Rees sees many patients with allergies, for example, insect bite hypersensitivity (Photo 1). Atopic dermatitis and environmental
allergies to dust, molds and poor-quality hay are most common. Hives are a common allergic response that can be caused by
such things as diet, insect bites, dust, mold and medications. Hives can occur anywhere on the body, but most typically on
the face, neck, chest and upper legs (Photo 2). They may or may not be itchy. For horses that get hives (urticaria), Rees
has found immunotherapy, such as allergy vaccines, to be effective.
Photo 1: A horse with marked insect bite hypersensitivity.
"I do a lot skin testing, though horses are a bit different in that they can have both delayed as well as immediate hypersensitivity
reactions," Rees says. "So I usually do readings at 15 minutes and 30 minutes and a delayed-reaction reading at four to six
Photo 2: Generalized hives, a common allergic response, in a horse.
Owners can be advised to minimize an inhalant allergy by reducing exposure and ensuring good-quality, clean bedding is provided
and changed often. Wetting the bedding can reduce dust, although clients should be cautious of creating greater mold growth.
In a horse with inhalant allergies, it's also important to ensure that the hay is mold-free and that a confined horse has
sufficient air exposure.
Bathing horses often to reduce the pollen on skin also is helpful, Rees advises. "Unfortunately, some horses are so allergic—so
bad off—that they need antihistamines, allergy shots or corticosteroids such as dexamethasone. In horses, one has to be careful
with excessive use of steroids due to the concern for hoof issues and laminitis," Rees says.
Allergic contact dermatitis
A particular type of allergic reaction, allergic contact dermatitis occurs when irritating substances, such as fly sprays,
shampoos, liniments or other substances, come into direct contact with the skin of hypersensitive horses. Almost anything
used in the barn may be suspect, even substances in saddle pads. Their chemical composition may provide an irritant leading
to allergy. If a fly spray is applied, such an offending chemical may become trapped between equipment and sweaty skin, exacerbating
the situation. Certain plants may act as contact allergens, affecting the skin of the tongue or muzzle (Photo 3). Signs may
include mild redness or flaky, itchy skin, severe hair loss, skin thickening, pain and, occasionally, skin sloughing.
Photo 3: Contact allergy on a horse's tongue.