Identification of the onchocerciasis vector in the Kakoi-Koda focus of the Democratic Republic of Congo

Abstract

BackgroundThe objective of this study was to characterise the vector in a small hyper-endemic focus of onchocerciasis (the Kakoi-Koda focus) which has recently been discovered on the western slopes of the rift valley above Lake Albert. Methodology/Principal FindingsAquatic stages of blackflies were collected by hand from streams and rivers, and anthropophilic adult females were collected by human landing catches. Using a combination of morphotaxonomy and DNA barcoding, the blackflies collected biting humans within the focus were identified as Simulium dentulosum and S. vorax, which were also found breeding in local streams and rivers. Simulium damnosum s.l., S. neavei and S. albivirgulatum were not found (except for a single site in 2009 where crabs were carrying S. neavei). Anthropophilic specimens from the focus were screened for Onchocerca DNA using discriminant qualitative real-time triplex PCR. One specimen of S. vorax was positive for Onchocerca volvulus in the body, and out of 155 S. dentulosum, 30% and 11% were infected and infective (respectively). Conclusions/SignificanceSimulium dentulosum currently appears to be the main vector of human onchocerciasis within the Kakoi-Koda focus, and S. vorax may be a secondary vector. It remains possible that S. neavei was the main (or only) vector in the past having now become rare as a result of the removal of tree-cover and land-use changes. Simulium vorax has previously been shown to support the development of O. volvulus in the laboratory, but this is the first time that S. dentulosum has been implicated as a vector of onchocerciasis, and this raises the possibility that other blackfly species which are not generally considered to be anthropophilic vectors might become vectors under suitable conditions. Because S. dentulosum is not a vector in endemic areas surrounding the Kakoi-Koda focus, it is probable that the Kakoi-Koda focus is significantly isolated. AUTHOR SUMMARYRiver blindness (=onchocerciasis) is a severely debilitating disease caused by the nematode parasite Onchocerca volvulus, and in Africa it is known to be transmitted from person to person by blood-sucking blackflies (Diptera: Simuliidae) of the Simulium damnosum complex, the S. neavei group and (rarely) by S. albivirgulatum. Using classical morphological characteristics and DNA analysis we have unexpectedly identified the vector blackfly as S. dentulosum (and possibly S. vorax) in a small endemic area (the Kakoi-Koda focus) which has been recently discovered on the western slopes of the rift valley in the Democratic Republic of Congo above Lake Albert. In the surrounding endemic areas, the vectors are S. damnosum complex and/or S. neavei (as normally expected), and because S. dentulosum is not a vector in these surrounding areas, it follows that this focus is entomologically isolated from immigrant blackfly species which might otherwise have carried new infections into the Kakoi-Koda focus (and vice-versa). This is of local importance, because it makes elimination of the parasite easier, but our findings have wider significance across Africa, because they raise the possibility that under the right conditions, other common and widely-distributed blackfly species might unexpectedly become man-biters and significant vectors.