Working equids in tropical and subtropical climates might be full of intestinal parasites, but deworming those horses to render them worm-free might not be the best strategy. According to Mexican researchers, massive deworming might do more harm than good by opening the door for anthelmintic-resistant parasite development.

Miguel Ángel Alonso-Díaz, PhD, DVM, of the Faculty of Veterinary Medicine of the Independent National University of Mexico, in Veracruz, explained that because the number of parasites doesn’t appear related to body condition in individual working horses, mules, and donkeys, treating all affected working equids with anthelminthics might not make a noticeable difference in many animals’ health status. It could, however, encourage anthelminthic-resistant parasites in those areas; resistant parasites are already posing a major problem in other regions of the world, Alonso-Díaz added.

“We suggest only deworming the animals that show clinical signs of parasite infection (such as diarrhea, colic, and emaciation), or those with low body condition scores, as a preventive or control treatment,” Alonso-Díaz said. He noted, however, that all the horses should have access to enough good-quality forage and grains, which will help improve their health and immune systems.

In the 140 working equids in central Veracruz that Alonso-Díaz and his fellow researchers monitored during the study, body condition averaged a “moderate” score (about 2.5 out of 5), he said. In horses, specifically, the average score was 2.8. Of all the equids, 90% had parasite infestations and none had been treated for worms in recent months