Changes in cytoforms of Simulium damnosum sensu lato (Diptera: Simuliidae) and onchocerciasis transmission zones in northern Cameroon with possible implications for onchocerciasis transmission elimination

Cytoforms (cytospecies and cytotypes) of the Simulium damnosum complex are the major onchocerciasis vectors in Africa; but they differ in vectorial efficiency. Transmission elimination of onchocerciasis requires understanding the vector dynamics and delineation of transmission zones, for monitoring and evaluation, and for stop-treatment decisions. This study sought to understand the vector population dynamics and to delineate Operational Transmission Zones in the 3 administrative regions (Far North, North, and Adamawa) of northern Cameroon. Using precontrol and relief maps, larvae were sampled in both rainy and dry seasons (July 2021 to February 2023) and fixed in Carnoy's solution. Chromosomal inversions were identified by cytotaxonomy. Karyotypes and inversion frequencies were compared between larval populations. Preimaginal stages were strictly seasonal in Far North, mostly seasonal in North, and mostly perennial in Adamawa. Five cytoforms (4 cytospecies and 1 cytotype) were recorded with 2 of them being reported for the first time (including S. yahense Vajime & Dunbar, an efficient forest zone vector), while 2 previously reported cytoforms were absent in all samples. These cytoform changes may impact the duration to transmission elimination. Also, there was redistribution of inversions in some populations and occurrence of new inversions at very high frequencies, suggesting genetic shifts probably caused by climate change. The 3 regions were distinct based on their vector phenology, cytoform composition, inversions and inversion frequencies, thereby enabling their delineation as separate Operational Transmission Zones.