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Ectoparasites in Horses

Horses can be affected by a number of ectoparasites including flies, midges and lice. The main problems associated with these ectoparasites are listed below.


There are several ‘types’ of fly which can prove a torment to horses during spring and summer months. Biting flies can pierce the horse’s skin and feed on its blood while nuisance flies lay secretions in and around the horse’s eye, mouth, nose and other sensitive areas. Aside from the threat of an allergic reaction and the annoyance, flies can carry diseases, which they can spread from horse to horse.

A very common type of fly is the Horsefly (approximately 8-10 mm), which typically comes out in June and July, especially around woodlands. They tend to bite the horse’s underside, legs, neck and withers and can cause painful lumps. Horse flies will also bite humans.

Black flies are another common pest and are small in size (approximately 2-5 mm). These breed in rapidly moving water and are most noticeable at dawn and dusk.  These flies commonly feed around the face, particularly inside the ears, where they trigger allergic skin reactions to their saliva, and distract the horse. They also feed on the horse’s neck and underside.

Midges and sweet-itch in horses

Sweet-itch is a common skin disease that affects many horses and ponies in the UK and at present there is no cure. Once a horse develops the allergy it will generally last for life, so it is the responsibility of every horse owner to be vigilant in order to aid early detection, and to take preventative measures to prevent unnecessary suffering.

Sweet-itch is caused by a reaction to the saliva of biting midges during the summer months. It causes horses to rub their manes, tails and sometimes their undersides too. In the United Kingdom, it is the antigens present in the saliva of the Culicoides midge and to a lesser extent, a member of the black fly family called Simulium equinum that horses are allergic to. The severity of the condition varies from horse to horse; some will only rub occasionally, while others will rub themselves bald, causing open sores.

Sweet itch can cause horses to rub their manes, tails and undersides.
Sweet itch can cause horses to rub their manes, tails and undersides.

Treatment revolves around anti- inflammatory therapy which is often unsatisfactory and can have serious side effects if used long term. Preventative measures are therefore crucial to avoid the condition and limit the suffering which can arise from the intense and unrelenting itch.

Lice in horses

Lice are wingless, flattened insects, often specific to host and usually 2–4 mm long. The claws of the legs are adapted for clinging to and moving among hairs. Lice thrive where they can keep warm and are often found at the roots of the forelock and mane but they can be found anywhere on the body particularly if the coat is thick.

The symptoms of a lice infestation include:

  • A dull, listless coat
  • Patchy hair loss
  • Matting of body hairs, mane and tail
  • Itching and rubbing against posts and stable walls
  • Biting at the skin
  • Loss of condition in more severe cases
  • In heavy infestations anaemia may occur due to blood loss
  • Lice and eggs visible on the surface of the skin and in the coat.

Lice are readily passed from one horse to another by physical contact, and can also be spread by the sharing of brushes and equipment between horses. Therefore the sharing of equipment is not recommended.


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