A cross-sectional study of Simulium damnosum sensu lato breeding sites and species distribution in Sudan savanna, mixed savanna–forest and rainforest regions in Cameroon

Abstract

Background The presence of breeding sites and distribution of species of Simulium damnosum sensu lato are critical in understanding the epidemiology of onchocerciasis and evaluating the impact of elimination interventions. Reports on breeding sites and species distribution of members of S. damnosum s.l. in Cameroon are scarce and the few ones available date back to more than three decades. The aim of this study is to provide information on S. damnosum breeding sites across the rainy (RS) and dry (DS) seasons and the species composition in three different regions in Cameroon: Southwest (SW), Northwest (NW) and North (N). Methods A cross-sectional two-season study was carried out in three regions with different ecological characteristics (SW—rainforest; NW—mixed forest–Guinea savanna; N—Sudan savanna). Pre-control onchocerciasis endemicity, relief maps and historical entomological information were used to identify potential rivers for purposive sampling. Sampled larvae were fixed in Carnoy’s solution and sorted, and S. damnosum s.l. larvae were stored until identification by cytotaxonomy. Geographical coordinates of potential breeding sites were recorded to produce maps using ArcGIS, while Chi-square tests in SPSS were used to test for any differences between black fly seasonal breeding rates. Results A total of 237 potential breeding sites were sampled (RS = 81; DS = 156) and 72 were found positive for S. damnosum s.l. The SW had the most positive sites [67 (RS = 24; DS = 43)], with a significant difference in the rate of breeding between the seasons (P < 0.05). Among 68 sites visited in both seasons, 16 (23.5%) were positive in one of the two seasons with more sites positive in DS(11) than RS(05), 14 (20.6%) and 38 (55.9%) respectively positive and negative in both seasons. Simulium damnosum sensu stricto and S. sirbanum were the main species in the N, while S. squamosum and S. mengense were the predominant species in the NW and SW. Simulium soubrense and S. yahense were uniquely recorded in the SW. Conclusions A comprehensive mapping of breeding sites requires rainy and dry seasons sampling. This study demonstrates that a breeding site survey of S. damnosum s.l. is achievable in forest as well as savanna zones. Not all potential breeding sites are actual breeding sites. Observation of S. soubrense in the SW indicates changes in species composition over time and could affect onchocerciasis epidemiology in this area. Graphical abstract

Onchocerciasis in the Cameroon-Chad border area after more than twenty years of annual mass ivermectin distribution

“…This study has shown that black y vectors are still abundant in this area, with de nite seasonality in their human biting activities in the study area. Similar to previous observations [17], black ies biting in the dry season are exclusively concentrated around the major perennial rivers while in rainy season they mostly occur in the seasonal tributaries of these rivers. Hence, we recommend taking this information into consideration when planning for vector surveillance in the region, as well as when scheduling CDTI (as treatment is recommended to be done during peak transmission, which is generally assumed to coincide with peak y densities).…”

“…Many seasonal rivers (locally called Mayo) criss-cross the study area, some of which support black ies breeding when owing in the rainy season. The main reported vector cytospecies at the different sites are S. damnosum s.s. and small proportion of S. sirbanum, and often with seasonal occurrence of small numbers of S. mengense and S. squamosum (17,19).…”

“…Collection of black y larvae from the breeding sites and identi cation by cytotaxonomy was performed as described by Ayisi et al [17]. Brie y, larval developmental (breeding) sites near the sites where adult black ies were collected were prospected and larvae collected and stored in Carnoy's solution (3-parts absolute ethanol and 1-part glacial acetic acid) and kept cold until identi cation by cytotaxonomy.…”

“…Savanna vectors play major role in onchocerciasis transmission in North region of Cameroon; where Simulium damnosum sensu stricto apparently serves as the major vector and S. sirbanum likely plays a secondary role [17]. A small number of larvae of forest vectors -S. squamosum cytotype A and S. mengense -periodically occur in the area [18,19], possibly contributing to overall transmission.…”

Taxonomy and molecular phylogeny of black flies (Diptera: Simuliidae) in Africa and Europe by using mitochondrial Cox1 and nuclear ITS2 markers

“…S. damnosum sensu stricto and S. sirbanum were the main species in the north, whereas S. squamosum and S. mengense were the predominant species in the north west and south west. Simulium soubrense and S. yahense were uniquely recorded in the south west[42]. Studying the Onchocerca ochengi transmission dynamics and the correlation of O. ochengi microfilaria density in cattle with transmission potential, Achukwi et al[43] observed a high biting rate of Simulium s.l.…”