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 T parumpilosus. Both species are distributed worldwide. D equi is a small louse, 1–2 mm long, and 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.
Stressors such as high stocking density, poor feed quality, gestational status, and underlying health issues are often contributing factors to susceptibility and degree of infestation. Longer body hair, whether a winter coat or feathering, appears to allow for 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 site predilection on host by species of louse, see Table: Site Predilection of Equine Lice.
Site Predilection of Equine Lice
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 USA.
Pyrethrin and pyrethroid sprays are the most popular method for lice control on horses in the USA. 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 intended, especially with powdered formulations. When using sprays, certain formulations require soaking the hair to the skin, including mane and tail, whereas others may require only a light, misting application. Label instructions should be read carefully, and the manufacturer consulted if necessary.
Depending on the severity of infestation, the coat may be clipped. Long hair such as feathers on certain breeds, winter coats, or resultant from endocrine disorder (eg, pituitary pars intermedia dysfunction, see Hypertrichosis Associated with Adenomas of the Pars Intermedia) may make treatment more difficult.
Husbandry issues (overcrowding, poor feed quality, etc) and underlying health conditions should be addressed. Treatment will be most effective if trailers, stalls, wash racks, and other areas where horses have contacted are cleaned and treated with an appropriate premise spray.