To infest is to attack, overrun, or establish a large-scale parasitic presence on or within a host organism or its environment. The term is applied to macroscopic parasites—ectoparasites living on the body surface and endoparasites residing in body cavities or tissues—whereas the parallel term microscopic pathogens that infect is reserved for microscopic pathogens such as bacteria, viruses, and fungi. In horses, common infestations include bot fly larvae (Gasterophilus spp.) in the stomach, strongyle worms in the large intestine, pinworms (Oxyuris equi) in the rectum, and ectoparasites such as ticks, lice (Damalinia and Haematopinus spp.), mites (mange), and culicoid midges on the skin and coat.
The clinical impact of an infestation depends on parasite species, load, and host immune status. Heavy strongyle burdens in young horses produce protein-losing enteropathy, poor body condition, and colic; cyathostome larvae mass-emerging from the colon wall (larval cyathostominosis) cause acute diarrhea and rapid deterioration that can be fatal. Bot larvae attach to the gastric mucosa and, at high loads, can cause ulceration and impaired digestion. Ectoparasite infestations cause pruritus, skin excoriation, mane and tail hair loss (sweet itch from Culicoides hypersensitivity is one of the most common equine skin problems in temperate climates), and, in heavy tick infestations, anemia and transmission of tick-borne infections such as equine piroplasmosis and Lyme disease.
Fecal egg count (FEC) monitoring is the evidence-based standard for managing internal parasite infestations; it replaces calendar-based universal targeted anthelmintic treatment guided by egg counts protocols that accelerated anthelmintic resistance by treating horses that did not carry significant parasite loads. Horses shedding more than 500 eggs per gram are considered high shedders and dewormed strategically; horses consistently below 200 eggs per gram receive treatment only at peak seasonal risk periods. Pasture hygiene—regular manure removal from the field, rotational grazing, and avoiding stocking densities that concentrate fecal contamination—is the non-pharmacological complement to drug treatment because it reduces the infective larval population that horses ingest while grazing. A clean pasture with low contamination pressure is the single most effective tool for keeping parasite burdens at subclinical levels.
Further Reading
- infestation (Wikipedia)
- For parasite identification and control protocols: horse parasites and flies — MSD Veterinary Manual