In areas where biting midges are found a common cause of persistent and recurrent mane and tail itching during warm weather is an allergic reaction to midge bites. In particular the culicoides midge is responsible. Culicoides hypersensitivity (CH) is a recurring warm season itching that without treatment or management will worsen each year. Where common this disease process goes by a number of local names including Sweet Itch and Queensland Itch. In the literature it is also called Insect Bite Hypersensitivity (IBH).

Culicoides is a small biting midge that prefers to feed on the head, tail, mane, and ventral midline of the horse. The bite itself is irritating but once a horse develops the allergy the skin irritation and pruritus (itchiness) worsen to the point that the mutilates himself with rubbing. Hair loss, redness, and small sores can develop. This article discusses the diagnosis, how the allergy forms, and treatment of this frustrating problem.

Culicoides is a common cause of the typical history and clinical signs listed below where ever culicoides occur with any frequency. They have nearly a world wide distribution but will be much worse anywhere there is regular standing water. Currently there are no dependable commercial tests for differentiating the causes of hypersensitivity. These tests suffer from nonspecificity, false positives, and false negatives. Allergic disease with itchiness in horses is diagnosed primarily based on history, clinical signs, and ruling out other causes of skin irritation and pruritus. Occasionally skin biopsy can be helpful ruling out autoimmune diseases and parasitic causes of pruritus.

As you review the history and symptoms one should always keep in mind other possible causes of similar clinical signs in horses ...more.

History is helpful with the diagnosis:

• The number of allergic individuals with an allergy to culicoides goes up in areas where there are more culicoides midges. Coastal, lowland, and wetland areas are particularly prone to the problem.
• A history of recurring pruritus with the typical distribution pattern during the warm months is essential to the diagnosis.
• A pattern of pruritus that effects the mane, tail, and ventral midline the worse.
• The pruritic worsens as years go by.

Lesions are typical for any pruritic disease:

• Papules, erect hair early in course
• Excoriations, crusts, scabs, scaling
• Alopecia
• Dorsal distribution of broken hairs in mane and base of tail

If there are reasons to believe other factors may be at cause by all means remove exposure and see what happens. Give any potential allergen at least six weeks before deciding that is or is not the cause of the itching. Often diagnosis will be based on response to management and treatment. Note that following effective management it may take up to 6 weeks to see improvement it not treated aggressively. Also keep in mind that many causes of pruritus will respond to antihistamine of corticosteroid therapy.

Testing for Culicoides Hypersensitivity