientific community, thus, is challenged with balancing the benefits and costs of aquatic Diptera, while maintaining sustainable populations as more species face extinction.

Neurological manifestations in Onchocerca volvulus infection: A review

Human onchocerciasis, caused by infection by the filarial nematode Onchocerca volvulus, is a major neglected public health problem that affects millions of people in the endemic regions of sub-Saharan Africa and Latin America. Onchocerciasis is known to be associated with skin and eye disease and more recently, neurological features have been recognized as a major manifestation. Especially the latter poses a severe burden on affected individuals and their families. Although definite studies are awaited, preliminary evidence suggests that neurological disease may include the nodding syndrome, Nakalanga syndrome and epilepsy but to date, the exact pathophysiological mechanisms remain unclear. Currently, the only way to prevent Onchocera volvulus associated disease is through interventions that target the elimination of onchocerciasis through community distribution of ivermectin and larviciding the breeding sites of the Similium or blackfly vector in rivers. In this review, we discuss the epidemiology, potential pathological mechanisms as well as prevention and treatment strategies of onchocerciasis, focusing on the neurological disease.

Onchodermatitis: Where Are We Now?

Onchocerciasis causes debilitating pruritus and rashes as well as visual impairment and blindness. Prior to control measures, eye disease was particularly prominent in savanna areas of sub-Saharan Africa whilst skin disease was more common across rainforest regions of tropical Africa. Mass drug distribution with ivermectin is changing the global scene of onchocerciasis. There has been successful progressive elimination in Central and Southern American countries and the World Health Organization has set a target for elimination in Africa of 2025. This literature review was conducted to examine progress regarding onchocercal skin disease. PubMed searches were performed using keywords 'onchocerciasis', 'onchodermatitis' and 'onchocercal skin disease' over the past eight years. Articles in English, or with an English abstract, were assessed for relevance, including any pertinent references within the articles. Recent progress in awareness of, understanding and treatment of onchocercal skin disease is reviewed with particular emphasis on publications within the past five years. The global burden of onchodermatitis is progressively reducing and is no longer seen in children in many formerly endemic foci.

Modelling the impact of larviciding on the population dynamics and biting rates of Simulium damnosum (s.l.): implications for vector control as a complementary strategy for onchocerciasis elimination in Africa

BACKGROUND

In 2012, the World Health Organization set goals for the elimination of onchocerciasis transmission by 2020 in selected African countries. Epidemiological data and mathematical modelling have indicated that elimination may not be achieved with annual ivermectin distribution in all endemic foci. Complementary and alternative treatment strategies (ATS), including vector control, will be necessary. Implementation of vector control will require that the ecology and population dynamics of Simulium damnosum (sensu lato) be carefully considered.

METHODS

We adapted our previous SIMuliid POPulation dynamics (SIMPOP) model to explore the impact of larvicidal insecticides on S. damnosum (s.l.) biting rates in different ecological contexts and to identify how frequently and for how long vector control should be continued to sustain substantive reductions in vector biting. SIMPOP was fitted to data from large-scale aerial larviciding trials in savannah sites (Ghana) and small-scale ground larviciding trials in forest areas (Cameroon). The model was validated against independent data from Burkina Faso/Côte d'Ivoire (savannah) and Bioko (forest). Scenario analysis explored the effects of ecological and programmatic factors such as pre-control daily biting rate (DBR) and larviciding scheme design on reductions and resurgences in biting rates.

RESULTS

The estimated efficacy of large-scale aerial larviciding in the savannah was greater than that of ground-based larviciding in the forest. Small changes in larvicidal efficacy can have large impacts on intervention success. At 93% larvicidal efficacy (a realistic value based on field trials), 10 consecutive weekly larvicidal treatments would reduce DBRs by 96% (e.g. from 400 to 16 bites/person/day). At 70% efficacy, and for 10 weekly applications, the DBR would decrease by 67% (e.g. from 400 to 132 bites/person/day). Larviciding is more likely to succeed in areas with lower water temperatures and where blackfly species have longer gonotrophic cycles.

CONCLUSIONS

Focal vector control can reduce vector biting rates in settings where a high larvicidal efficacy can be achieved and an appropriate duration and frequency of larviciding can be ensured. Future work linking SIMPOP with onchocerciasis transmission models will permit evaluation of the impact of combined anti-vectorial and anti-parasitic interventions on accelerating elimination of the disease.

