A new study has found anthelmintic medicines are not working as effectively as they used to in treating worm infections on UK stud farms.

According to researchers, implications from the study – published in the International Journal for Parasitology – highlight the “impending threat” of multi-drug resistance in worm (cyathostomin) populations infecting horses.

Academics from the Moredun Research Institute in Edinburgh – in collaboration with the universities of Liverpool and Bristol – investigated the efficacy of the four available broad-spectrum anthelmintic drugs on 16 thoroughbred stud farms in the UK.

The team calculated the percentage reduction in worm egg counts found in the faeces of horses after treatment and examined the time to egg re-appearance following treatment. The results showed while ivermectin and moxidectin showed acceptable efficacy at two weeks post-treatment, time to the reappearance of worm eggs in the faeces of the horses was shorter than previously observed. Worm resistance, meanwhile, measured two weeks after treatment, was observed against the other two anthelmintic products examined, fenbendazole and pyrantel. Resistance to ivermectin was also identified in the common roundworm of foals, Parascaris equorum.

Deputy director of Moredun Jacqueline Matthews said: “These results from a comprehensive study suggest the common anthelmintic products are working less effectively than measured previously, and indicate the presence of multi-drug resistance.

“It is important more sustainable strategies for worm control be deployed in practice. These involve evidence-based protocols using worm egg count analysis to direct treatments at certain times of year, such as spring and summer, and should be designed with input from a veterinary surgeon or suitably qualified person.”

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