Onchocerciasis in the Ntui Health District of Cameroon: epidemiological, entomological and parasitological findings in relation to elimination prospects

Abstract

Background Despite decades of community-directed treatment with ivermectin (CDTI), onchocerciasis transmission persists in Cameroon and has been associated with increased risk for epilepsy in endemic communities. We investigated the onchocerciasis situation in the Ntui Health District (a known onchocerciasis focus in Cameroon where the Sanaga River constitutes the main source of black fly vectors) using parasitological, entomological and serological parameters. Methods In July 2021, community-based surveys were conducted in four villages (Essougli, Nachtigal, Ndjame and Ndowe). Onchocerciasis was diagnosed via microscopic examination of skin snips. Using rapid diagnostic tests, we screened children aged 3–6 years for Ov16 antibodies as a proxy for recent onchocerciasis transmission. Monthly black fly biting rates were obtained from the two riverside villages (Nachtigal and Essougli) for 12 consecutive months (July 2021 to June 2022) using the human landing catch technique. Some black flies were dissected each month to check for infection. Results Overall, 460 participants were recruited; mean age was 32.1 (range: 3–85) years with 248 (53.9%) being males. Among skin snipped participants (n = 425), onchocerciasis prevalence was 14.6%. Participants with epilepsy (n = 25) were more often skin snip positive (45.8% vs 12.7%; P < 0.001) and had higher microfilarial loads (9.2 ± 22.0 vs 0.7 ± 3.5 microfilariae/skin snip; P < 0.001) compared to their peers without epilepsy. Eight (6.5%) of the 123 tested children were Ov16 seropositive. The breeding sites we investigated along the Sanaga River during the current study harbored fewer vectors (annual biting rates reaching 530,322 vs 606,370 in the Mbam River) and exhibited lower black fly infection rates (annual transmission potentials reaching 1479 vs 4488 in the Mbam River) when compared to recent entomological reports in Cameroon. Conclusion Despite substantial biting rates, black fly infection rates (by microscopy) in the Ntui Health District were rather low resulting in overall low transmission potentials in study villages. Thanks to CDTI, O. volvulus infection in both humans and insects is on the decrease. However, there is evidence that O. volvulus is still endemic in these communities. Reducing the vector population will further accelerate onchocerciasis elimination prospects. Graphical Abstract

Epidemiology of epilepsy and relationship with onchocerciasis prevalence in villages of the Ntui Health District of Cameroon

“…The 50.9% therapeutic coverage of the 2021 CDTI session in our study population, similar to the 54.2% coverage obtained in these same villages without using a door-to-door approach [ 25 ], falls way below the optimal threshold of 85% needed to rapidly achieve onchocerciasis elimination [ 34 ]. This may be indicative of regular sub-optimal coverage rates over the years, explaining why onchocerciasis transmission persists in these villages despite more than two decades of annual mass treatment with ivermectin.…”

“…This is consistent with previous findings from endemic communities in the Mbam valley in Cameroon where only one case of Nakalanga syndrome was found after surveying five villages [ 30 ], suggesting that although onchocerciasis transmission persists in Cameroonian communities, the infectious context (for instance: O. volvulus parasitic levels and chronicity of infection) may be enough just to trigger seizures, but not suffice to induce Nakalanga features in the affected individuals. Indeed, the reported community microfilarial loads in our study sites are rather low (values below 2 were reported [ 25 ]), supporting ongoing onchocerciasis transmission but with few parasites in circulation.…”

“…Based on the prevalence of onchocerciasis which we obtained in the four study villages during a previous survey [ 25 ], we found a strong correlation between epilepsy prevalence and onchocerciasis prevalence, with borderline statistical significance: Spearman Rho = 0.949, p = 0.051 (see Supplementary Material S3 ). Furthermore, we observed a trend whereby the distance of the village from the Sanaga river (blackfly breeding site) was inversely related to the prevalence of both onchocerciasis (Spearman Rho = –0.949, p = 0.051) and epilepsy (Spearman Rho = –0.889, p = 0.111); see Fig.…”

“…The study was conducted in March 2022 in the Ntui Health District, a known onchocerciasis focus in Cameroon with blackfly breeding sites along the Sanaga river [ 25 ]. In a previous hospital-based survey in the suburban city of Ntui by our team, we suggested the occurrence of onchocerciasis-associated epilepsy (OAE) in this area [ 23 ].…”

Cameroonian blackflies (Diptera: Simuliidae) harbour a plethora of (RNA) viruses

“…The gathering took place from 7:00 AM to 6:00 PM over three consecutive days in July 2021. Nachtigal is a rural community of the Ntui Health District, an onchocerciasis-endemic area with a high prevalence of OAE (Siewe Fodjo et al . 2022), in the forest-savannah transition zone in the Centre Region of Cameroon.…”