Equine Internal Parasites Study Guide

Overview and Clinical Importance

Internal parasites represent one of the most significant health challenges in equine medicine, affecting horses worldwide regardless of management system. These parasites cause substantial economic losses through decreased performance, poor body condition, colic, and death. Understanding the biology, pathogenesis, diagnosis, and treatment of equine parasites is essential for the NAVLE and clinical practice.

The primary internal parasites of clinical concern in horses include: large strongyles (Strongylus spp.), small strongyles (cyathostomins), ascarids (Parascaris spp.), tapeworms (Anoplocephala perfoliata), pinworms (Oxyuris equi), and bots (Gasterophilus spp.). The emergence of widespread anthelmintic resistance has transformed parasite control strategies from interval-based deworming to evidence-based targeted selective treatment.

Large Strongyles (Strongylus Species)

Three species of large strongyles infect horses: Strongylus vulgaris (the most pathogenic), Strongylus edentatus, and Strongylus equinus. While effective anthelmintics have significantly reduced their prevalence, S. vulgaris is considered a re-emerging pathogen due to changes in deworming practices and anthelmintic resistance concerns.