Horse anti-worm treatments can reduce faecal parasite eggs but quicken egg-reappearance

21 Jan 2015

Take-home message: Treating worm-infected horses with ivermectin or moxidectin can reduce the number of faecal parasitic eggs, but does shorten the egg re-appearance period. The current preferred method of worm management in grazing horses is to treat the whole herd with cheap and easy-to-use anthelmintics. However this is contributing to the development of anthelmintic resistance (AR). AR in small strongles -intestinal parasitic worms of grazing horses - has been reported worldwide for two types of anthelmintics: benzimidazoles and pyrantel. However, treatment with ivermectin (IVM) or moxidectin (MOX) is reported to still be effective. Previous studies have focused on evaluating these anthelmintics only 10-21 days after treatment, but a new study has evaluated the efficacy of IVM or MOX treatment over a longer period. The study, conducted by a team of researchers across Europe, examined 320 naturally-infected horses on 32 farms in Italy, Belgium and The Netherlands. The scientists performed  faecal egg count reduction tests and monitored the egg reappearance period after treatment. Faecal samples were collected at least every fortnight during 56 days after treatment with IVM and during 84 days after MOX treatment. Faecal egg count reduction tests on day 14 indicated that IVM and MOX were 100% effective efficacy in 59 of the 64 treatment groups, and more than 92% effective in the remaining five groups. However, the egg reappearance period (the time from treatment to when the animal is infected again) was reduced on around half of the study sites for both treatments. To avoid the development of AR and shortened egg reappearance period, the researchers recommend that pasture management and selective individual treatment approaches are used. They suggest that based on this study, for routine efficacy evaluation on horse farms, additional screening is required around day 35-42 for IVM and day 56 for MOX. Although this approach requires frequent monitoring, the researchers say diagnostic tools are now available to quickly identify horses requiring treatment and the envisaged increased work load and cost is no longer such a hurdle. Geurden, T., van Doorn, D., Claerebout, E., Kooyman, F., De Keersmaecker, S., Vercruysse, J., Traversa, D. (2014). Decreased strongyle egg re-appearance period after treatment with ivermectin and moxidectin in horses in Belgium, Italy and The Netherlands. Veterinary Parasitology, 204(3-4), 291–6. Joanne Stocks, PhD, University of Nottingham