Interruption of onchocerciasis transmission in Bioko Island: Accelerating the movement from control to elimination in Equatorial Guinea

Background

Onchocerciasis, also known as river blindness, is a parasitic disease. More than 99 percent of all cases occur in Africa. Bioko Island (Equatorial Guinea) is the only island endemic for onchocerciasis in the world. Since 2005, when vector Simulium yahense was eliminated, there have not been any reported cases of infection. This study aimed to demonstrate that updated WHO criteria for stopping mass drug administration (MDA) have been met.

Methodology/Principal findings

A cross-sectional study was conducted from September 2016 to January 2017. Participants were 5- to 9-year-old school children. Onchocerciasis/lymphatic Filariasis (LF, only in endemic districts) rapid diagnostic tests (RDTs) were performed. Blood spots were collected from RDT positive children and 10 percent of the RDT negatives to determine Ov16 and Wb123 IgG4 antibodies through enzyme-linked immunosorbent assay (ELISA). Skin snips were collected from RDT positives. Filarial detection was performed by PCR in positives and indeterminate sera. Black fly collection was carried out in traditional breeding sites. A total of 7,052 children, ranging from 5 to 9 years of age, were included in the study. Four children (0.06%) were Ov16 IgG4 RDT positives, but negative by ELISA Ov16, while 6 RDT negative children tested positive by ELISA. A total of 1,230 children from the Riaba and Baney districts were tested for LF. One child was Wb123 RDT positive (0.08%), but ELISA negative, while 3 RDT negative children were positive by Wb123 ELISA. All positive samples were negative by PCR for onchocerciasis and LF (in blood spot and skin snip). All fly collections and larval prospections in the traditional catching and prospection sites were negative.

Conclusions/Significance

WHO criteria have been met, therefore MDA in Bioko Island can be stopped. Three years of post-treatment surveillance should be implemented to identify any new occurrences of exposure or infection.

Onchocerciasis, commonly called river blindness, is a chronic parasitic disease particularly prevalent in Africa. It is transmitted through the bites of infected Simulium blackflies. Onchocerciasis is endemic in Equatorial Guinea. Huge achievements have been made in human and vector control during the last two decades, especially on Bioko Island. Eliminating onchocerciasis transmission on Bioko is feasible given its isolation from other landmasses, which also reduces the risk of reinvasion by the disease vector. Recently updated WHO guidelines for stopping mass drug administration (MDA) and verifying elimination of human onchocerciasis (2016) established a new critical threshold to verify elimination of onchocerciasis transmission based on novel serological tests. We applied these techniques in a representative sample of 5- to 9-year-old school children. An entomological assessment was also carried out. We found no evidence of current infection or recent transmission. There was no evidence of onchocerciasis vectors, and our results from the sample population meet the current WHO serologic criteria for stopping MDA. Based on these results, we recommended to the Ministry of Health and Social Welfare of Equatorial Guinea that MDA on Bioko Island be stopped and that 3 years of post-treatment surveillance should be undertaken to identify any new occurrences of exposure or infection.

Geographical distribution and species identification of human filariasis and onchocerciasis in Bioko Island, Equatorial Guinea

Human filariae are vector-borne parasites and the causative agents of various diseases, including human onchocerciasis and lymphatic filariasis. Onchocerciasis causes a spectrum of cutaneous and ophthalmologic manifestations (including blindness) and has long been a major public health problem in Bioko Island (Equatorial Guinea). Bioko Island has been included in the WHO's Onchocerciasis Control Program since 1987. In Bioko Island, the specificity and sensitivity of clinical Onchocerca volvulus diagnosis is key. The objective of this work was to update onchocerciasis elimination progress in Bioko Island, after 18 years of mass ivermectin intervention, and the general filariasis situation through a rapid and accurate molecular method. A cross-sectional study was conducted in Bioko Island from mid-January to mid-February 2014. A total of 543 subjects were included in the study. Whole blood and one skin snip (from lumbar regions) were analysed with a real time PCR assay. Two other skin biopsies were analysed by an expert microscopist. All positive samples were confirmed by sequencing. Traditional microscopic examination of the skin biopsies failed to detect any microfilariae. However, 11 (2.03%) infections were detected using PCR assay, including one O. volvulus, two Mansonella streptocerca, seven Mansonella perstans and one Loa loa infections. PCR assays in blood detected 52 filariae-positive individuals (9.6%) which harboured M. perstans or L. loa. The low prevalence of O. volvulus confirms the success of the Onchocerciasis Control Programme and suggests that Mass Drug Administration in Bioko Island can be interrupted in the near future. The very high prevalence of M. perstans found in skin snips assays raises doubts about the reliability of microscope-based diagnosis of O. volvulus infections.

