Equine Environmental Allergy Testing

Allergy tests can help veterinarians identify possible allergens for inclusion in a horse’s immunotherapy program.
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testing horse for allergies
Allergy testing can be performed via blood test or intradermal skin test. | iStock

Horses develop a number of conditions attributed to allergies, but two of the more common are pruritus (itching), which might manifest as rubbing or biting, and recurrent hives. Although biting insects account for many of these allergies, they can also be caused by sensitivities to environmental allergens such as grass, trees, weeds, and mites. Environmental allergens should be considered when clinical signs are seasonal and persistent, despite elimination of common causes (e.g., biting flies).

If an environmental allergy is suspected, allergy testing can be a helpful tool to identify the cause. Allergy testing can be performed via two different methods. The first is known as 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 a simple blood collection is all that is needed; sedation is not required, and 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 and multiple injections of different known allergens to be administered under the skin and monitored for allergic reactions. This form of testing offers several advantages over blood testing. Specifically, it can test for more allergens, reactions are detected in the skin where the problem is manifesting, and the results are immediately available, unlike blood results that might take a week to receive.

It is important to remember there are several major limitations regardless of the test performed. First, neither method definitively confirms nor diagnoses a patient with an environmental allergy, because both methods can have false positive and false negative test results. The results should be considered as a guide of potential causes but must be interpreted 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, because most 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).

The main reason for performing 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 educating and training owners on how to administer injections and identify adverse reactions and therapeutic responses.

Editor’s note: This is an excerpt from Equine Disease Quarterly, Vol. 31, Issue 4, funded by underwriters at Lloyd’s, London, brokers, and their Kentucky agents. It was written by Darren Berger, DVM, Dipl. ACVD, of Iowa State University.

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