Benzimidazole-resistant Cooperia curticei from Cornwall, England

Hidden in plain sight - Multiple resistant species within a strongyle community

•

PCR speciation highlighted parasite species diversity on a commercial UK sheep farm.

•

Species diversity confounded interpretation of faecal egg count data and bioassays.

•

These tests detected only moderate resistance to benzimidazoles and ivermectin.

•

Post-treatment populations were composed almost entirely of Teladorsagia circumcincta.

•

Ivermectin strongly selected for a highly dual-resistant and pathogenic species.

Ovine parasitic gastroenteritis is a complex disease routinely treated using anthelmintics. Although many different strongyle species may contribute to parasitic gastroenteritis, not all are equally pathogenic: in temperate regions, the primary pathogen is Teladorsagia circumcincta. In this study we investigated benzimidazole and ivermectin resistance on a commercial sheep farm in southeast Scotland. We assessed the impact of species diversity on the diagnosis of resistance using the faecal egg count reduction test and in vitro bioassays, and correlated the results with the frequency of benzimidazole resistance-associated genotypes measured in the T. circumcincta population by pyrosequencing of the β-tubulin isotype-1 gene.

Faecal egg count reduction test results showed efficacies of 65% for albendazole and 77% for ivermectin, indicating moderate resistance levels on the farm. However, PCR speciation of the same populations pre- and post-treatment revealed that removal of susceptible species had masked the presence of a highly resistant population of T. circumcincta. Less than 25% of individuals in the pre-treatment populations were T. circumcincta, the remainder consisting of Cooperia curticei, Chabertia ovina, Oesophagostomum venulosum and Trichostrongylus spp. In contrast, post-treatment with albendazole or ivermectin, the majority (88% and 100% respectively) of the populations consisted of T. circumcincta. The egg hatch test for benzimidazole resistance and the larval development test for ivermectin resistance were carried out using eggs obtained from the same populations and the results were broadly consistent with the faecal egg count reduction test. Thirty individual T. circumcincta from each sampling time point were assessed for benzimidazole resistance by pyrosequencing, revealing a high frequency and diversity of resistance-associated mutations, including within the population sampled post-ivermectin treatment.

These results highlight the potential diversity of parasite species present on UK farms, and their importance in the diagnosis of anthelmintic resistance. On this particular farm, we demonstrate the presence of a highly dual-resistant population of T. circumcincta, which was strongly selected by treatment with either benzimidazoles or ivermectin, while other potentially less pathogenic species were removed.

Anthelmintic resistance in Northern Ireland (I): prevalence of resistance in ovine gastrointestinal nematodes, as determined through faecal egg count reduction testing

The prevalence of anthelmintic resistance in Northern Ireland sheep flocks was evaluated between July and October 2011. Sampling kits were sent to 172 flock owners and returns were received from 91. Within this survey population, 27 flock owners used benzimidazole products, 10 used levamisole products, 15 used avermectin products, 26 used milbemycin products and 4 flock owners used the amino acetonitrile derivative, Monepantel. The remaining 9 flock owners used combination drenches (broad spectrum wormer plus fasciolicide). However, 15 sets of samples were ineligible for faecal egg count reduction testing due to either too low an egg count or insufficient faecal volume. Treatment efficacy below 95%, indicating significant resistance, was detected in 81% (n=24) of flocks tested for benzimidazole resistance; in 14% (n=1) of flocks tested for levamisole resistance; and in 50% (n=7) and 62% (n=13) of flocks tested for avermectin and milbemycin resistance, respectively. Monepantel resistance was absent in all (n=3) flocks tested. Combination products (broad spectrum nematocide plus flukicide) containing levamisole were entirely effective, while treatment efficacy below 95% was detected in 60% (n=3) of flocks where the nematocide in the combination product was a benzimidazole. Where parasite identification based on coproculture was completed, Trichostrongylus was the dominant genus detected in all cases post-treatment, indicating the occurrence of anthelmintic-resistant Trichostrongylus spp. populations. Benzimidazole efficacy was highest in treating Trichostrongylus spp. (51%) and lowest when treating Teladorsagia spp. Levamisole was 100% effective in treating Cooperia, but ineffective (0%) in treating Trichostrongylus spp. Avermectin efficacy was highest when treating Haemonchus contortus (100%) and Teladorsagia spp. (73%), with a marginally lower efficacy against Trichostrongylus spp. (71%). Moxidectin efficacy was 33% against Trichostrongylus spp., 68% against Teladorsagia spp., 97% against Cooperia spp. and 100% against Haemonchus contortus infections.

Anthelmintic resistance

Anthelmintic resistance is widespread in nematode parasites of sheep, goats and horses. Resistance is also developing in nematode parasites of cattle and has been detected in pig parasites. Benzimidazole, levamisole/morantel and ivermectin resistances occur in nematodes of sheep and goats and closantel resistance has been found in Haemonchus contortus. Anthelmintic resistance is likely to develop wherever anthelmintics are frequently used and be detected if it is investigated. Worm count or egg count reduction after treatment are useful for the detection of all types of anthelmintic resistances. More economical, faster and more sensitive in vitro assays for the detection of anthelmintic resistance have been developed. Some, such as the egg hatch assay are specific for a particular class of anthelmintic, whilst others such as larval development assays can be used with most anthelmintics. Improvements in our understanding of the biochemistry and molecular genetics of anthelmintic actions should lead to the development of more sensitive assays for the detection of anthelmintic resistance in individual nematodes. Levamisole/morantel resistance appears to be associated with alterations in cholinergic receptors in resistant nematodes. Ivermectin appears to act by binding to a glutamate receptor of a membrane chloride channel. This receptor has been expressed in vitro so that further studies of the interaction of ivermectin with this receptor and its possible alteration in ivermectin resistance will be feasible. Benzimidazole resistance in nematodes and fungi appears to be associated with an alteration in beta-tubulin genes which reduces or abolishes the high affinity binding of benzimidazoles for tubulin in these organisms. This knowledge can be exploited for DNA probes for benzimidazole resistance/susceptibility in individual organisms.