Co-infection withOnchocerca volvulusandLoa loamicrofilariae in central Cameroon: are these two species interacting?

Abstract

Ivermectin treatment may induce severe adverse reactions in some individuals heavily infected withLoa loa. This hampers the implementation of mass ivermectin treatment against onchocerciasis in areas whereOnchocerca volvulusandL. loaare co-endemic. In order to identify factors, including co-infections, which may explain the presence of highL. loamicrofilaraemia in some individuals, we analysed data collected in 19 villages of central Cameroon. Two standardized skin snips and 30 μl of blood were obtained from each of 3190 participants and the microfilarial (mf) loads of bothO. volvulusandL. loawere quantified. The data were analysed using multivariate hierarchical models. Individual-level variables were: age, sex, mf presence, and mf load; village-related variables included the endemicity levels for each infection. The two species show a certain degree of ecological separation in the study area. However, for a given individual host, the presence of microfilariae of one species was positively associated with the presence of microfilariae of the other (OR=1·79, 95% CI [1·43–2·24]). Among individuals harbouringLoamicrofilariae, there was a slight positive relationship between theL. loaandO. volvulusmf loads which corresponded to an 11% increase inL. loamf load per 100O. volvulusmicrofilariae. Co-infection withO. volvulusis not sufficient to explain the very highL. loamf loads harboured by some individuals.