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.

Evaluation of onchocerciasis seroprevalence in Bioko Island (Equatorial Guinea) after years of disease control programmes

BACKGROUND

Onchocerciasis or "river blindness" is a chronic parasitic disease caused by the filarial worm Onchocerca volvulus, transmitted through infected blackflies (Simulium spp.). Bioko Island (Equatorial Guinea) used to show a high endemicity for onchocerciasis. During the last years, the disease control programmes using different larvicides and ivermectin administration have considerably reduced the prevalence and intensity of infection. Based on this new epidemiological scenario, in the present work we aimed to assess the impact of the strategies applied against onchocerciasis in Bioko Island by an evaluation of IgG4 antibodies specific for recombinant Ov-16 in ELISA.

METHODS

A cross-sectional study was conducted in Bioko Island from mid-January to mid-February, 2014. Twenty communities were randomly selected from rural and urban settings. A total of 140 households were chosen. In every selected household, all individuals aged 5 years and above were recruited; 544 study participants agreed to be part of this work. No previous data on onchocerciasis seroprevalence in the selected communities were available. Blood samples were collected and used in an "ELISA in-house" prepared with recombinant Ov-16, expressed and further purified. IgG4 antibodies specific for recombinant Ov-16 were evaluated by ELISA in all of the participants.

RESULTS

Based on the Ov-16 ELISA, the onchocerciasis seroprevalence was 7.9 %, mainly concentrated in rural settings; samples from community Catedral Ela Nguema (# 16) were missed during the field work. Among the rural setups, communities Inasa Maule (# 7), Ruiché (# 20) and Barrios Adyacentes Riaba (# 14), had the highest seropositivity percentages (29.2, 26.9 and 23.8 %, respectively). With respect to the urban settings, we did not find any positive case in communities Manzana Casa Bola (# 3), Colas Sesgas (# 6), Getesa (# 8), Moka Bioko (# 9), Impecsa (# 10), Baney Zona Baja (# 12) and Santo Tomás de Aquino (# 1). No onchocerciasis seropositive samples were found in 10-year-old individuals or younger. The IgG4 positive titles increased in older participants.

CONCLUSIONS

A significant decline in onchocerciasis prevalence was observed in Bioko Island after years of disease-vector control and CDTI strategy. The seroprevalence increased with age, mainly in rural settings that could be due to previous exposure of population to the filarial parasite, eliminated by the control programmes introduced against onchocerciasis. A new Ov-16 serological evaluation with a larger sample size of children below 10 years of age is required to demonstrate the interruption of transmission of O. volvulus in the human population of Bioko Island (Equatorial Guinea) according to the WHO criteria.

Evidence for Suppression of Onchocerciasis Transmission in Bioko Island, Equatorial Guinea

Onchocerciasis or "river blindness" is a chronic parasitic neglected tropical disease which is endemic both in mainland and insular Equatorial Guinea. We aim to estimate the current epidemiological situation of onchocerciasis in Bioko Island after vector elimination in 2005 and more than sixteen years of Community Directed Treatment with Ivermectin (CDTI) by using molecular and serological approaches for onchocerciasis diagnosis. A community-based cross-sectional study was carried out in Bioko Island from mid-January to mid-February 2014. A total of 544 study participants were recruited. A complete dermatological examination was performed and three skin snips were performed in every participant for parasitological and molecular assessments. Blood spots were also taken for determination of Ov16 IgG4 antibodies trough an “in-house” ELISA assay. Overall, we found 15 out of 522 individuals suffering any onchocerciasis specific cutaneous lesions and 16 out of 528 (3.0%) with onchocercal nodules in the skin. Nodules were significantly associated with age, being more common in subjects older than 10 years than in younger people (3.9% vs. 0%, p = 0.029). Regarding the onchocerciasis laboratory assessment, no positive parasitological test for microfilaria detection was found in the skin snips. The calculated seroprevalence through IgG4 serology was 7.9%. No children less than 10 years old were found to be positive for this test. Only one case was positive for Onchocerca volvulus (O. volvulus) after skin PCR. The present study points out that the on-going mass ivermectin treatment has been effective in reducing the prevalence of onchocerciasis and corroborates the interruption of transmission in Bioko Island. To our knowledge, this is the first time that accurate information through molecular and serological techniques is generated to estimate the onchocerciasis prevalence in this zone. Sustained support from the national program and appropriate communication and health education strategies to reinforce participation in CDTI activities are essential to ensure progress towards onchocerciasis elimination in the country.

