Why onchocerciasis transmission persists after 15 annual ivermectin mass drug administrations in South-West Cameroon

Abstract

Introduction: Onchocerciasis is targeted for elimination mainly with annual community-directed treatment with ivermectin (CDTI). High infection levels have been reported in South-West Cameroon, despite ≥15 years of CDTI. The aim of this study was to assess factors associated with continued onchocerciasis transmission and skin disease.

Methods: A large-scale cross-sectional study was conducted in 2017 in 20 communities in a loiasis-risk area in South-West Cameroon. A mixed-methods approach was used. Associations between infection levels, skin disease and adherence to CDTI were assessed using mixed regression modelling. Different community members' perception and acceptability of the CDTI strategy was explored using semi-structured interviews.

Results: Onchocerciasis prevalence was 44.4% among 9456 participants. 17.5% of adults were systematic non-adherers and 5.9% participated in ≥75% of CDTI rounds. Skin disease affected 1/10 participants, including children. Increasing self-reported adherence to CDTI was associated with lower infection levels in participants aged ≥15 years but not in children. Adherence to CDTI was positively influenced by perceived health benefits, and negatively influenced by fear of adverse events linked with economic loss. Concern of lethal adverse events was a common reason for systematic non-adherence.

Conclusion: CDTI alone is unlikely to achieve elimination in those high transmission areas where low participation is commonly associated with the fear of adverse events, despite the current quasi absence of high-risk levels of loiasis. Such persisting historical memories and fear of ivermectin might impact adherence to CDTI also in areas with historical presence but current absence of loiasis. Because such issues are unlikely to be tackled by CDTI adaptive measures, alternative strategies are needed for onchocerciasis elimination where negative perception of ivermectin is an entrenched barrier to community participation in programmes.

Keywords: control strategies; epidemiology; onchocerciasis; parasitology; public health.

Figure 1

O. volvulus prevalence in 20 study communities (A) and O. volvulus prevalence and CMFL before and after 15 rounds of CDTI in six study villages (B). A: Geographical location of, and O. volvulus prevalence in, the 20 study villages. Data were obtained from 9456 participants aged 5 years and over in a cross-sectional survey conducted in 2017 in 20 villages of South-West Region, Cameroon. This map has been produced in ArcGIS 10.5 specifically for this study. B: O. volvulus prevalence and CMFL before and after 15 rounds of CDTI in six of the study villages. Pre-control data were obtained from. Data after 15 rounds of CDTI were obtained from a cross-sectional survey conducted in 2017 and including 3062 participants aged 5 years and over (prevalence) and 1703 participants aged 20 years and over (CMFL), living in six villages of South-West Cameroon. CDTI, community-directed treatment with ivermectin; CMFL, community microfilarial load; O. volvulus, Onchocerca volvulus.

Figure 2

Mean proportion of CDTI rounds participated in (A) and proportion of high adherers (B), by age and gender. High adherers are defined as participants who took ivermectin in ≥75% of rounds. Denominators for the mean proportion of rounds taken are the maximum number of rounds an individual could have participated in given their age. Data were obtained from 9164 participants aged 5 years and over, with available CDTI adherence data, in a cross-sectional survey conducted in 2017 in 20 villages of South-West Region, Cameroon. CDTI, community-directed treatment with ivermectin.

Figure 3

O. volvulus skin mf prevalence (A and D), infection intensity (B and E) and nodule prevalence (C and F) by adherence level and age. Those predictions were obtained using the multivariate models presented in table 2. Age groups 15 to 19 and 20 to 39 years had similar infection risk and intensity across adherence levels and were grouped in a larger category (ie, 15 to 29 years) to increase the precision of estimates. Data were obtained from a cross-sectional survey conducted in 2017, including 9115 participants with complete data aged 5 years and over living in 20 communities of South-West Cameroon. CDTI, community-directed treatment with ivermectin; IVM, ivermectin; mf, microfilariae; O. volvulus, Onchocerca volvulus.