Onchocerciasis

Bradley, Joshua; Whitworth, James; Basáñez, María-Gloria

doi:10.1002/9780470688618.taw0204

Cited by 14 publications

(19 citation statements)

References 145 publications

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“…volvulus depends on transmission by biting flies of the genus Simulium . Persistent infection can lead to gradual loss of visual acuity, eventually leading to blindness, and to the development of a variety of severe and debilitating skin lesions [ 4 , 5 ]. The infection is also a cause of excess human mortality both indirectly through blindness and directly through (still poorly understood mechanisms caused by) heavy infection in sighted individuals [ 6 , 7 ].…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2016

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BackgroundThe 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 FindingsPre-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 SignificanceThis 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.

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Section: Introductionmentioning

confidence: 99%

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

et al. 2016

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“…Onchocerciasis is a chronic and cumulative skin and ocular disease caused by infection with the filarial nematode Onchocerca volvulus Leuckart and transmitted among humans through the bites of blackfly species of the genus Simulium Latreille. The embryonic stages of the parasite (microfilariae, mf) migrate through the skin and cause severe itching, skin disease, and ocular lesions, with the severity of the clinical manifestations depending on the length of exposure to blackfly bites and the density of mf in the skin [ 1 , 2 ]. Visual loss and blindness can result from exposure to heavy parasite loads in the human host over time [ 2 ].…”

Section: Introductionmentioning

confidence: 99%

“…The embryonic stages of the parasite (microfilariae, mf) migrate through the skin and cause severe itching, skin disease, and ocular lesions, with the severity of the clinical manifestations depending on the length of exposure to blackfly bites and the density of mf in the skin [ 1 , 2 ]. Visual loss and blindness can result from exposure to heavy parasite loads in the human host over time [ 2 ]. Since blackflies breed in fast flowing rivers, the disease is also known as ‘river blindness’, although in the Americas it is called Robles’ Disease after Rodolfo Robles, who described it one hundred years ago in Guatemala [ 3 ].…”

Section: Introductionmentioning

confidence: 99%

Evidence of suppression of onchocerciasis transmission in the Venezuelan Amazonian focus

et al. 2016

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BackgroundThe World Health Organization (WHO) has set goals for onchocerciasis elimination in Latin America by 2015. Most of the six previously endemic countries are attaining this goal by implementing twice a year (and in some foci, quarterly) mass ivermectin (Mectizan®) distribution. Elimination of transmission has been verified in Colombia, Ecuador and Mexico. Challenges remain in the Amazonian focus straddling Venezuela and Brazil, where the disease affects the hard-to-reach Yanomami indigenous population. We provide evidence of suppression of Onchocerca volvulus transmission by Simulium guianense s.l. in 16 previously hyperendemic Yanomami communities in southern Venezuela after 15 years of 6-monthly and 5 years of 3-monthly mass ivermectin treatment.MethodsBaseline and monitoring and evaluation parasitological, ophthalmological, entomological and serological surveys were conducted in selected sentinel and extra-sentinel communities of the focus throughout the implementation of the programme.ResultsFrom 2010 to 2012–2015, clinico-parasitological surveys indicate a substantial decrease in skin microfilarial prevalence and intensity of infection; accompanied by no evidence (or very low prevalence and intensity) of ocular microfilariae in the examined population. Of a total of 51,341 S. guianense flies tested by PCR none had L3 infection (heads only). Prevalence of infective flies and seasonal transmission potentials in 2012–2013 were, respectively, under 1 % and 20 L3/person/transmission season. Serology in children aged 1–10 years demonstrated that although 26 out of 396 (7 %) individuals still had Ov-16 antibodies, only 4/218 (2 %) seropositives were aged 1–5 years.ConclusionsWe report evidence of recent transmission and morbidity suppression in some communities of the focus representing 75 % of the Yanomami population and 70 % of all known communities. We conclude that onchocerciasis transmission could be feasibly interrupted in the Venezuelan Amazonian focus.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-016-1313-z) contains supplementary material, which is available to authorized users.

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“…damnosum sensu stricto / S . sirbanum ) are responsible for the most severe sequelae of onchocerciasis [ 37 , 38 ]. The model captures age- and sex-specific host exposure to biting blackfly vectors ( Fig 2A ), calibrated to reproduce observed pre-control microfilarial load (infection intensity) age profiles ( Fig 2B ) in Cameroon [ 32 ], epidemiological patterns which are also seen in forest areas of Cameroon [ 35 ] and elsewhere in foci under vector control in the Onchocerciasis Control Programme in West Africa (OCP) area [ 39 ].…”

Section: Methodsmentioning

confidence: 99%

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

et al. 2015

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BackgroundCurrently, the predominant onchocerciasis control strategy in Africa is annual mass drug administration (MDA) with ivermectin. However, there is a consensus among the global health community, supported by mathematical modelling, that onchocerciasis in Africa will not be eliminated within proposed time frameworks in all endemic foci with only annual MDA, and novel and alternative strategies are urgently needed. Furthermore, use of MDA with ivermectin is already compromised in large areas of central Africa co-endemic with Loa loa, and there are areas where suboptimal or atypical responses to ivermectin have been documented. An onchocerciasis vaccine would be highly advantageous in these areas.Methodology/Principal FindingsWe used a previously developed onchocerciasis transmission model (EPIONCHO) to investigate the impact of vaccination in areas where loiasis and onchocerciasis are co-endemic and ivermectin is contraindicated. We also explore the potential influence of a vaccination programme on infection resurgence in areas where local elimination has been successfully achieved. Based on the age range included in the Expanded Programme on Immunization (EPI), the vaccine was assumed to target 1 to 5 year olds. Our modelling results indicate that the deployment of an onchocerciasis vaccine would have a beneficial impact in onchocerciasis–loiasis co-endemic areas, markedly reducing microfilarial load in the young (under 20 yr) age groups.Conclusions/SignificanceAn onchocerciasis prophylactic vaccine would reduce the onchocerciasis disease burden in populations where ivermectin cannot be administered safely. Moreover, a vaccine could substantially decrease the chance of re-emergence of Onchocerca volvulus infection in areas where it is deemed that MDA with ivermectin can be stopped. Therefore, a vaccine would protect the substantial investments made by present and past onchocerciasis control programmes, decreasing the chance of disease recrudescence and offering an important additional tool to mitigate the potentially devastating impact of emerging ivermectin resistance.

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Onchocerciasis

Cited by 14 publications

References 145 publications

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

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

Evidence of suppression of onchocerciasis transmission in the Venezuelan Amazonian focus

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

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