Impact of seasonal changes on human onchocerciasis transmission and disease elimination

The current guidelines for stopping treatment and verifying elimination of onchocerciasis recommend entomological and epidemiological surveys to be performed during the peak transmission season, which is generally assumed to occur during the rainy season, although this may not necessarily hold true in all transmission zones (TZs). This review has synthesized data, from a literature search, on the effects of seasonality on transmission in West and Central Africa. Seasonality affects vector population dynamics by altering terrestrial and aquatic ecology. In TZs with perennial rivers, peak transmission and peak vector density may either coincide or occur at different periods. These peaks could occur in either the rainy or the dry season in both forest and savanna zones, and the periods may change between years. Therefore, countries should establish updated local biting patterns at a TZ prior to vector collection. Also, while using peak biting density as a convenient indicator for when to commence vector collection (because of the need for a sufficient sample size), the survey period should include months in both seasons to increase the chances of collecting during peak transmission. Seasonality significantly impacts onchocerciasis transmission elimination; and it also offers critical avenues for effective intervention and transmission evaluation.