South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
pruritic medications is often necessary: Prednisolone, 0.5 mg/kg, orally, q 24 h as tablets mixed in
grain or dexamethasone, 0.05-0.1 mg/kg, orally, q 24 h (the liquid injectable formulation is 60-70%
bioavailable when squirted in the mouth) can be used until pruritus and self-trauma are controlled, then
the dose is tapered to the lowest every other day amount that controls pruritus. Adverse effects of
corticosteroids in the horse can include laminitis and altered mentation (e.g., hyperexcitability or
placidity) but these are rare. Antihistamines appear to be largely ineffective as a primary treatment for
equine insect hypersensitivity. However, hydroxyzine, 1-2 mg/kg, orally, q 8-12 h or doxepin, (a
tricyclic antidepressant with strong antihistaminic effect) 0.5–0.75 mg/kg, orally, q 12 h can be used to
decrease the dose of corticosteroids needed to control pruritus. Dietary supplementation with an omega6/omega-3 fatty acid product may also allow the dosage of corticosteroids to be decreased. In horses
with concurrent pyoderma/folliculitis, treatment with a trimethoprim-sulfonamide combination (30
mg/kg, orally, q 12-24 h) for the initial 2-4 weeks of therapy may also be indicated. Efficacy of
allergen-specific immunotherapy (hyposensitization) in the management of insect hypersensitivity has
been reported to range from 0 to 90%. The author has successfully used immunotherapy to treat horses
with insect hypersensitivity, specifically in cases of fly bite hypersensitivity.
Atopic dermatitis (environmental allergies) is a common problem with insect hypersensitivity. Clinical
signs are similar and diagnosis is made with a compatible history and by ruling out other causes of pruritus.
Treatment with antihistamines and corticosteroids are identical to the treatment of insect hypersensitivity
and immunotherapy is an effective treatment for environmental allergies, with about 80% efficacy reported.
Allergen-specific immunotherapy appears to be safe in horses, is not cost prohibitive, and the horse owner
or barn personal can easily administer injections of the extract mixture. Immunotherapy should be
considered strongly when the following criteria are met: 1) clinical signs for more than 4 months of the
year; 2) lack of a satisfactory response to management changes and use of anti-pruritic drugs; 3) higher risk
of adverse drug effects (e.g., use of corticosteroids in a horse with pre-existing laminitis); and 4) concurrent
environmental allergies. Finally, because there is limited evidence that there may be a hereditary
predisposition to allergic dermatitis, owners of affected horses should be counseled about the risks of using
affected horses for breeding.jmc
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