Vector dynamics and Onchocerca infectivity status of Simulium damnosum complex at Arinta and Erin-Ijesha waterfalls, Southwest Nigeria.

Abstract

Background objectives: Arinta and Erin-Ijesha are popular waterfalls in Southwest Nigeria where preliminary assessments identified blackfly activity, yet data on vector dynamics and Onchocerca infectivity remain unavailable. This study investigated these parameters to assess the risk of onchocerciasis transmission at both sites.

Methods: Adult black flies were collected for three consecutive days in a month by two dark complexioned adult male human landing catchers from September 2023 to August 2024. For each hour of sampling, 20% of the catches were randomly selected and dissected in real-time to assess parity and infectivity using standard procedure. The remaining flies were morphologically identified as savannah and forest flies and were preserved for O-150 pool screening.

Results: A total of 1,090 Simulium flies were caught, Arinta (n=1012) and Erin-Ijesha (n=78). The peak monthly biting rate (MBR) were 3160.0 (Arinta, September) and 289.3 (Erin-Ijesha, October) bites/person/month while the lowest MBRs were 72.3 (Arinta, July) and 0.0 (Erin-Ijesha, January to March, May, July and August) bites/person/month. The MBR at Arinta was significantly higher than at Erin-Ijesha (p=0.02). The Annual biting rate (ABR) at Arinta (10,231.8 bites/person/year) exceeded the WHO threshold of 1,000 bites/person/year, while below at Erin-Ijesha (790 bites/person/year). All morphologically examined flies were identified as forest species. No Onchocerca infectivity was detected in dissected or pool-screened (n = 857) flies.

Interpretation conclusion: Findings suggest low risk of onchocerciasis transmission at both waterfalls, with blackflies merely constituting biting nuisance. Continued public health education of tourist is recommended to allay their fears on the public health risk posed by blackfly bites, along with the use of protective clothing and repellents.