Ivermectin selection on beta-tubulin: evidence in Onchocerca volvulus and Haemonchus contortus

Ivermectin resistance is common in trichostrongylid nematodes of livestock, such as Haemonchus contortus. This anthelmintic is the only drug approved for mass administration to control onchocerciasis caused by the nematode parasite, Onchocerca volvulus. In parts of West Africa up to 18 rounds of ivermectin treatment have been administered to communities and there are reports of poor parasitological responses to treatment. Understanding ivermectin resistance and ivermectin selection is an important step to reduce selection pressure for resistance, and to develop molecular markers which can be used to monitor the development of resistance and its spread. Here we report evidence that ivermectin selection changes the frequency of beta-tubulin alleles in both the sheep parasite, H. contortus, and the human parasite, O. volvulus. In O. volvulus we have been able to look at the frequency of beta-tubulin alleles in O. volvulus obtained before any ivermectin was used in humans in Africa, and following its widespread use. In H. contortus, we have been able to look at the frequency of beta-tubulin alleles in a strain which has not seen any anthelmintic selection and in an ivermectin selected strain derived from the unselected strain. We have found ivermectin selects on beta-tubulin in both of these nematode species. In the case of O. volvulus, we had previously reported that ivermectin selects for specific single nucleotide polymorphisms in the O. volvulus beta-tubulin gene. This polymorphism results in three amino acid changes in the H3 helix of beta-tubulin, as well as deletions in an associated intron. We report a simple PCR assay to detect the amplicon length polymorphism, resulting from these intronic deletions, which can be used to monitor the frequency of the beta-tubulin allele selected for by ivermectin in O. volvulus.

A comparison of genetic polymorphism in populations of Onchocerca volvulus from untreated- and ivermectin-treated patients

An analysis of the polymorphism of 16 genes from Onchocerca volvulus was undertaken, in two populations of worms from either ivermectin-naïve patients or patients who had been repeatedly treated with ivermectin, in Ghana. Six genes were selected for analysis because studies in other nematodes had suggested a possible association with ivermectin resistance. The other 10 genes were included as control genes and have not been associated with ivermectin resistance. Twelve of the 16 genes were polymorphic, including five of the candidate genes and seven of the control genes. In all of the control genes and four of the candidate genes, there were no differences in genetic polymorphism between the untreated and ivermectin treatment worms. However, there were statically significant differences (chi2=0.05) in allelic frequencies between the untreated and treatment derived worms for P-glycoprotein and beta-tubulin genes; both genes which have been previously associated with ivermectin resistance in other nematodes. These genes were in Hardy-Weinberg equilibrium in the untreated population. However, the P-glycoprotein alleles, in the worms from the patients under treatment were not in Hardy-Weinberg equilibrium, and analysis of the allele frequencies of beta-tubulin suggested that this gene may have also been under selection in the worms from the ivermectin-treated patients. This data provides evidence of genetic selection by ivermectin on O. volvulus and indicates that investigations should be made to determine whether ivermectin resistance is developing. The beta-tubulin and P-glycoprotein genes may prove useful for monitoring for possible development of ivermectin resistance.