A community-based vector control intervention "Slash and Clear" implemented in two onchocerciasis-endemic foci in South Sudan.

Background: Despite several rounds of ivermectin treatment, onchocerciasis transmission persists in Mvolo and Mundri West Counties (Western Equatoria State, South Sudan). A community-based "Slash and clear" (S&C) vector control method was recently introduced, but its effectiveness remains unclear.

Methods: Between October 2023 and November 2024, entomological studies were conducted to map blackfly breeding sites along River Naam (Mvolo County) and River Yei (Mundri West County). Following baseline assessment of daily biting rates at multiple catching sites, one round of S&C was implemented at selected intervention sites. Monthly biting rates (MBR) were monitored over a follow-up period of 8-14 months and compared between intervention and control sites.

Results: Thirteen breeding sites of Simulium damnosum, sensu lato. were found on the Naam and Yei Rivers. Biting rates were consistently higher during the rainy season than the dry season across both Counties. On the Naam River (14 months of follow-up), Mann-Kendall trend tests showed non-significant reductions in MBRs at both intervention (tau = -0.038, p-value = 0.881) and control sites (tau = -0.135, p-value = 0.313). Similar non-significant changes were observed on the Yei River (8 months follow-up), with tau = 0.046 (p-value = 0.820) and tau = 0.163 (p-value = 0.363) for intervention and control sites, respectively. Generalized additive models (GAM) regression analysis indicated that seasonality was the only significant predictor of MBR, with increased biting rates during the rainy season (p < 0.001). A single round of S&C at baseline did not result in significant reductions in MBRs (p-value = 0.651 in Mvolo and p-value = 0.531 in Mundri West).

Conclusion: Blackfly biting rates in Mvolo and Mundri West Counties are strongly influenced by seasonal variations, peaking during the rainy season. Our findings indicate that a single round of S&C is insufficient to reduce blackfly biting in the medium term. Repeated and strategically timed annual implementation of S&C is likely required to achieve significant and lasting vector control impacts.