Onchocerciasis or “river blindness” is a chronic parasitic disease which is mainly found in Sub-Saharan Africa. Onchocerciasis is endemic in both mainland and insular Equatorial Guinea. Huge achievements have been made on onchocerciasis control in Bioko Island in the last years, and the country is moving fast towards elimination. In the new elimination context, monitoring and evaluation activities with more sensitive diagnostic tools become especially necessary in order to confirm that transmission has been interrupted. Previous data on the epidemiological situation of onchocerciasis in Bioko Island are mainly based on microfilaria (MF) skin snip assessments. We aim to create evidence towards the fact that onchocerciasis transmission might have been achieved in Bioko Island after more than sixteen years of onchocerciasis control activities by using molecular and serological technics for onchocerciasis diagnosis.

Relationship between oral declaration on adherence to ivermectin treatment and parasitological indicators of onchocerciasis in an area of persistent transmission despite a decade of mass drug administration in Cameroon

Background

Onchocerciasis control for years has been based on mass drug administration (MDA) with ivermectin (IVM). Adherence to IVM repeated treatment has recently been shown to be a confounding factor for onchocerciasis elimination precisely in rain forest areas where transmission continues and Loa loa co-exists with Onchocerca volvulus. In this study, participants’ oral declarations were used as proxy to determine the relationship between adherence to IVM treatment and parasitological indicators of onchocerciasis in the rain forest area of Cameroon with more than a decade of MDA.

Methods

Participants were recruited based on their IVM intake profile with the aid of a semi-structured questionnaire. Parasitological examinations (skin sniping and nodule palpation) were done on eligible candidates. Parasitological indicators were calculated and correlated to IVM intake profile.

Results

Of 2,364 people examined, 15.5 % had never taken IVM. The majority (40.4 %) had taken the drug 1–3 times while only 18 % had taken ≥ 7 times. Mf and nodule prevalence rates were still high at 47 %, 95 % CI [44.9–49.0 %] and 36.4 %, 95 % CI [34.4–38.3 %] respectively. There was a treatment-dependent reduction in microfilaria prevalence (r_s =−0.986, P = 0.01) and intensity (r_s =−0.96, P = 0.01). The highest mf prevalence (59.7 %) was found in the zero treatment group and the lowest (33.9 %) in the ≥ 7 times treatment group (OR = 2.8; 95 % CI [2.09–3.74]; P < 0.001). Adults with ≥ 7 times IVM intake were 2.99 times more likely to have individuals with no microfilaria compared to the zero treatment group (OR = 2.99; 95 % CI [2.19–4.08], P < 0.0001). There was no clear correlation between treatment and nodule prevalence and intensity.

Conclusion

Adherence to ivermectin treatment is not adequate in this rain forest area where L. loa co-exists with O. volvulus. The prevalence and intensity of onchocerciasis remained high in individuals with zero IVM intake after more than a decade of MDA. Our findings show that using parasitological indicators, reduction in prevalence is IVM intake-dependent and that participants’ oral declaration of treatment adherence could be relied upon for impact studies. The findings are discussed in the context of challenges for the elimination of onchocerciasis in this rain forest area.

Hypo-endemic onchocerciasis hotspots: defining areas of high risk through micro-mapping and environmental delineation

Background

Onchocerciasis (river blindness) caused by the parasite Onchocercavolvulus and transmitted by riverine Simulium spp. (Black flies) is targeted for elimination in Africa. This is a significant change in strategy from the ‘control’ of meso- and hyper-endemic areas through mass drug administration (MDA) with Mectizan® (ivermectin), to the ‘elimination’ in all endemic areas where a range of interventions may be required. The most significant challenges of elimination in low transmission or hypo-endemic areas are two-fold. First, there are vast remote areas where the focality of low transmission is relatively undefined. Second, the treatment with ivermectin increases the risk of serious adverse events (SAEs) in individuals with high parasitaemias of Loa loa, a filarial parasite widespread in Central and West Africa, which causes Tropical eye worm and transmitted by Chrysops spp. (Deer flies).

