Identification of the onchocerciasis vector in the Kakoi-Koda focus of the Democratic Republic of Congo
Authors: Post Rory J , Laudisoit Anne , Mandro Michel , Lakwo Thomson , Laemmer Christine , Pfarr Kenneth , Hoerauf Achim , Tortosa Pablo , Gomard Yann , Ukety Tony , Mande Claude , Farovitch Lorne , Amazigo Uche , Bakajika Didier , Oguttu David W , Awaca Naomi , Colebunders Robert 
Published: July 2022

Background

The 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 Findings

Aquatic 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 asSimulium dentulosumandS. vorax, which were also found breeding in local streams and rivers.Simulium damnosums.l.,S. neaveiandS. albivirgulatumwere not found (except for a single site in 2009 where crabs were carryingS. neavei). Anthropophilic specimens from the focus were screened forOnchocercaDNA using discriminant qualitative real-time triplex PCR. One specimen ofS. voraxwas positive forOnchocerca volvulusin the body, and out of 155S. dentulosum, 30% and 11% were infected and infective (respectively).

Conclusions/Significance

Simulium dentulosumcurrently appears to be the main vector of human onchocerciasis within the Kakoi-Koda focus, andS. voraxmay be a secondary vector. It remains possible thatS. neaveiwas 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 voraxhas previously been shown to support the development ofO. volvulusin the laboratory, but this is the first time thatS. dentulosumhas 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. BecauseS. dentulosumis not a vector in endemic areas surrounding the Kakoi-Koda focus, it is probable that the Kakoi-Koda focus is significantly isolated.

AUTHOR SUMMARY

River blindness (=onchocerciasis) is a severely debilitating disease caused by the nematode parasiteOnchocerca volvulus, and in Africa it is known to be transmitted from person to person by blood-sucking blackflies (Diptera: Simuliidae) of theSimulium damnosumcomplex, theS. neaveigroup and (rarely) byS. albivirgulatum. Using classical morphological characteristics and DNA analysis we have unexpectedly identified the vector blackfly asS. dentulosum(and possiblyS. 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 areS. damnosumcomplex and/orS. neavei(as normally expected), and becauseS. dentulosumis 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.