Lice Infestation in Horses

By PetMD Editorial on Sep. 4, 2009

Pediculosis Infection In Horses

There are two main types of lice: those that feed through biting and those that feed through sucking. Lice are small, flat-bodied insects. Fully grown, they may be only 2 – 4 millimeters in length, making them difficult to detect in early infestation. They feed from the waste given off by the skin as well as the bodily fluids that can be extracted from it. They breed in the thick coats that horses grow during the colder winter months and reside in various areas of the horse’s body, from the coat to the mane and the tail.

Horses and donkeys may be infested by 2 species of lice, Haematopinus asini (H asini), the horse sucking louse, and Damalinia equi (D equi), the horse biting louse. Both species are worldwide in distribution. Normally, H asini is found at the roots of the forelock and mane, around the base of the tail, and on the hairs just above the hoof. D equi prefers to oviposit on the finer hairs of the body and is found on the sides of the neck, the flanks, and the base of the tail.

Lice are wingless, flattened insects, usually 2-4 mm long. The claws of the legs are adapted for clinging to hairs or feathers. Mallophaga have ventral chewing mandibles and they feed on epidermal products, primarily skin scales and scurf. The head of the mallophagan is wider than the prothorax. Anoplura are blood feeders. When not in use, their mouthpart stylets are retracted within the head.

Louse eggs or nits are glued to hairs of mammallian hosts near the skin surface and are pale, translucent, and suboval. The three nymphal stages, of increasing size, are smaller than adults but otherwise resemble them in habits and appearance. About 3-4 weeks are required to complete one generation, but this varies with species.

Symptoms and Types

Pediculosis is manifest by pruritus and dermal irritation with resultant scratching, rubbing, and biting of infested areas. A generally unthrifty appearance, rough coat, and lowered production in farm animals are common. In severe infestations, there may be loss of hair and local scarification. Extreme infestation with sucking lice can cause anemia.

  • There are two main types of lice, including:
    • Biting lice
    • Sucking lice
  • The symptoms of a lice infestation include:
    • Loss of luster to horse’s coat
    • Loss of hair from neck
    • Loss of hair from shoulders
    • Matting of body hairs
    • Mane and tail become matted
    • Itching of the skin
    • Rubbing against poles, walls, etc.
    • Biting at the skin
    • Loss of condition in more severe cases
    • Anemia dues to ongoing blood loss occurs in more severe case
    • Lice may be seen on the surface of the skin


Those horses that are less healthy are more prone to severe infestation in lice. Areas where large numbers of horses are kept together are a more prominent breeding ground for lice and will result in a higher likelihood of infestation in any horse, even the healthiest ones. The cycle is brief, as nits are laid on the hairs and hatch within ten days, normally a short enough period of time for the condition to go completely unnoticed.


A veterinarian does not need to be called in to diagnose lice, but it may be a good idea to consult one when it comes to detecting lice and treating them. It often helps to have some background information when it comes to dealing with lice, as those who don’t know what they are dealing with may not have much of a chance against them when it comes to getting rid of them.

Diagnosis is based on the presence of lice. The hair should be parted, and the skin and proximal portion of the coat examined with the aid of light if indoors. The hair of large animals should be parted on the face, neck, ears, topline, dewlap, escutcheon, tail base, and tail switch. On smaller animals, the ova are readily seen. Occasionally, when the coat is matted, the lice can be seen when the mass is broken apart. Biting lice are active and can be seen moving through the hair. Sucking lice usually move more slowly and are often found with mouthparts embedded in the skin.


Clipping of the coat is the best first step in treating lice in horses. This is because the longer the coat is, the more welcoming it is to lice--this is a great way to discourage them from migrating to and breeding on your horse. The thicker the coat the larger the numbers of lice; this is why the winter months are a better time for them to breed in large numbers.

The lice should be treated every two weeks as the life cycle is so short; treatment any less frequent can give the lice ample time to regrow in numbers and become an issue once again. An insecticidal medication may be the best method of treatment; a type of lice killing powder may also be used to dust the horse’s coat and kill the parasites as well.

Living and Management

Keeping the horse clipped when possible is a good way to slow down lice. It is also important to make sure that the treatment is administered often enough until the lice are eradicated.

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