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Allergic skin diseases of the horse (Proceedings)

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Aug 01, 2010

Insect Hypersensitivity

This is the most common allergic skin disease of the horse, caused by a hypersensitivity to the bites of common insects. There are a variety of common names for this condition including "sweet itch" and "summer eczema". This is thought to be a type 1 hypersensitivity reaction involving allergen specific IgE, but a delayed type hypersensitivity reaction may also be involved. Familial involvement can often be documented.

      A. Etiology: Culicoides sp. (no-see-ums, midges), black flies, horse flies, horn flies, deer flies, mosquitoes, and stable flies. Culicoides sp. are the most common insects involved. There are as many as 1000 different species of Culicoides worldwide. The different species have different feeding preferences. Culicoides feed most at dusk and dawn and prefer environments with much standing water, decaying vegetation, and manure. They are poor fliers, and have a limited flight range (1-2 km).

      B. Clinical Signs: whether due to Culicoides or other insects, this is characterized by a seasonal pruritus. Distribution of lesions may differ depending on the species of insect involved. Different species of Culicoides gnats prefer different areas of the body. The most common being the topline (mane, tail, ears, back) and ventrum. Horn flies tend to feed on the ventral abdomen, mosquitoes on the lateral aspects of the body, black flies on the head, ears, and ventrum, and stable flies on the lower limbs, ventrum, chest, and back. The dermatitis is characterized initially by crusted papules or wheals followed by by excoriation, scaling, and self trauma; the mane and tail may become broken and matted from self trauma. Secondary infections are common.

      C. Diagnosis:
           1. History of seasonal occurrence
           2. Clinical signs
           3. Response to insect control
           4. Biopsy: Nondiagnostic . . . typical allergic response
           5. Intradermal allergy testing for each individual species of insect.

      D. Differential Diagnoses:
           1. Atopic dermatitis
           2. Dermatophytosis
           3. Dermatophilosis
           4. Onchocerciasis
           5. Ectoparasites (mange, chiggers, etc)

      E. Treatment: Culicoides Avoidance Measures "Separate the bug from the horse"
           1. Insect control: For Culicoides - Stable during dawn and dusk with very fine meshed netting ( smaller than mosquito netting).
           2. Decrease standing water or move horse further than 2 km from standing water
           3. Frequent spraying of horse with insect repellants (pyrethroids, Avon skin so soft) or timer operated spray misters in the stalls.
           4. Box fans in the stalls – Culicoides gnats are poor fliers.
           5. Co2 producing insect traps (Mosquito magnet)
           6. Full body suits
           7. Cattle tags with fenvalerate or pyrethrins (extralabel)

      F. Treatment: Symptomatic
           1. Corticosteroids: Prednisone 1 mg/kg PO SID x 1-2 weeks then taper off.
           2. Antihistamines and fatty acids may be of benefit (see atopic dermatitis section)
           3. Immunotherapy – very successful in one study, not effective in another.