Equine Allergy Testing

If an environmental allergy is suspected in a horse, equine allergy testing can be a helpful tool to identify the cause.

Darren Berger, DVM, DACVD, Iowa State University

A chestnut horse stands in a field in front of a tree. If an environmental allergy is suspected, equine allergy testing can be a helpful tool to identify the cause.
If an environmental allergy is suspected, equine allergy testing can be a helpful tool to identify the cause.

Horses develop a number of conditions attributed to allergies. Two of the more common are pruritis (itching), which might manifest as rubbing or biting, and recurrent hives. Biting insects account for many of these allergies. However, they can also result from sensitivities to environmental allergens such as grass, trees, weeds and mites. Veterinarians should consider environmental allergens when clinical signs are seasonal and persistent, despite elimination of common causes (e.g., biting flies). If the veterinarian suspects an environmental allergy, equine allergy testing can be a helpful tool to identify the cause.

Equine Allergy Testing Methods

Allergy testing can be performed via two different methods. The first method is serum or in vitro allergy testing. This test is performed by drawing a blood sample and sending it to a laboratory for interpretation. This route of testing offers several advantages in that it only requires a simple blood collection. Sedation is not required. Stoppage of ongoing treatments, such as antihistamines or steroids, is typically not necessary.

The second method is performed via an intradermal skin test. An intradermal skin test requires the horse’s neck to be shaved. The veterinarian then administers multiple injections of different known allergens under the skin and monitors for allergic reactions. This form of testing offers several advantages over blood testing. Specifically, it can test for more allergens. Further, reactions are detected in the skin where the problem is manifesting. Finally, the results are immediately available, unlike blood results that might take a week to receive.

Limitations of Allergy Testing

It is important to remember that there are several major limitations regardless of the equine allergy test performed. First, neither method definitively confirms nor diagnoses a patient with an environmental allergy. This is because both methods can have false positive and false negative test results. The results should be a guide of potential causes. The veterinarian must interpret them in context with the horse’s clinical signs, region where it lives, seasonality, abundancy of the allergen in the environment and severity of the allergenic reaction.

Avoidance to suspected allergens can be impractical and nearly impossible in some circumstances. This is because more pollens or allergens are ubiquitous or can travel great distances in the air. To use an oak allergy as an example, unless you remove every oak tree for 100 miles around, then removal of oak trees from the pasture where the horse is housed is futile. Finally, the testing modality does not determine future treatment success. To date, studies have not shown one method provides better results when used to guide desensitization therapy (allergen-specific immunotherapy or allergy shots).

Allergen-Specific Immunotherapy

The main reason for performing equine allergy tests is to identify possible allergens for inclusion in allergen-specific immunotherapy. These injections are initially given frequently and then spaced over time as the concentration and amount of the allergens is increased. Allergy shots have been shown in numerous studies to be helpful for alleviating allergic conditions in the horse that are triggered by environmental allergens. Most horses that show a response to therapy do so in the first six to 12 months. As previously stated, the route of testing does not impact therapeutic success, but client adherence to treatment protocol is of utmost importance. Given the importance of client participation with allergen-specific immunotherapy, sufficient time should be spent on educating and training owners on how to administer injections and identify adverse reactions and therapeutic responses.

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