Evidence-based considerations for control of Parascaris spp. infections in horses

Parascaris spp. infection is virtually ubiquitous in young foals and worm burdens can achieve high numbers. The most important disease manifestation is impaction of the small intestine, which occurs in a small proportion of infected foals but is associated with a guarded prognosis for survival. Control of Parascaris spp. is complicated by emerging resistance to currently available anthelmintic drug classes. Resistance to macrocyclic lactones has been reported worldwide and a few studies have also documented signs of resistance to pyrantel salts and benzimidazoles. Foals generally develop immunity to Parascaris spp. parasites around age 6 months, but a proportion of weanlings and yearlings can harbour smaller burdens at age 8-10 months. Older horses have occasionally been reported with substantial ascarid burdens as well. Qualitative detection of ascarid eggs has good diagnostic value whereas an actual egg count does not correlate well with the size of the worm burden. A recent investigation documented the applicability of a transabdominal ultrasound technique for semiquantitatively monitoring ascarid burdens in foals. Control of ascarids is complicated by the limited drug classes available for treating this parasite, and by the fact that foals are often concurrently infected with strongyles. In many cases, none of the 3 available anthelmintic classes are simultaneously effective against both parasite groups, so close monitoring is required to select the most appropriate anthelmintic in each case.