Discussion

We therefore propose novel mapping approaches using remote sensing satellite and modelled environmental data to be used in combination with rapid field surveys to help resolve the problems of targeting the expansion of onchocerciasis elimination activities in L. loa co-endemic areas. First, we demonstrate that micro-stratification overlap mapping (MOM) of available onchocerciasis and loiasis prevalence maps can be used to identify 12 key high risk areas, where low O. volvulusand high L. loa transmission overlap, which we define as “hypo-endemic hotspots”. Second we show that integrated micro-mapping of prevalence data, and the use of environmental data to delineate riverine and forest risk factors associated with Simulium spp. and Chrysops spp. vector habitats can further help to define target intervention areas i.e. secondary hotspots within hotspots, to help avoid the risk of SAEs.

Summary

These mapping examples demonstrate the value of bringing prevalence, entomological and ecological information together to develop maps for planned implementation and targeted strategies. This is critical as better mapping may the reduce costs and lower the L. loa associated risks, especially if there are extensive areas of low endemicity that may require treatment with ivermectin or alternative strategies. Novel cost-effective approaches are necessary if elimination of O.volvulus transmission in Africa is to be achieved in an efficient and safe way by the goal of 2025.

Electronic supplementary material

The online version of this article (doi:10.1186/s40249-015-0069-6) contains supplementary material, which is available to authorized users.

Nodding syndrome since 2012: recent progress, challenges and recommendations for future research

We aim to review the current epidemiology of nodding syndrome (NS) and discuss relevant gaps in research. NS and convulsive epilepsy of unknown aetiology are clustered within the same villages and families in onchocerciasis-endemic areas. They are therefore potentially different clinical expressions of the same disease. It has been difficult to perform full autopsies on NS patients who die in remote villages. Adequate fixation of tissue immediately after death is critical for the examination of brain tissue. Therefore, post-mortem transsphenoidal brain biopsies, performed immediately after death by trained nurses, will provide the best option for obtaining tissue for analysis. We suspect that certain blackflies in onchocerciasis-endemic areas may transmit a novel pathogen that could cause NS and epilepsy. This is supported by a recent drop in the number of new NS cases coinciding with vector control activities aimed at reducing blackfly populations in northern Uganda. We propose that metagenomic studies of human samples, blackflies and microfilariae are conducted to screen for pathogens, and that a clinical trial is planned to evaluate the impact of larviciding against NS and epilepsy epidemics.

New tools and insights to assist with the molecular identification of Simulium guianense s.l., main Onchocerca volvulus vector within the highland areas of the Amazonia onchocerciasis focus

Following the success of the Onchocerciasis Elimination Programme for the Americas (OEPA), there is now just one Latin American onchocerciasis focus where onchocerciasis transmission is described as 'on-going:' the Amazonia Onchocerciasis focus. In the hyperendemic highland areas of the Amazonia focus, Simulium guianense s.l. Wise are the most important vectors of the disease. Populations of S. guianense s.l. are, however, known to vary in their cytogenetics and in a range of behaviours, including in their biting habits. In the hypoendemic lowland areas of the Amazonia focus, for example, S. guianense s.l. are generally regarded as zoophilic and consequently unimportant to disease transmission. Robust tools, to discriminate among various populations of S. guianense s.l. have, however, not yet been developed. In the work reported here, we have assessed the utility of a ribosomal DNA sequence fragment spanning the nuclear ribosomal ITS-1, ITS-2 and 5.8S sequence regions and a ∼850 nucleotide portion of the mitochondrial cytochrome oxidase gene (CO1) for species-level identification and for resolving the within species substructuring. We report here how we have generated 78 CO1 sequences from a rich set of both zoophilic and anthropophilic populations of S. guianense s.l. that were collected from eight sites that are broadly distributed across Brazil. Consistent with previous findings, our analysis supports the genetic isolation of Simulium litobranchium from S. guianense s.l. In contrast with previous findings, however, our results did not provide support for the divergence of the two species prior to the radiation of S. guianense s.l. In our analysis of the S. guianense s.l. ribosomal DNA sequence trace files we generated, we provide clear evidence of multiple within-specimen single nucleotide polymorphisms and indels suggesting that S. guianense s.l. ribosomal DNA is not a good target for conventional DNA barcoding. This is the first report of S. guianense s.l. within individual ribosomal DNA variation and thus the first evidence that the species is not subject to the normal effects of concerted evolution. Collectively, these data illustrate the need for diverse sampling in the development of robust molecular tools for vector identification and suggest that ribosomal DNA might be able to assist with resolving S. guianense s.l. species substructuring that C01 barcoding has hitherto failed to.

