Displacements of Simulium damnosum and strategy of control against onchocerciasis

Abstract

Regular aerial treatment of 14 000 km of watercourses has achieved and maintained, over an area of 700 000 km2 of West African savannah, a very high degree of control of the larvae of Simulium damnosum sensu stricto and S. sirbanum, the vectors of onchocerciasis in this area. However, particular and relatively restricted parts of this area, mainly in northern Ivory Coast and neighbouring parts of Upper Volta, experience regular and prolonged reinvasions by parous female vectors, which have already taken bloodmeals (and many of them carrying the parasites) and arrive from unknown sources probably hundreds of kilometres away, from directions probably between southwest and north. This reinvasion, now experienced in three successive years, represents the outstanding scientific, epidemiological and logistic problem still facing the WHO Onchocerciasis Control Programme. An outline is presented of the multidisciplinary investigations being undertaken to find a solution.

Effect of a single dose of 8 mg moxidectin or 150 μg/kg ivermectin on O. volvulus skin microfilariae in a randomized trial: Differences between areas in the Democratic Republic of the Congo, Liberia and Ghana and impact of intensity of infection

“…If that is the case and if CDTI selects for SOR, this selection in the Pru, Daka and Black Volta/Tombe river basins could be reflected in the parasite population in our study participants. Long range transmission of parasites through infected/infective vector movement was a challenge for the Onchocerciasis Control Programme in West Africa, resulting in extensions of the original programme area [ 71 – 74 ]. Recently long range vector movement has been implicated in renewed transmission of O .…”

Cytotaxonomic characterization and estimation of migration patterns of onchocerciasis vectors (Simulium damnosum sensu lato) in northwestern Ethiopia based on RADSeq data

“…While CDTI has been increased to three times per year in the Metema hotspot [ 20 ], one concern is that blackfly immigration from Metekel could be responsible for ongoing transmission in the hotspot and/or re-introduce transmission of onchocerciasis into Metema should CDTI be stopped. Whether adult flies from Metekel are migrating into the cleared areas of the Metema focus is unknown, but the distance between the two foci is certainly within the known distance for long-range wind-assisted migration [ 10 , 12 , 21 , 22 ]. The epidemiological risk of blackfly migration should be incorporated into decisions about whether to stop CDTI in these areas, but there are currently no routinely employed methods by which the pattern and extent of vector migration can be measured.…”

“…Infective vector migration might interfere with progress towards elimination where endemic areas are geographically closer together or continuously distributed, as is the case for much of sub-Saharan Africa [9]. Long-distance migration by infective adult female blackflies of some vector species in West Africa brought significant numbers of parasites into areas where they had been controlled by larviciding during the WHO Onchocerciasis Control Programme (1975-2022) (before the widespread adoption of CDTI) and were considered a serious threat to the control of onchocerciasis [10][11][12]. Migration by infective vectors could similarly contribute to failure to achieve elimination of transmission by CDTI, or might result in re-establishment of endemism in areas where onchocerciasis was previously considered to have been eliminated ( [13][14][15]; see also e.g., [16][17][18] with regards to mosquito vectors).…”

Synthesizing environmental, epidemiological and vector and parasite genetic data to assist decision making for disease elimination

“…In part, this may be because endemic communities in the Americas tend to be relatively isolated from each other, while prevalence is continuous across large geographic areas in sub‐Saharan Africa (Zouré et al., 2014), facilitating movement between communities and across internal and external geopolitical borders. For example, research by the Onchocerciasis Control Programme (OCP; 1974–2002) suggested that the failure to achieve elimination of onchocerciasis transmission in West Africa via larviciding of vector breeding sites could be attributed to the rapid spread of insecticide resistance alleles through vector migration (Cheke & Garms, 1983; Cupp et al., 2011; Le Berre et al., 1979; Service, 1980). Migration of the parasites via infected humans has been linked to re‐emergence of onchocerciasis transmission in areas of Burkina Faso where transmission had been stopped (Koala et al., 2017, 2019; Nikièma et al., 2018).…”