A parasite is any organism that lives on or within a host organism and derives nutrients or shelter at the host’s expense, typically causing some degree of harm. Equine parasites are divided into ectoparasites (living on the body surface) and endoparasites (living internally). The most clinically significant internal parasites of horses are strongyles (large and small), ascarids (Parascaris equorum), tapeworms (Anoplocephala perfoliata), pinworms (Oxyuris equi), and bots (Gasterophilus spp. larvae). Large strongyles, particularly Strongylus vulgaris, were historically the leading cause of parasite-associated colic through larval migration in mesenteric arteries, but their prevalence has declined substantially in populations with strategic deworming programs. Small strongyles (cyathostomins) have emerged as the primary clinical concern because of their ability to encyst in the intestinal wall and develop resistance to benzimidazole anthelmintics. Fecal egg counts (FECs) and fecal egg count reduction tests (FECRTs) guide targeted selective treatment, replacing calendar-based deworming. Ectoparasites of importance include skin-burrowing ectoparasites, ticks, and lice, each capable of causing dermatitis, anemia, or acting as vectors for other pathogens. Regular fecal-egg-count-guided anthelmintic strategy based on fecal egg counts is the current best-practice approach. For pasture management that reduces larval exposure see the reducing larval exposure through pasture management.
Further Reading
- parasite (Wikipedia)
- For the clinical overview of equine internal parasite control: gastrointestinal parasites of horses — Merck Veterinary Manual