Onchocerca volvulus infection prevalence and intensity in Logo and Nyarambe Health Zones in Ituri, Democratic Republic of the Congo in 2010 and in 2021-2023: results of screening for clinical trials of moxidectin versus ivermectin

BACKGROUND: In Ituri Province, 7576 and 1056 volunteers living in Logo and Nyarambe Health Zone (HZ), respectively, were screened in 2021-2023 for two studies comparing moxidectin and ivermectin in individuals with ≥ 0 Onchocerca volvulus skin microfilariae density (SmfD, microfilariae/mg skin). Site selection was based on the trial capacity established for the moxidectin Phase 3 study and SmfD measured among 1373 and 36 individuals screened in HZ Logo and Nyarambe, respectively, in 2010. We compared the SmfD measured in 2010 and 2021-2023 in Logo HZ where ivermectin mass administration was never implemented and provide descriptive statistics for SmfD from Nyarambe HZ. METHODS: Four skin snips from each consenting/assenting individual ≥ 12 years old were weighed and incubated in isotonic saline for ≥ 8 h. Emerged microfilariae were counted and SmfD calculated as the mean of the microfilariae/mg skin of each snip. Other data collected included age, gender, village of residence, and history of ivermectin treatment. RESULTS: In 2010 and 2021-2023, respectively, adults (18-93 years old) represented 92.1% and 73.2%, and women 36.9% and 46.6% of the 1373 and 7547 volunteers from Logo HZ without reported prior ivermectin treatment. Among these adults and adolescents (12-17 years), no microfilariae were detected in snips from 23.3% and 26.9% in 2010 and 89.8% and 96.8% in 2021-2023, respectively, with mean ± standard deviation SmfD being 24.30 ± 35.52 and 11.8 ± 18.37 in 2010 and 1.1 ± 6.44 and 0.3 ± 2.62 in 2021-2023, respectively. CONCLUSIONS: Given that the reduction in infection prevalence and intensity in Logo HZ cannot be attributed to ivermectin distribution, it has to be due to reduction in infective vector biting rates, possibly linked to a recently proposed change in vector species triggered by land-use changes. Because SmfD reflects transmission events approximately 2-15 years earlier, infective vector biting rate assessment is needed to determine current transmission rates. Reduced transmission shifts macrofilariae age distribution toward older macrofilariae with lower reproductive capacity. Comparison of the results from the Phase 3 and the ongoing efficacy study might help determine whether drug susceptibility changes significantly with macrofilariae age. Should that be the case, transmission models evaluating the impact of mass drug administrations could be adjusted.