Phenotypic and Genotypic Analysis of Onchocerca Volvulus Response To Ivermectin Treatment

Abstract

Ivermectin remains the only potent drug for the control and mass treatment of onchocerciasis. Nonetheless, recent studies indicate sub-optimal responses and genetic changes in some populations of the adult Onchocerca volvulus. Hence, more studies are required to determine whether resistance is developing. This requires analysis of phenotypic and genotypic responses of O. volvulus to ivermectin treatment to determine the association between worm phenotype and genotype after treatment. Forty archived O. volvulus female worms were obtained from patients hailing from three Ghanaian endemic communities: Asubende, Kyingakrom and Agborlekame. These communities were known to harbour both good and poor ivermectin response groups. The female worms were retrieved from nodules removed from patients who previously had undergone 16-17 rounds of (150 µg/kg) ivermectin treatment with one year interval between treatments. The samples were obtained 3 months after the last round of treatment. Genetic analysis of the beta-tubulin gene of the adult worms and their pooled microfilariae (MF) on a 538 bp DNA fragment of the beta-tubulin gene as well as restriction was done to determine mutations in the beta-tubulin gene. It was detected that 70% (28/40) of the worms harboured greater than 70% of normal stretched MF in their genital tracts; these were considered as poor responders to ivermectin treatment. On the other hand, 30% (12/40) of the worms that harboured less than 10% of the stretched MF in their genital tracts were considered as good responders to ivermectin treatment. Molecular analysis indicated single nucleotide polymorphism (SNP) for both adult worm and their corresponding MF with the heterozygote mutant adult worms showing strong association with the MF. In conclusion, it is found in this study that some of the worms have responded poorly or sub-optimally to ivermectin treatment. This development stems from mutations in the beta-tubulin gene of the adult female worms that have been passed on to their progenies