Genetic Selection of Low Fertile Onchocerca volvulus by Ivermectin Treatment

Abstract

Background Onchocerca volvulus is the causative agent of onchocerciasis, or “river blindness”. Ivermectin has been used for mass treatment of onchocerciasis for up to 18 years, and recently there have been reports of poor parasitological responses to the drug. Should ivermectin resistance be developing, it would have a genetic basis. We monitored genetic changes in parasites obtained from the same patients before use of ivermectin and following different levels of ivermectin exposure.Methods and Findings O. volvulus adult worms were obtained from 73 patients before exposure to ivermectin and in the same patients following three years of annual or three-monthly treatment at 150 µg/kg or 800 µg/kg. Genotype frequencies were determined in β-tubulin, a gene previously found to be linked to ivermectin selection and resistance in parasitic nematodes. Such frequencies were also determined in two other genes, heat shock protein 60 and acidic ribosomal protein, not known to be linked to ivermectin effects. In addition, we investigated the relationship between β-tubulin genotype and female parasite fertility. We found a significant selection for β-tubulin heterozygotes in female worms. There was no significant selection for the two other genes. Quarterly ivermectin treatment over three years reduced the frequency of the β-tubulin “aa” homozygotes from 68.6% to 25.6%, while the “ab” heterozygotes increased from 20.9% to 69.2% in the female parasites. The female worms that were homozygous at the β-tubulin locus were more fertile than the heterozygous female worms before treatment (67% versus 37%; p = 0.003) and twelve months after the last dose of ivermectin in the groups treated annually (60% versus 17%; p<0.001). Differences in fertility between heterozygous and homozygous worms were less apparent three months after the last treatment in the groups treated three-monthly.ConclusionsThe results indicate that ivermectin is causing genetic selection on O. volvulus. This genetic selection is associated with a lower reproductive rate in the female parasites. We hypothesize that this genetic selection indicates that a population of O. volvulus, which is more tolerant to ivermectin, is being selected. This selection could have implications for the development of ivermectin resistance in O. volvulus and for the ongoing onchocerciasis control programmes.

Identifying sub-optimal responses to ivermectin in the treatment of River Blindness

“…This assumption is based on lack of published evidence for heterogeneity in life-history parameters of O. volvulus (e.g., differences in life expectancy)-but see recent work by Bourguinat et al (32).…”

“…Estimation of the host's skin repopulation rate provides a method of quantifying an individual host's response to treatment which can be correlated with the genotypic information of parasites obtained from that host (microfilariae and adult worms). A number of possible molecular markers (including beta-tubulin and Pglycoprotein among others) have been identified which could provide useful tools in identifying anthelmintic resistance before it becomes widely spread (32,33). Linking phenotypic responses with molecular genetics markers will be required to prove that suboptimal responses are in fact due to drug-resistant parasites and not to host-related factors (3).…”

Efficacy of 5-week doxycycline treatment on adult Onchocerca volvulus

“…Doxycycline may also be developed as a reserve drug if the concerns regarding the potential development of ivermectin resistance (Bourguinat et al 2007;Osei-Atweneboana et al 2007) are proven to be correct and quick action is needed to prevent spreading of resistant worms. Finally, anti-wolbachial chemotherapy may become an option for areas that are co-endemic for loiasis and where ivermectin treatment has resulted in the occurrence of encephalopathy (Thomson et al 2004).…”

“…Further, it has been reported that some populations of the intestinal nematode Haemonchus contortus in sheep have developed a certain degree of ivermectin resistance (Coles et al 2005). Since genetic selection of low fertile Onchocerca volvulus by ivermectin treatment has been observed, it was hypothesized that O. volvulus populations more tolerant to ivermectin may also be selected (Bourguinat et al 2007). Epidemiological hints for a suboptimal performance of ivermectin against O. volvulus populations have been detected in northern Ghana (Awadzi et al 2004a,b;Osei-Atweneboana et al 2007).…”

Onchocerciasis control: biological research is still needed

“…These observations may indicate a reduced embryostatic effect of the drug in some patients. In addition, it has been shown that IVM induces a selection for the P-glycoprotein (PgP), P-glycoprotein-like protein (PLP) and β-tubulin genes in adult worms, and that a relationship exists between the genotype of the adult worms for the β-tubulin gene and their fertility (Prichard, 2007;Bourguinat et al, 2007). These results are a matter of concern, even if it has also been shown that IVM remains as efficient on O. volvulus mfs (the pathogenic stage) as it was 20 years ago.…”