Investigating impact of slash and clear vector control strategy on blackfly population and onchocerciasis transmission in a hotspot in Nigeria

Abstract

BACKGROUND: The Edo-Ondo border remains a hotspot for onchocerciasis transmission and has been designated as a special intervention zone by Nigeria's National Onchocerciasis Elimination Committee because of its persistent transmission and high blackfly biting rates. This study evaluated the effectiveness of the "slash-and-clear" vector control strategy and annual ivermectin distribution to interrupt transmission of Onchocerca volvulus in the area. METHODS: This study was implemented in two phases. Phase 1 (July-August 2023) involved slash-and-clear in four communities along the Ose River and its tributaries, while four others served as controls. Baseline biting rates were recorded for eight days, after which interventions were implemented in designated sites and fly collections continued for 29 days. Phase 2 (September 2023-June 2025) annual slash-and-clear was extended to 76 communities, including the original eight, where fly populations were continuously monitored. All collected blackflies were screened for O. volvulus infection using Ov ND5 qPCR pool screening. RESULTS: In Phase 1, biting rates in intervention communities decreased significantly by 51.1-91.9% (p < 0.05) along tributaries but increased in communities located directly on the Ose River or within 5 km. No significant changes were observed at the control sites. In Phase 2, sustained slash-and-clear across 76 communities resulted in significant reductions in blackfly densities, ranging from 39.7%-94.5%. Analysis of the flies showed infectivity for Ov ND5 genes in designated communities in the main Ose River and its tributaries indicating the presence of reservoirs of human Onchocerca volvulus microfilariae of onchocerciasis in the area. CONCLUSION: The results of this study demonstrate the possibility of significant reduction in Simulium damnosum complex biting rates with sustained slash-and-clear activity in the region (most especially the tributaries). Therefore, we recommend further studies to determine drivers of transmission aside from flies to fast-track the elimination of onchocerciasis in the area.