Insect Bite Hypersensitivity/Sweet Itch

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During these hot, humid summer days horses may suffer from a skin condition called seasonal recurrent Sweet Itchdermatitis. Also referred to as sweet itch, it is an allergy to the saliva of the Culicoides midge or gnat. Allergies to other insects may also be involved and so it is often referred to as insect bite hypersensitivity (IBH).

Affected horses develop an allergic reaction at the site of the bites, usually along the base of the mane and tail head, around the ears and head, and along the back, center of the chest, and abdomen. Horses may be agitated and exhibit signs of restlessness such as tail switching and stomping at the ground. Itching can be intense sometimes leading to self-trauma when the horse rolls or rubs up against solid objects, resulting in broken hairs, bald patches, and even bleeding and oozing skin. The resulting round patches are sometimes confused with hives, ringworm or rain rot but are relatively easy to distinguish for a veterinarian.

The best treatment for insect bite hypersensitivity is prevention of further insect bites. Turnout during dawn and dusk hours when midges are most active should be avoided. Horses should be moved away from stagnant water sources (bogs, puddles, old tires, etc.). Pastures that have access to a good breeze are preferred, as midges are unable to feed in the wind currents.

Placing fans in the stable creates air movement that makes it more difficult for midges to feed. Applying oils (citronella, tea tree, baby oil) to the coat especially under the mane can also discourage the gnats from biting.

Fly masks and fly sheets can be used, though a full body blanket that covers the belly and neck must be used to be effective. Permethrin and citronella based insect repellents also have variable efficacies. Permethrin based sprays with sunscreen may be longer lasting, but most repellents must be applied daily.

Treatment of affected horses involves a variety of topical ointments, sprays, and shampoos. These may contain a combination of corticosteroid (hydrocortisone, betamethasone, triamcinolone), anesthetics (pramoxine, lidocaine), antiseptics (chlorhexidine, benzoyl peroxide), emollients (aloe, colloidal oatmeal), and repellants (citronella oil). More severely affected horses are treated with short acting corticosteroid injections, tapering courses of oral corticosteroids (dexamethasone or prednisolone), and anti-histamines. For deeper more generalized skin infections, antibiotics may be prescribed.

Anecdotally the addition of fatty acids in the form of flax seed has resulted in improvement of symptoms, although controlled studies have not supported these claims. Allergen specific immunotherapy has been attempted, but must be started prior to the onset of clinical signs continued for at least a year. This can be expensive and results have been mixed.

In summary, horses with insect bite hypersensitivity can be managed with strategic turnout, environmental management, insect repellants, protective masks and blankets, and dietary supplementation. Once clinical signs are noted, symptomatic topical and/or systemic anti-inflammatory therapy is prescribed.

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