Epidemiological and entomological assessment of persistent onchocerciasis transmission after long-term ivermectin treatment in communities along the Kéllé river, Cameroon

Abstract

Despite two decades of annual Community-Directed Treatment with Ivermectin (CDTI) in the Eséka health district, onchocerciasis transmission persists in frontline villages along the Kéllé River. This study assessed the current epidemiological and entomological status to guide elimination strategies. We conducted cross-sectional clinical and parasitological surveys in two first-line villages (Pouth-Kéllé and Mapan) during June and July 2021. Skin snips were examined for Onchocerca volvulus microfilariae, and clinical manifestations were assessed. Monthly blackfly collections by human landing catches (HLC) were performed at four river sites over one year. Flies were dissected to detect O. volvulus larvae. Microfilarial prevalence was 18.7% (14/75) in Pouth-Kéllé and 15.6% (5/32) in Mapan. Palpable nodules were found in 10% and 6.3% of adults, respectively, reflecting chronic lesions despite recent ivermectin distribution. Reduced visual acuity was frequent (46.7% in Pouth-Kéllé, 47% in Mapan) and was significantly associated with microfilarial positivity in Pouth-Kéllé ( p = 0.04). Entomological surveys yielded a total of 9,163 blackflies collected across the four capture sites, of which 6,036 were dissected to detect O. volvulus larvae. Analysis of biting rates showed that collected point I (P1) exhibited the highest annual biting rate (40,500 bites/person/year), with marked seasonality peaking in April. Transmission risk persisted at collected point II (P2 –annual transmission potential: 14.3 infective larvae/person/year) and collected point IV (P4 − 10.0 infective larvae/person/year). Onchocerciasis transmission remains active in the Kéllé River basin despite two decades of ivermectin treatment. These findings support the urgent need for complementary and/or alternative strategies to achieve elimination of onchocerciasis.