Simulium larvae susceptibility to temephos and the effect of 10 weeks of treatment of the Mbende tributary in the Nkam-Wouri River drainage of Cameroon on larval density and adult fly biting rates

Background: Despite over 18 years of annual ivermectin mass drug administration (MDA) in Cameroon’s Nkam-Wouri River drainage, onchocerciasis transmission persists. Several reasons, including multiple breeding sites and abundant vector populations, contribute to ongoing transmission. High vector abundance also causes a biting nuisance to local populations. The change in paradigm from onchocerciasis control to elimination may not be achieved if alternative control measures are not used. There is a need to complement ivermectin MDA with other strategies. This study tested the susceptibility of Simulium larvae to temephos insecticide and monitored the effect of 10 weeks of ground larviciding on the larval density and black fly population. 

Methods: Simulium breeding sites along the course of three rivers within the Solle transmission zone in the Nkam-Wouri River drainage were identified. Seven temephos concentrations (0.001–0.1 mg/l) were tested on freshly collected Simulium larvae for susceptibility. Simulium biting rates were monitored using human landing catches before and during 10 weeks of ground larviciding. Fishing was used to assess the abundance and diversity of large aquatic fauna, while the presence and diversity of small invertebrate fauna were assessed during the collection of larvae, as they are usually found on the same substrates in the river. Ground larviciding was conducted using the spraying method at two dosing points. 

Results: Six breeding sites were identified. Larval mortality decreased with temephos concentration, with 100% mortality observed at 0.1–0.025 mg/l. The non-target fauna included various fish species, crabs, crayfish, and small invertebrates. Ground larviciding cleared larvae from identified substrates and reduced adult fly biting rates by 82.8% (from 900 flies/man/day at the beginning to 180 flies/man/day at the end), a statistically significant decrease (χ² = 1351.5, P < 0.001). 

Conclusions: Simulium larvae showed susceptibility to temephos. Clearance of larvae from traps and identified natural substrates, and a significant reduction in the Simulium biting rates were observed.