The transmission of onchocerciasis at a forest village in Sierra Leone. I.<i>Simulium damnosum</i>s.l. biting densities and infection with<i>Onchocerca volvulus</i>at five representative sites

Bockarie, Moses J.; Davies, J.B.; Thomson, Madeleine C.; Morgan, H. G.

doi:10.1080/00034983.1990.11812514

Cited by 9 publications

(3 citation statements)

References 8 publications

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“…However, Renz et al (1987) found biting density to drop to 10% of its original level at distances of 5km from riverine breeding sites in Sudan-savanna focus of Northern Cameroon [ 81 ]. Other studies have shown that people may be bitten by female blackflies in locations distant from any known breeding site [ 82 – 84 ] and that re-invasions of parous flies into areas previously cleared through larviciding can happen from breeding sites as far as 300km away [ 85 ]. This lends strong epidemiological and ecological support to the operation of a spatial trend operating over this distance.…”

Section: Discussionmentioning

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: Discussionmentioning

confidence: 99%

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

et al. 2016

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“…Coordinates of collection sites not shown on the map Fig. 1 prior to intervention measures such as in Sierra Leone (Bockarie et al 1990), Liberia (Garms 1973(Garms , 1987, Ivory Coast (Quillévéré et al 1977), and Togo and Benin (Garms and Cheke 1985). Considering the high biting densities almost everywhere in the area and the fact that 1 out of 46 bites was theoretically infective, a person could have received one or more infective bites per week.…”

Section: Discussionmentioning

confidence: 99%

Assessments of the transmission of Onchocerca volvulus by Simulium sanctipauli in the Upper Denkyira District, Ghana, and the intermittent disappearance of the vector

et al. 2015

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Following studies on the transmission of Onchocerca volvulus (Leuckart) by Simulium sanctipauli Vajime & Dunbar (Diptera, Simuliidae) in Upper Denkyira District in Ghana in 2001 and 2002 (Kutin et al., Med Vet Ent 18:167-173, 2004), further assessments were carried out in 2006 and 2013/2014 to determine whether transmission parameters had changed since community-directed ivermectin treatment (CDTI) began in 1999. There were no marked changes of the transmission intensities in 2006. Only slight, but non-significant, reductions were observed in infection rates of parous flies with larval stages (L1-L3) of O. volvulus from 44.1 % (of 1672 parous flies) in 2001/2002 to 42.1 % (506) in 2006 and from 6.5 to 5.9 % of flies carrying infective larvae in their heads. This suggested that there was an ongoing transmission in the area and the parasite reservoir in the human population was still high. Unexpectedly, further assessments conducted in October 2013 and March and October 2014 revealed that the vector S. sanctipauli had apparently disappeared and transmission had ceased, probably as a result of intensified gold mining activities along the rivers Ofin and Pra. The water of both rivers was extremely turbid, heavily loaded with suspended solids, probably preventing the development of blackfly larvae. Some breeding and biting of Simulium yahense Vajime & Dunbar was observed in a small tributary of the Pra, the Okumayemfuo, which is not affected by gold mining. However, the infection rate of flies was low, only 3.7 % of 163 parous flies were infected with first stage (L1) larvae of O. volvulus.

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“…In Liberia, for example, S. soubrense was found biting and transmitting O. volvulus 11 km away from its breeding sites in the St. Paul river (Garms, 1973). Bockarie et al . (1990) showed that, in Sierra Leone, the highest risk areas for the transmission of onchocerciasis by S. sanctipauli s.l.…”

Section: Discussionmentioning

confidence: 99%

Efficiency of Simulium sanctipauli as a vector of Onchocerca volvulus in the forest zone of Ghana

Kutin

Kruppa

Brenya

et al. 2004

Medical Vet Entomology

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The role of Simulium sanctipauli Vajime & Dunbar (Diptera: Simuliidae) as a vector of Onchocerca volvulus (Leuckart) (Spirurida: Onchocercidae) in the forest zone of central Ghana was studied in the Upper Denkyira district, where onchocerciasis is hyperendemic. Simulium sanctipauli was found to be a highly efficient vector, with a mean of 377 infective (L3) larvae in the heads of 1000 parous and 122 in the heads of 1000 biting flies. The overall infection rate of 44% of the parous flies with L1, L2 and L3 stages of O. volvulus (identity confirmed by polymerase chain reaction) demonstrates marked anthropophily. Female flies dispersed over a wide area and can transmit onchocerciasis up to at least 10 km away from their breeding sites. Annual community-directed treatments with ivermectin did not have a noticeable effect on the infection rates and parasitic loads of fly populations, which were as high 2 months after as 3 months before the distribution of ivermectin. This failure can be attributed to poor coverage, with treatment taken by only 24.4% of the population of the six study villages.

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The transmission of onchocerciasis at a forest village in Sierra Leone. I.Simulium damnosums.l. biting densities and infection withOnchocerca volvulusat five representative sites

Cited by 9 publications

References 8 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

Assessments of the transmission of Onchocerca volvulus by Simulium sanctipauli in the Upper Denkyira District, Ghana, and the intermittent disappearance of the vector

Efficiency of Simulium sanctipauli as a vector of Onchocerca volvulus in the forest zone of Ghana

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