Lice infestation, also known as pediculosis, can occur in horses of all ages. Horses are affected by both sucking (Anoplura) and biting (Mallophaga) lice. These lice are host specific, meaning they only cause infestation in equids. The life cycle of a louse consists of three phases: egg (nit), larvae (nymph) and adult. All three phases of this life cycle need to occur on the horse or other equids. Adult females deposit eggs near the skin where they develop into nymphs and later adults.
Transmission occurs through direct contact between horses or indirectly though brushes, blankets, saddle pads and other grooming equipment and tack. The majority of transmission occurs quickly as lice cannot live off of the host for more than a few days. Infestations appear most commonly in the winter because lice cannot survive the warm body temperatures that occur in summer when horses are exposed to sunlight. There is some evidence of “carriers,” or animals that can remain infected throughout the summer months, that lead to re-infestation of the other members of the herd come fall. Immunocompromised, under-conditioned or sick equids are more likely to become affected.
The most common sign associated with lice infection is itching or pruritis. Depending on the severity of the infestation, hair loss and skin lesions (dry, ulcerated or crusty) may be present. Horses may cause trauma to themselves attempting to itch and loss of body condition and/or anemia (low red blood cell count) can occur with more severe infestations. Lice are often most apparent in the mane and tail, and infestations occur more commonly in horses with heavy hair coats.
Diagnosis is made by visually identifying the lice. Infestations will range in severity and the number of lice present can vary dramatically. Lice tend to migrate toward the tip of the hair shaft when the horse is warm or sweaty, so exercise can sometimes exacerbate the number of lice visible.
Individualized treatment recommendations should be made by the owner's primary veterinarian. The type of treatment can be dependent on the type of lice with multiple oral deworming products or topical insecticides (shampoos, sprays, wipes, powders) available. It is important that treatment is done at proper intervals, and more than once, as no products are effective against the egg stage.
Management strategies include continual surveillance of the herd while paying closer attention to any new or sick or immune-compromised horses. Once an infestation is identified, the affected animals, and those in contact, should be isolated and treated. Additionally, cleaning and applying an insecticide to brushes, tack and saddle pads is recommended.
For more information on lice, visit the American Association of Equine Practitioners website.
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