A louse (plural: lice) is a wingless, dorsoventrally flattened parasitic insect of the order Phthiraptera that completes its entire life cycle — egg, nymph, and adult — on the body surface of a specific host species. Lice are highly host-specific; equine lice infest only horses, donkeys, and mules and do not transfer to humans or other mammalian species. The adult equine louse is 1 to 3.5 millimeters in length depending on species, with clawed legs adapted for grasping individual hair shafts and a mouthpart structure configured either for chewing skin debris or piercing skin to access blood.
The two species that infest horses differ in feeding biology and preferred body site. Damalinia equi is a biting or chewing louse that consumes desquamated skin cells, sebum, and hair fragments; it is most abundant along the back, sides, mane, and tail base. Haematopinus asini is a sucking louse equipped with piercing mouthparts for blood feeding; it concentrates around the head, neck, inner legs, and the base of the forelock where skin is thinner and warmer. The sucking louse can cause anemia in heavily infested young, elderly, or immunocompromised horses. Eggs are cemented to hair shafts close to the skin surface and hatch in 1 to 2 weeks; nymphs reach adulthood in approximately 3 weeks, completing the cycle entirely on the host.
Louse infestation is seasonal, peaking in late winter and early spring when coat density is highest and horses are in close housing proximity. Diagnosis is by direct visual inspection — parting the coat and examining the skin surface reveals adults or attached nits. Treatment with topical pyrethroid or organophosphate products is effective but must be repeated after 14 days to kill nymphs hatching from eggs that survived the first application. All in-contact horses should be treated simultaneously, and shared equipment should be cleaned and treated to eliminate transfer between animals. A thorough quarantine protocol for newly arriving horses, which includes louse inspection alongside deworming baseline assessment, prevents introduction to an established herd.
Further Reading: The two equine louse species are covered in their respective Wikipedia entries: Damalinia (the biting louse genus) and Haematopinus (the sucking louse genus), both of which infest horses and are discussed with their host range and feeding biology.