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

Abstract

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.

Deciphering the fine nucleotide diversity of full HLA class I and class II genes in a well‐documented population from sub‐Saharan Africa

“…As DQB1*03:19 probably behaves like DQB1*03:01 regarding peptide presentation (their unique amino acid difference being in the β2 domain), a likely hypothesis is thus to consider DQB1*03:19 as a protective allele to O. volvulus , which would have led to a strong selective sweep increasing the frequency of the whole haplotype DRB1*13:04~DQA1*05:05:01~DQB1*03:19 in the Mandenkalu. The fact that this West African population lives in an area where O. volvulus is highly prevalent strongly supports this hypothesis. The hypothesis formerly proposed by Hill et al in 1992 of recent directional selection to account for the high frequency of DRB1*13:04 in Gambia finds here a more precise explanation.…”

“…Interestingly, the combination of DQB1*03:01 and DQA1*05:01 (whose peptide‐binding region is also identical to that of DQA1*05:05 ) was reported by Meyer et al to be more frequent in individuals putatively immune to onchocerciasis disease, suggesting a role of these 2 alleles in the immune defence against the filarial parasitic worm Onchocerca volvulus ( O. volvulus) . Onchocerciasis, or “river blindness,” is a highly prevalent disease in West Africa, including Eastern Senegal . Although it does not directly kill the infected people, it leads to severe disabilities and decreases life expectancy due to a reduced immunity.…”

“…Onchocerciasis, or "river blindness," is a highly prevalent disease in West Africa, including Eastern Senegal. 91 Although it does not directly kill the infected people, it leads to severe disabilities and decreases life expectancy due to a reduced immunity. As DQB1*03:19 probably behaves like DQB1*03:01 regarding peptide presentation (their unique amino acid difference being in the β2 domain), a likely hypothesis is thus to consider DQB1*03:19 as a protective allele to O. volvulus, which would have led to a strong selective sweep increasing the frequency of the whole haplotype DRB1*13:04D QA1*05:05:01~DQB1*03:19 in the Mandenkalu.…”

Situation analysis of onchocerciasis in Cameroon: a protocol for systematic review of epidemiological studies and impact of disease control interventions

“…Indeed, maps of interventions against onchocerciasis as well as maps of different environmental parameters (climate, vegetation, hydrography, soil … ) will be superimposed on point prevalence and intensity of infection maps to elucidate the impact of these factors on onchocerciasis trends in Cameroon. Kriging and/or model-based geostatistical methods will then be used to obtain smooth prevalence maps throughout the country [25,26].…”

Research for new drugs for elimination of onchocerciasis in Africa

“…The vast majority of these countries are in Africa, where more than 100 Million people live in onchocerciasis endemic areas (Noma et al., 2014, Zoure et al., 2014, O’Hanlon et al., 2016). Around 0.56 Million people were estimated to live in the 13 small endemic foci in 6 countries in Central and South America (Center for Disease Control (CDC), 2013) and around 0.3 Million in Yemen (Mackenzie et al., 2012) Onchocerciasis control started in 1974 with the Onchocerciasis Control Programme in West Africa (OCP) which conducted large scale larviciding of vector breeding sites in the West African Savannah.…”