Equine Science Update
Reports from the world of  equine research.
Chorioptic mange treatment.
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Chorioptic mange is a common skin problem in horses. It is more often seen in draft breeds, especially those with hairy legs (“feather”). The lower legs are the most common sites of infestation, but the base of the tail and body can also be affected.

The chorioptic mites live on the skin surface. Their mouthparts are adapted for chewing and they feed on skin debris. Although they do not burrow into the skin, they can cause considerable irritation.

Affected horses rub, stamp, and bite their legs and kick, especially at night. Mild cases may show patchy hair loss and the skin may become thickened and scaly. In severe cases the skin may become raw, with secondary bacterial infection. The disease tends to be seen more commonly in the winter - possibly as a result of animals being housed close to each other.

Interestingly, some horses can be infected without showing signs, and they may act as a source of re-infestation for treated animals. The mites are also able to survive in the environment, probably for several weeks. So, as well as treating affected animals, it is important to replace the bedding and treat in-contact horses to get the problem under control.

In many countries, including the UK, no products are licensed to treat the condition. Several drugs have been used “off-label” to treat affected animals including fipronil and doramectin.

Fipronil is widely used to treat fleas and lice in small animals. It accumulates in the sebaceous glands of the hair follicles from where it is released over a several weeks. It is marketed, for small animal use, both as a spray and as a topical (“spot-on”) preparation. Doramectin is used against internal and external parasites in cattle and sheep. It is stored in body fat and released slowly to give prolonged activity. A recent study considered the use of these two drugs for treating chorioptic mange in horses.

Mr David Rendle and colleagues at the Glasgow vet school assessed the response to treatment of seventeen cases of chorioptic mange in horses. They treated eight cases with doramectin (0.3 mg/kg, two doses by subcutaneous injection 14 days apart) and nine others with fipronil (0.25%, sprayed onto the lower legs.)

After two weeks, four of the doramectin-treated horses and eight of those treated with fipronil had improved. By four weeks none of the horses showed any behavioural signs of skin irritation. "I am unconvinced that one treatment is better than the other" Mr Rendle adds. "I find both equally effective."

No adverse reactions were noted in any animals. Some of the horses with heavily feathered legs were treated without being clipped. Despite that the treatments appeared to be effective. However, because of the small number if horses involved, that does not prove that clipping is unnecessary.

This work should provide encouragement for vets in practice when faced with this irritating condition.

For more information see:

Comparative study of doramectin and fipronil in the treatment of equine chorioptic mange.
DI Rendle, J Cottle, S Love, KJ Hughes.
Vet Rec (2007) 161, 335 - 338.
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© Copyright Equine Science Update  2007