Lice are small, wingless, parasitic insects. They can infest a variety of hosts, including cats, birds, horses, dogs, and people. However, lice are host specific, which means that the species of lice that infest humans, for example, don’t infest other types of hosts. Therefore, humans can’t be infested with lice from animals, including horses. Two species of lice can infest horses. Damalinia (Werneckiella or Bovicola) equi is a biting louse that grasps onto a host’s hair and eats skin debris and secretions. Haematopinus asini is a blood-sucking louse that similarly attaches to the host’s hair and uses its mouthparts to pierce the skin and drink blood.

The clinical signs associated with lice can vary in severity and may be limited to skin
problems. The clinical signs include the following:

  • Scratching, rubbing, and biting
  • Stress due to restlessness and irritation
  • Hair loss
  • Skin wounds and raw areas (from self-trauma)
  • A rough coat and an unthrifty appearance

If a horse is heavily infested with blood-sucking lice, the parasites can drink enough blood to cause anaemia. Horses can also become very stressed and annoyed by lice, leading to weight loss and contributing to an unthrifty appearance.

Horses most commonly get lice from being in contact with other horses that are infested. Horses in a crowded environment can very easily spread the parasites to each other. Lice can also be transmitted by fomites—objects such as combs, brushes, or blankets that, if shared, can help spread lice from horse to horse. Lice can also live for a few hours on stalls or fences, so if a horse has been rubbing against such an area, another horse can become infested.

However, lice can only live in the environment for a very short time, so transmission generally occurs from direct contact between horses. Sick, old, or debilitated horses are also more likely to become infested. Lice infestations in horses are more common in the winter and spring, when horses spend more time in close quarters. Also, longer hair during the cold weather gives lice an excellent place to hide, allowing them to go undetected.

Keeping your horse healthy including regular worming, vaccinations, dentistry and nutrition can help to prevent the lice from finding your horse a suitable host. Isolating new horses to
the property is imperative to prevent introducing lice into your herd. If one horse is treated for lice in your stable or paddock, it is preferable that all in-contact horses are also treated and the affected horse isolated. Despite lice being host specific, humans can act as a form of transport for the lice between horses. Affected horses should be retreated 3-5 weeks after the first treatment to ensure the complete life cycle is eliminated.