Currently there are two types of allergy testing, blood testing and skin testing. Recent research has shown that both types of testing have led to creation of successful injection regimens. However, the skin testing has long been thought to be superior to blood-based testing.
Intradermal skin testing is considered an imperfect but helpful method for identifying substances that cause allergies. It tends to be a last resort after many other therapies have been tried. It is more useful for determining inhaled and contact allergens, as opposed to food allergens.
In this test, the horse is usually sedated and the skin on one side of the neck is clipped. Several dozen potential allergens are injected into the skin in a large grid pattern. A key tracks which antigen has been injected in what location. A positive response is the increased size of the wheal. Some allergic reactions are immediate, while some are delayed. For this reason, horses are monitored over a 24 hour period and the reactions charted over that time.
Reactions may be subtle and hard to read for a vet who does not perform the test frequently. For this reason, this diagnostic is usually performed by a board certified veterinary dermatologist who is trained in the performance and interpretation of this test.
The skin test is typically read at 15 minutes, 30 minutes, four to six hours, and 24 hours. Some horses have an immediate reaction only, others have a delayed reaction only, and some have both immediate and delayed reactions. The skin test is thought to be more sensitive than a blood test.
Reasons to UseRelated Observations
This test gives the most reliable information available regarding the allergy-causing substances. This information can be used to include the correct antigens for an allergy shot, which desensitizes the horse to that substance over time.
It also can provide information as to what substances to try to avoid.
Care must be taken in the interpretation of the results. Allergens vary greatly geographically and the antigens supplied for intradermal testing are usually commercially supplied. There may be a disconnect between the allergens a horse is reacting to and those supplied in the antigen kit. The test tends to work poorly for detecting feed allergies.
The horse must stop oral medication prior to testing. The horse cannot receive steroids for two weeks prior to testing, and cannot receive antihistamines at least one week prior to testing.
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