Atopic dermatitis is a common type of skin disorder that begins when foreign proteins (allergens) invade the immune system. When the body responds, the result is a hypersensitivity reaction typified by itching and hives.

Causes

Horses who suffer from this type of allergic response are ones that have been repeatedly exposed to allergens that are typically found within the environment, such as:

• Pollens
• Grasses
• Trees
• Weeds
• Flowers
• Dust
• Molds
• Feathers
• Fibers, such as cotton and wool

Clinical Signs

The three most common clinical signs of atopic dermatitis include:

• Urticaria (hives)
• Hives with pruritus
• Itching alone

Additional clinical signs that arise from the animal’s intensified biting, chewing, and rubbing at the irritated area include:

• Alopecia (hair loss)
• Excoriations (skin abrasions caused from rubbing)
• Erythema (Skin redness)
• Lichenification (thickened, leathery skin)
• Hyperpigmentation (skin darkening)

Hives

The most visually alarming sign of atopic dermatitis is hives, which occur as when the animal comes into contact with allergens through inhalation or ingestion. The appearance of hives is very similar from one animal to the next, and includes:

• Round, raised wheals of up to several inches in diameter on the skin surface
• Wheals that pit when pressed
• Swelling of the face or eyelids

Hives typically recover spontaneously without medical intervention. However, in severe cases with swelling and itching, short-acting corticosteroids and antihistamines may need to be considered for treatment.

In cases where relief is necessary, the corticosteroids most commonly used include:

• Prednisolone
• Dexamethasone

Antihistamine use may include:

• Hydroxyzine pamoate
• Cetirizine
• Doxepin

Recurrent Cases

When an animal presents with recurrent allergic reactions, it is in its best interest to pinpoint the reason for the problem. Since certain drugs and feeds can also set off allergic reactions that include symptoms similar to atopic dermatitis, it is important to determine the accurate cause of the irritation. Along with a comprehensive clinical examination and environmental assessment, information should be gathered on the animal that details patient history and includes:

• Change in diet
• Change in activities
• Contact with other animals
• Exposure to skin irritants
• Environmental influences, such as seasonal and climate changes

Testing

Two types of tests commonly used for allergen identification include:

• Intradermal Testing (IDT)
• Provides information for diagnoses of skin and food allergies
• Requires patient preparation, may require transport to clinic and sedation
• Subcutaneous injections of specific allergens in neck of horse followed by grading of the animal’s response
• Previous use of steroids and antihistamines can alter results
• Existing skin conditions can cause interference with procedure
• Has been known to produce false positives or false negative results
• Serum Allergy Testing (SAT)
• Measures for elevated antibody levels in the blood to specific allergens
• Requires small amount of patient preparation
• Can produce occasional false positive results
• Not influenced by existing medications
• Indicates presence of antibody, does not prove allergy

Treatment

Once the allergen is identified, the next step to consider is how to eliminate it from the horse’s environment. If elimination is not practical or feasible, desensitizing injections may be considered. These injections offer immunotherapy treatment that gradually builds the horse’s immunity through administered injections containing increasing amounts of the allergens that are causing the hypersensitivity reaction.

Sources:
http://www.greendale.co.uk/pdf/allergy_immunotherapy_testing.pdf
http://www.thehorse.com/articles/36122/atopic-dermatitis-in-horses
http://www.thehorse.com/articles/28967/allergic-dermatitis-in-horses-a-review