New molecular identifiers for Simulium limbatum and Simulium incrustatum s.l. and the detection of genetic substructure with potential implications for onchocerciasis epidemiology in the Amazonia focus of Brazil

The Amazonia onchocerciasis focus of southern Venezuela and northern Brazil is the larger of the two remaining Latin American onchocerciasis foci where disease transmission still occurs and is often regarded as the most challenging of all the Latin American foci to eliminate onchocerciasis. The site is home to a population of over 20,000 semi-nomadic, hunter-gatherer Yanomami people and is made-up of a mosaic of rainforest and savannah ecologies, which are influenced by the area's undulating terrain and rich geological diversity. At least six blackfly vectors have been implicated in onchocerciasis transmission in this focus; however, because of the difficulty in their routine identification the relative importance of each has been obscured. Simulium limbatum and Simulium incrustatum s.l. have both been recorded as vectors in the Amazonia focus, but they are difficult to discriminate morphologically and thus the ecological range of these species, and indeed the presence of S. limbatum in the Amazonia focus at all, have remained controversial. In the work described here, we report 15 S. incrustatum s.l. CO1 sequences and 27 S. limbatum sequences obtained from field-caught adult female blackflies collected from forest and savannah localities, inside and just outside the Amazonia focus. Phylogenetic analysis with the sequences generated in this study, showed that both the S. limbatum and the S. incrustatum s.l. CO1 sequences obtained (even from specimens living in sympatry) all fell into discrete species-specific bootstrap-supported monophyletic groups and thus confirmed the utility of the CO1 gene for identifying both these species inside the Amazonia focus. As the S. limbatum-exclusive cluster included CO1 sequences obtained from forest-caught and morphologically identified specimens these results provide the clearest evidence yet of the presence of S. limbatum inside the Amazonia focus. The question, however, of whether S. limbatum is actually a vector in the focus still remains unanswered as the data presented here also suggest that S. limbatum found in the savannahs adjacent to, but outside the Amazonia focus (and which represent the only S. limbatum population to be unambiguously incriminated as a host of Onchocerca volvulus), are genetically distinct from those living inside the focus. These findings highlight the need for a clearer picture of the vector taxonomy inside the Amazonia onchocerciasis focus.

Preventive chemotherapy as a strategy for elimination of neglected tropical parasitic diseases: endgame challenges

Global efforts to address neglected tropical diseases (NTDs) were stimulated in January 2012 by the London declaration at which 22 partners, including the Bill & Melinda Gates Foundation, World Bank, World Health Organization (WHO) and major pharmaceutical companies committed to sustaining and expanding NTD programmes to eliminate or eradicate 11 NTDs by 2020 to achieve the goals outlined in the recently published WHO road map. Here, we present the current context of preventive chemotherapy for some NTDs, and discuss the problems faced by programmes as they consider the 'endgame', such as difficulties of access to populations in post-conflict settings, limited human and financial resources, and the need to expand access to clean water and improved sanitation for schistosomiasis and soil-transmitted helminthiasis. In the case of onchocerciasis and lymphatic filariasis, ivermectin treatment carries a significant risk owing to serious adverse effects in some patients co-infected with the tropical eye worm Loa loa filariasis. We discuss the challenges of managing complex partnerships, and maintain advocacy messages for the continued support for elimination of these preventable diseases.

