Overview of Gastrointestinal Parasites of Horses

Horses are hosts to a multitude of intestinal helminth parasites. Most of these are acquired while grazing on pasture, and parasitism should be regarded as a normal state in healthy horses. However, parasitic diseases can occur, and some of these conditions can be life-threatening. This chapter aims to describe the most common helminth parasites infecting horses and the clinical conditions they can cause. Furthermore, the chapter will provide information about diagnostic tests and describe appropriate treatment considerations.

Anthelmintic resistance is described with increasing frequency in equine helminths. Cyathostomins and ascarids are widely resistant to several anthelmintic drug classes across the world, and cases of complete anthelmintic failure have been described. Furthermore, equine pinworms are reported to be widely resistant to macrocyclic lactones, and treatment failure of praziquantel and pyrantel pamoate has been described in equine tapeworms.

Recommendations for equine parasite control have moved away from traditional prophylactic treatments that often employed a rotational schedule to surveillance-based approaches that consider the types of parasites present, their anthelmintic resistance profile, and local climatic conditions. These guidelines are summarized in the American Association for Equine Practitioners (AAEP) Internal Parasite Control Guidelines.