Lice and Horses

Horses are affected by two species of lice, the sucking louse (Haematopinus asini) and the biting louse (Damalinia equi). Horse sucking lice are the more common louse species infesting horses. The adult lice lay their eggs on the horse hair, which hatch into nymphs in approximately 1 week.

It takes approximately 3 weeks before the nymphs mature into adults. Nymphs appear as a smaller version of adult lice and feed on the horse the same way as the adults do. Sucking lice are usually found in the mane, forelock, tail base and around the coronary bands. Biting lice are usually found on the body, mainly the backline, head, neck and flanks. Severe infestations may involve the whole body and can even be serious enough to result in anaemia. The lice are easily visualized in heavy infestations, lighter infestations can be more tricky to identify. It is important to quickly lift up or part the mane hair or forelock as lice quickly move away from the exposed area. The horse biting louse is approximately 3mm long, reddish-brown in colour, with a yellow and brown striped abdomen. The horse sucking louse is just under 3mm long and is grey coloured with a broad abdomen and narrow head.

Lice feed on sloughed skin, hair, and skin secretions. This causes a stark, roughened coat, skin irritation, itching and self mutilation. There is usually significant hair loss due to the horse rubbing and scratching. Infection is predominantly in winter and spring as long winter coats and lack of rugging and grooming predispose horses to infestation. Lice are spread between horses through direct physical contact, or indirectly through grooming equipment or tack, or the environment. Lice can only survive a few days in the environment, as they need to feed from horses to survive. Horses that are old, in poor condition or immune-compromised are more susceptible to infection. Lice are species specific and therefore humans and other animals can not be infected with horse lice.

Lice are usually readily controlled with insecticides, however, it is important to also address underlying predisposing factors such as poor body condition. All horses should be treated and re-examined in 2 weeks to check that newly hatched lice are not developing. It is also important that contaminated gear is treated and equipment is not used for this period, as lice do not survive in the environment. There is a pour-on treatment available for horses. Care should be taken if the skin is significantly broken or irritated as severe inflammation can result. Horses should be checked again 1-2 months after application as this treatment works by retarding the growth of the developing lice rather than by a direct kill. Cattle or sheep pour-on medications are not recommended for horses as they can result in severe inflammation.