High infection rates at low transmission potentials in West African 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.

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.

Model-Based Geostatistical Mapping of the Prevalence of Onchocerca volvulus in West Africa

BACKGROUND

The initial endemicity (pre-control prevalence) of onchocerciasis has been shown to be an important determinant of the feasibility of elimination by mass ivermectin distribution. We present the first geostatistical map of microfilarial prevalence in the former Onchocerciasis Control Programme in West Africa (OCP) before commencement of antivectorial and antiparasitic interventions.

METHODS AND FINDINGS

Pre-control microfilarial prevalence data from 737 villages across the 11 constituent countries in the OCP epidemiological database were used as ground-truth data. These 737 data points, plus a set of statistically selected environmental covariates, were used in a Bayesian model-based geostatistical (B-MBG) approach to generate a continuous surface (at pixel resolution of 5 km x 5km) of microfilarial prevalence in West Africa prior to the commencement of the OCP. Uncertainty in model predictions was measured using a suite of validation statistics, performed on bootstrap samples of held-out validation data. The mean Pearson's correlation between observed and estimated prevalence at validation locations was 0.693; the mean prediction error (average difference between observed and estimated values) was 0.77%, and the mean absolute prediction error (average magnitude of difference between observed and estimated values) was 12.2%. Within OCP boundaries, 17.8 million people were deemed to have been at risk, 7.55 million to have been infected, and mean microfilarial prevalence to have been 45% (range: 2-90%) in 1975.

CONCLUSIONS AND SIGNIFICANCE

This is the first map of initial onchocerciasis prevalence in West Africa using B-MBG. Important environmental predictors of infection prevalence were identified and used in a model out-performing those without spatial random effects or environmental covariates. Results may be compared with recent epidemiological mapping efforts to find areas of persisting transmission. These methods may be extended to areas where data are sparse, and may be used to help inform the feasibility of elimination with current and novel tools.

A research agenda for helminth diseases of humans: modelling for control and elimination

Mathematical modelling of helminth infections has the potential to inform policy and guide research for the control and elimination of human helminthiases. However, this potential, unlike in other parasitic and infectious diseases, has yet to be realised. To place contemporary efforts in a historical context, a summary of the development of mathematical models for helminthiases is presented. These efforts are discussed according to the role that models can play in furthering our understanding of parasite population biology and transmission dynamics, and the effect on such dynamics of control interventions, as well as in enabling estimation of directly unobservable parameters, exploration of transmission breakpoints, and investigation of evolutionary outcomes of control. 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. A research and development agenda for helminthiasis modelling is proposed based on identified gaps that need to be addressed for models to become useful decision tools that can support research and control operations effectively. This agenda includes the use of models to estimate the impact of large-scale interventions on infection incidence; the design of sampling protocols for the monitoring and evaluation of integrated control programmes; the modelling of co-infections; the investigation of the dynamical relationship between infection and morbidity indicators; the improvement of analytical methods for the quantification of anthelmintic efficacy and resistance; the determination of programme endpoints; the linking of dynamical helminth models with helminth geostatistical mapping; and the investigation of the impact of climate change on human helminthiases. It is concluded that modelling should be embedded in helminth research, and in the planning, evaluation, and surveillance of interventions from the outset. Modellers should be essential members of interdisciplinary teams, propitiating a continuous dialogue with end users and stakeholders to reflect public health needs in the terrain, discuss the scope and limitations of models, and update biological assumptions and model outputs regularly. It is highlighted that to reach these goals, a collaborative framework must be developed for the collation, annotation, and sharing of databases from large-scale anthelmintic control programmes, and that helminth modellers should join efforts to tackle key questions in helminth epidemiology and control through the sharing of such databases, and by using diverse, yet complementary, modelling approaches.

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.

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.

Diagnostic value of nodule palpation in onchocerciasis

Nodule palpation is the major diagnostic tool for determining the prevalence of infection in areas of the African Programme for Onchocerciasis Control (APOC) and is recommended for identifying communities at risk and selecting them for mass drug administration. The diagnostic value of palpation, however, has not been quantified in terms of sensitivity and predictive values. We derive these measures from the probability that a nodule is palpable, which has been estimated by stochastic simulations from an extensive pre-control database. We show that nodule palpation is only reliable in highly endemic areas and that false-positive diagnoses can lead to considerable misclassifications of regions where endemicity is actually low. Its diagnostic precision is poor because of large intra- and inter-individual variability. The findings underline the need for further development of available diagnostics that allow long-term monitoring when endemicity declines.