Refocusing vector assessment towards the elimination of onchocerciasis from Africa: a review of the current status in selected countries

Measures to control onchocerciasis have been in place for well over 30 years. Recently, programs have turned from disease control towards transmission elimination. The absence of infective larvae in the black fly Simulium sp. vector is central to defining elimination, and assessments of infectivity by O150 polymerase chain reaction in the vector not only provide valuable information to programs, but are also required for verification of elimination. The status of transmission in black flies was assessed in five countries in the African region during 2014 and 2015. Several of these countries were evaluated because of promising results from epidemiological studies in humans. No infective flies were found in two countries. Infective flies were found in the other three, despite the absence of infection in humans (as evaluated by skin-snip microscopy). Ongoing transmission as demonstrated in the black flies could be due to a variety of factors, including lack of treatment of hypo-endemic areas and cross-border issues. Challenges identified during the course of the entomological work suggest that there is a need for improved selection of vector collection sites and vector collection periods in order to improve fly catches. Two important challenges to achieving elimination identified were definition of the hypo-endemic zones and establishing the existence of areas of cross-border transmission occurring between countries.