A large percentage of horses affected with seasonal pruritic dermatitis are hypersensitive (allergic) to the bites of insects. Culicoides spp (gnats), black flies, stable flies, and horn flies are the most commonly implicated insects, although any biting insect can contribute to insect bite-induced hypersensitivity (IBH).

Insect bite hypersensitivity is characterized by intense pruritus (itching) that often leads to excoriation (abrasive skin damage), extensive hair loss, secondary infections, and chronically to hyperkeratosis and lichenification (thickened skin).

Many horses develop IBH in middle age or later, although horses with atopy (a tendency to be hyperallergic) could exhibit clinical signs as early as 1 year of age. Clinical signs often progress in each subsequent year. Diagnosis is made from the signalment, history, clinical signs, and ruling out of other possible diagnoses.

Lesion distribution on affected horses is dependent on the biting characteristics of the insect(s) responsible. Since there is still much to be learned about the identification and feeding habits of many insects implicated in allergic dermatoses, it might not be possible to identify the exact etiological agent(s).

Insect bite hypersensitivity typically improves and exacerbates seasonally, whereas clinical expression of allergies in atopic horses may occur at any time of the year. Urticaria (hives), commonly found in atopy, is not a diagnosis but a cutaneous reaction pattern that could be induced by a wide variety of causes, both immunologic and non-immunologic. Rule outs for urticaria include drug and vaccine reactions; stinging and biting insects (such as wasps) and arachnids; infections; contact irritation; vasculitis; and cold, stress, or exercise-induced lesions. Other diseases that exhibit pruritus are Oxyuris (pin worm) infestation, onchocerciasis, and mite (Chorioptes) and tick infestations. Diseases that occasionally are associated with pruritus include ringworm and pemphigus, an autoimmune disease.

In all cases of pruritic dermatitis, managerial procedures to decrease exposure to exciting agents as well as symptomatic therapy to reduce pruritus are warranted. When dealing with an IBH horse, procedures to decrease exposure to insects should be fully implemented. Farm sanitation to compost manure properly, eliminate wet areas with decomposing vegetation, and the use of fly predators or feed through fly inhibitors can greatly reduce fly populations. Heat, humidity, and solar radiation exacerbate pruritus, thus the provision of shade and wind currents by fans provide relief. Stabling to avoid the most significant insects (Table 1) and the use of fly sheets and face masks are helpful.

Because some horses develop contact allergies to topical products, judicious and cautious use of fly repellants is advisable. Simple feeds and whole grains are better than mixed, multiple-grain sweet feeds. Cool-water rinses and shampoos can rehydrate and sooth dry skin as well as reduce the amount of allergens on the skin. Colloidal oatmeal, pramoxine, and 1% hydrocortisone shampoos or leave-on hydrocortisone rinses can reduce pruritus and minimize or reduce the amount of systemic antipruritic medication needed. Systemic medications (steroids and antihistamines) may help interrupt the itch-scratch cycle, however long-term use, particularly of steroids, should be avoided.

A complete and detailed investigation into the etiology of the disease should occur simultaneously with general symptomatic care. Intradermal skin testing to determine antigens for allergen specific immunotherapy may be helpful, particularly in young animals with atopy.

Clients should understand that hypersensitivities and atopy are lifelong. Affected horses will need continuous management and/or therapy. Often a patient can be clinical-sign-free with low exposure to inciting antigens and symptomatic as the antigen load increases. Antigen exposure is additive, thus comprehensive management is needed to best control clinical disease.

CONTACT: Susan L. White, DVM, MS, Dipl. ACVIM—slwhite@uga.edu—706/296-8607—University of Georgia College of Veterinary Medicine Department of Large Animal Medicine, Athens

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This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.