Lice infestation of horses and donkeys occurs most often in animals with longer haircoats. Other predisposing factors include systemic compromise or inappropriate husbandry, as for other species. Pyrethrin-based treatments are effective.
Courtesy of Dr. Sameeh M. Abutarbush.
Horses and donkeys may be infested by two species of lice: Haematopinus asini (the horse bloodsucking louse) and Damalinia equi (the horse biting louse). D equi is also called Werneckiella equi and was formerly known as Bovicola equi, Trichodectes equi, and Trichodectes parumpilosus. Both species are distributed worldwide.
D equi is a small louse, 1–2 mm long; H asini is 3–3.5 mm long. There have been reports of poultry chewing lice (see Lice of Poultry) infesting horses when poultry and horses are housed in the same facilities. This problem is exacerbated when the poultry are removed without concurrent animal or premise treatments, leaving horses as the only host available.
Factors such as stocking density, feed quality, gestational status, and underlying health problems often contribute to susceptibility and extent of infestation. Longer body hair (winter coat or feathering) appears to enable higher densities of lice to be sustained because they have greater surface area to infest. Infestations are most common in winter and early spring. For predilection sites on the host by species of louse, see the table Predilection Sites of Equine Lice.
Pathogenesis and Disease Transmission of Lice in Horses and Donkeys
D equi and H asini are not known to vector any disease agents in the US.
Treatment of Lice in Horses and Donkeys
In the US, pyrethrin and pyrethroid sprays
In severe infestations, clipping the coat
A variety of compounds effectively control lice on horses, including synergized pyrethrins, synthetic pyrethroids, and the organophosphate coumaphos. Diazinon is no longer labeled for use on horses in the US.
Pyrethrin and pyrethroid sprays are the most popular method for lice control on horses in the US. Pyrethrin or pyrethroid formulations also are available in wipe-on, pour-on, and powder applications. Coumaphos is available as a powder or spray.
Caution is warranted in mares with foals at their side, because the foals may be exposed to larger amounts of the compound than are intended, especially with powdered formulations. Certain formulations of sprays require soaking the hair to the skin, including mane and tail; other formulations may require only a light, misting application. Label instructions should be read carefully, and the manufacturer consulted if necessary. Retreatment 14–21 days after the initial application is needed to control infestations.
Depending on the severity of infestation, the coat may need to be clipped. Long hair such as feathers on certain breeds, winter coats, or the result of an endocrine disorder (eg, pituitary pars intermedia dysfunction; see Hirsutism Associated With Adenomas of the Pars Intermedia) may make treatment more difficult.
Husbandry problems (overcrowding, poor feed quality, etc) and underlying health conditions should be addressed. Treatment is most effective if trailers, stalls, wash racks, and other areas contacted by horses are cleaned and treated with an appropriate premise spray.
Key Points
Lice infestations are more common in sick and debilitated horses than in healthy horses.
Clinical signs include pruritus, dermal irritation, and unthrifty appearance.
Repeat treatments are needed to control infestations.
For More Information
American Association of Equine Practitioners, Glossary/Terminology: Lice
Also see pet health content regarding lice in horses.
