The prevalence and intensity of internal parasites of horses in the U.S.A.

The research involved necropsying 55 adult horses over a 15-month period to identify, count, and classify internal parasite species. The study found high prevalence and benzimidazole anthelmintic resistance in 10 species, and noted less frequent occurrence of Strongylus vulgaris and related arterial lesions compared to previous surveys.

Methodology and Species Identified

• The study involved a post-mortem examination (necropsy) of 55 adult horses over a period of 15 months. During this necropsy, worm burdens were counted and identified or speciated.
• The research identified 21 species of Cyathostominae, a subfamily of small strongyles or redworms that are common internal parasites in horses.
• Out of these 21 species, 10 were found to comprise 98.9% of the total cyathostome burdens, including Cyathostomum catinatum, Cylicocyclus nassatus, Cylicostephanus longibursatus, Cyathostomum coronatum, Cylicostephanus goldi, C. calicatus, C. minutus, Cylicocyclus leptostomus, C. insigne and Cyathostomum pateratum.

Prevalence and Anthelmintic Resistance

• The same 10 species were found to display high prevalence in four previous studies, suggesting their dominant presence in the horse population studied.
• The study discovered that eight out of the 10 dominant species have shown resistance to benzimidazole anthelmintics, a class of deworming agents commonly used to control parasite burdens in horses.
• The research presents an interesting finding, noting that anthelmintic resistance doesn’t appear to affect the prevalence of a particular species. Instead, it was found to increase the relative abundance of that species in a mixed population, suggesting that resistance may allow these species to thrive amidst anthelmintic treatment.

Prevalence of Other Parasites

• The prevalence of other internal parasites found in the horses was largely in line with the findings of previous studies, suggesting a consistent pattern of parasitic colonization in these animals.
• The study noted that the prevalence of Strongylus vulgaris adults, a species of large strongyles, and the occurrence of arterial lesions related to this parasite were less common compared to surveys conducted in the past.