Proposed guidelines for the management of nodding syndrome

Nodding Syndrome is a poorly understood neurologic disorder of unknown aetiology that affects children and adolescents in Africa. Recent studies have suggested that the head nods are due to atonic seizures and Nodding Syndrome may be classified as probably symptomatic generalised epilepsy. As part of the Ugandan Ministry of Health clinical management response, a multidisciplinary team developed a manual to guide the training of health workers with knowledge and skills to manage the patients. In the absence of a known cause, it was decided to offer symptomatic care. The objective is to relieve symptoms, offer primary and secondary prevention for disability and rehabilitation to improve function. Initial management focuses on the most urgent needs of the patient and the immediate family until 'stability' is achieved. The most important needs were considered as seizure control, management of behavioural and psychiatric difficulties, nursing care, nutritional and subsequently, physical and cognitive rehabilitation. This paper summarises the processes by which the proposed guidelines were developed and provides an outline of the specific treatments currently being provided for the patients.

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.

Indices of onchocerciasis transmission by different members of the Simulium damnosum complex conflict with the paradigm of forest and savanna parasite strains

Onchocerciasis in savanna zones is generally more severe than in the forest and pathologies also differ geographically, differences often ascribed to the existence of two or more strains and incompatibilities between vectors and strains. However, flies in the forest transmit more infective larvae than their savanna counterparts, even in sympatry, contradicting expectations based on the forest and savanna strains paradigm. We analysed data on the numbers of Onchocerca volvulus larvae of different stages found in 10 different taxonomic categories of the Simulium damnosum complex derived from more than 48,800 dissections of flies from Sierra Leone in the west of Africa to Uganda in the east. The samples were collected before widespread ivermectin distribution and thus provide a baseline for evaluating control measures. Savanna species contained fewer larvae per infected or per infective fly than the forest species, even when biting and parous rates were accounted for. The highest transmission indices were found in the forest-dwelling Pra form of Simulium sanctipauli (616 L3/1000 parous flies) and the lowest in the savanna-inhabiting species S. damnosum/S. sirbanum (135) and S. kilibanum (65). Frequency distributions of numbers of L1-2 and L3 larvae found in parous S. damnosum/S. sirbanum, S. kilibanum, S. squamosum, S. yahense, S. sanctipauli, S. leonense and S. soubrense all conformed to the negative binomial distribution, with the mainly savanna-dwelling species (S. damnosum/S. sirbanum) having less overdispersed distributions than the mainly forest-dwelling species. These infection patterns were maintained even when forest and savanna forms were sympatric and biting the same human population. Furthermore, for the first time, levels of blindness were positively correlated with infection intensities of the forest vector S. yahense, consistent with relations previously reported for savanna zones. Another novel result was that conversion rates of L1-2 larvae to L3s were equivalent for both forest and savanna vectors. We suggest that either a multiplicity of factors are contributing to the observed disease patterns or that many parasite strains exist within a continuum.

A research agenda for helminth diseases of humans: health research and capacity building in disease-endemic countries for helminthiases control

Capacity building in health research generally, and helminthiasis research particularly, is pivotal to the implementation of the research and development agenda for the control and elimination of human helminthiases that has been proposed thematically in the preceding reviews of this collection. Since helminth infections affect human populations particularly in marginalised and low-income regions of the world, they belong to the group of poverty-related infectious diseases, and their alleviation through research, policy, and practice is a sine qua non condition for the achievement of the United Nations Millennium Development Goals. Current efforts supporting research capacity building specifically for the control of helminthiases have been devised and funded, almost in their entirety, by international donor agencies, major funding bodies, and academic institutions from the developed world, contributing to the creation of (not always equitable) North-South "partnerships". There is an urgent need to shift this paradigm in disease-endemic countries (DECs) by refocusing political will, and harnessing unshakeable commitment by the countries' governments, towards health research and capacity building policies to ensure long-term investment in combating and sustaining the control and eventual elimination of infectious diseases of poverty. The Disease Reference Group on Helminth Infections (DRG4), established in 2009 by the Special Programme for Research and Training in Tropical Diseases (TDR), was given the mandate to review helminthiases research and identify research priorities and gaps. This paper discusses the challenges confronting capacity building for parasitic disease research in DECs, describes current capacity building strategies with particular reference to neglected tropical diseases and human helminthiases, and outlines recommendations to redress the balance of alliances and partnerships for health research between the developed countries of the "North" and the developing countries of the "South". We argue that investing in South-South collaborative research policies and capacity is as important as their North-South counterparts and is essential for scaled-up and improved control of helminthic diseases and ultimately for regional elimination.

Elimination of onchocerciasis from Africa: possible?

