Cytotaxonomic Characterization and Seasonal Distribution of Simulium damnosum s.l. (Diptera: Simuliidae) along the Osun–Oyo Border, Southwestern Nigeria: Implications for Onchocerciasis Control

Abstract

The public health menace of black flies in the transmission of onchocerciasis remains alarming, particularly in sub-Saharan Africa. The present study was conducted to investigate the blackflies species using the cytotaxonomical method within communities along the border of Osun and Oyo States. Four border-line communities: Aro, Molosun, Olukeye and Jago were prospected for Simulium damnosum s.l larvae between November, 2024 to October, 2025. Larvae prospection was done bimonthly, and collected larvae were preserved in cold Carnoy’s fixative for laboratory identification using cytotaxonomic keys. Blackfly larvae were attached to both floating and submerged substrates, including the natural and artificial junks, creating obstructions in the river course.  Seasonal patterns showed that prospections were most efficient at the onset of the dry season, when receding water levels exposed breeding and reduced risks to field entomologists, whereas prospections during peak rains or late dry seasons could underestimate breeding sites due to inaccessible or dried-up sites. Cytotaxonomic analysis of larvae demonstrated the presence of Simulium damnosum s.s. and Simulium sirbanum across all the locations. Out of 339 larvae collected, 258 larvae were confirmed to be S. damnosum s.l. A total of 106 larvae were randomly selected from the sampled locations for cytological examinations. The chromosomal inversions observed for S. damnosum s.s, were IL 1.3/1.2.3, IL 1.3/1.3.2, IIL C/C, IIIL 2/2 and IS 3/3 accounting for 63.2% (67) of the identified larvae, while 19 larvae with chromosomal inversios IS 3/3 and IIIL 2/2 were also observed for S. sirbanum (36.8%). Thus, no new inversions were recorded. These findings highlight the ecological flexibility and genetic composition of vector populations along the Osun–Oyo border, emphasizing the need for seasonally timed entomological surveillance and evidence-based policy interventions for effective onchocerciasis control in both states.