Human onchocerciasis, a parasitic disease found in 28 African countries, six Latin American countries and Yemen, causes blindness and severe dermatological problems. In 1987, efforts to control this infection shifted from vector approaches to include the mass distribution of ivermectin - a drug donated by Merck & Co. for disease control in Africa and for disease elimination in the Americas. Currently, almost 25 years later, with the Americas being highly successful and now approaching elimination, new evidence points towards the possibility of successful elimination in Africa. We suggest several major changes in the programmatic approach that through focused goal-directed effort could achieve global elimination of onchocerciasis by 2025.

A guide to the Simulium damnosum complex (Diptera: Simuliidae) in Nigeria, with a cytotaxonomic key for the identification of the sibling species

Although approximately 40% of all the people blinded by Onchocerca volvulus are Nigerians, almost nothing was known about the various cytospecies of the blackfly vectors present in Nigeria until 1981. The activation of the Nigerian National Onchocerciasis Control Programme in 1986 (and that programme's initiation of mass distributions of ivermectin in 1991) provided a significant stimulus to understand the biology of the Nigerian vectors but the exploration of any possible differences between the cytospecies has been hampered by a lack of accessible taxonomic information. This review attempts to satisfy that need. There are nine different cytoforms reliably recorded from Nigeria (Simulium damnosum s.s. Nile form, S. damnosum s.s. Volta form, S. sirbanum Sirba form, S. sirbanum Sudanense form, S. soubrense Beffa form, S. squamosum A, S. squamosum B, S. squamosum C and S. yahense typical form), and three more are known from surrounding countries and might be reasonably expected to occur in Nigeria. All of these cytospecies are presumed to be vectors, although there have been almost no identifications of the vectors of O. volvulus in Nigeria. The biogeographical distribution of the cytoforms is broadly similar to that known in other parts of West Africa (although many of the cytoforms remain insufficiently studied). The physico-chemical hydrology of the Nigerian breeding sites of the cytospecies does not, however, correspond to that seen elsewhere in West Africa, and it is not clear whether this might be related to differences in the cytoforms. An illustrated cytotaxonomic key is presented to facilitate and encourage future studies.

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.

Onchodermatitis

PURPOSE OF REVIEW

This review is timely because awareness of the burden of disease from onchodermatitis has increased significantly over recent years. Recent progress in the field is reviewed with emphasis on publications within the past 2 years.

RECENT FINDINGS

Advances have been made in understanding immunopathogenesis and in diagnosis and treatment. The World Bank/WHO African Programme for Onchocerciasis Control (APOC), which uses annual community-directed treatment with ivermectin (CDTI) via the Mectizan Donation Programme, now covers 19 African countries. Development of ivermectin resistance is a concern. Unlike ivermectin, which is a microfilaricide, doxycycline, which targets Wolbachia endosymbiotic bacteria, sterilizes adult female worms and has a macrofilaricidal effect. Moxidectin, which sterilizes or kills adult worms has started a phase III trial with ivermectin. Additional primary healthcare interventions have been successfully integrated with CTDI. In Latin America, transmission has been interrupted in half of the original endemic foci and Colombia is the first nation to have achieved countrywide interruption of transmission. The first report of elimination using ivermectin in an African setting is a milestone. Two African foci using vector control plus CDTI have reported vector elimination.

SUMMARY

Results of the longer-term impact of large-scale ivermectin distribution by the APOC are awaited. Research is needed into new drug targets within Wolbachia's metabolic pathways. Elimination of transmission of disease is on the horizon but more research is needed on when and where ivermectin treatment can be stopped.

Neglected tropical diseases

INTRODUCTION

The neglected tropical diseases (NTDs) are infectious diseases that principally impact the world's poorest people. They have been neglected for decades, initially as part of a general disregard for the developing world, and more recently due to the intensity of focus on HIV/AIDS, tuberculosis and malaria.

SOURCES OF DATA

Primary research and review articles were selected for inclusion using searches of PubMed and our existing collections.

RESULTS

There have been recent notable successes in NTD control. Dracunculiasis is approaching eradication. Leprosy and onchocerciasis are in decline. There are ambitious plans to eliminate trachoma and lymphatic filariasis. Investment in NTD control has high rates of economic return.

CONCLUSION

Although there are proven strategies to control several NTDs, these diseases continue to cause a massive burden of morbidity. There is urgent need for more basic and operational research, drug and vaccine development, and greater prioritization by governments and international agencies.