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28th September 2009

Testing for Allergic Reactions

Allergic diseases in horses most commonly affect the skin and the respiratory system. They are often treated symptomatically with corticosteroids or antihistamines. Nut if the allergen (the substance to which the horse is allergic) can be identified, it may be possible to develop a specific treatment.

Perhaps the horse could be prevented from coming in contact with the allergen. Or a vaccine could be produced to try and stimulate a more normal response.

Two methods are commonly used to try and identify the offending allergen. Each has its advantages and disadvantages.

The intra-dermal skin test is the “gold standard” for investigating hypersensitivity reactions. Small amounts of antigen are injected into the skin and the reaction is measured at intervals from 30 minutes to 48 hours. Swelling at the site of injection indicates a positive result. The time at which the reaction is first noticed gives an indication of the types of hypersensitivity that is involved.

Reactions first seen after 30-60 minutes represent immediate type 1 allergic reactions involving IgE antibodies, while appearing after 4-10 hours probably involve IgG antibodies. Reactions occurring 24-48 hours involve cell-mediated (delayed type) hypersensitivity.

The intra-dermal skin test is time-consuming and requires a supply of the test antigens. It tends to be a blood sample. The analysis is carried out in the laboratory.

Another option is to measure the allergen specific IgE antibodies in the blood. This simply requires a blood sample. The analysis is carried out in the laboratory.

Owners are often reluctant to take samples for testing for allergens when the problem first arises because of the cost. So horses may be treated with corticosteroids or antihistamines before further investigation is carried out. But does that drug treatment interfere with diagnostic tests than are performed?

A Study carried out at the Michigan State University College of Veterinary Medicine looked at the effect of corticosteroid (dexamethasone) and anti-histamine (hydroxyzine) on skin and blood tests used to investigate allergies. The results were presented at the Sixth World Congress of Veterinary Dermatology in Hong Kong.

Skin tests and blood samples for IgE were performed on normal horses before the start of drug treatment and after treatment for a week with wither dexamethasone (20mg injected into muscle once daily) or hydroxyzine (500mg orally twice daily.)

The researchers found that the intradermal skin test response was reduced after treatment with dexamethasone for up to 14 days. Hydroxyzine limited the response for only three days.

Neither of the drug treatments had any effect on the serology results at the time. So, although skin test results were influenced by previous treatment, the results of the blood test were not affected.

Some of the horses had positive reactions to some allergens even though they showed no sign of allergic disease. This shows that a positive reaction, either to the skin test or to the blood allergen test, does not in itself indicate that a problem exists. So the test cannot be relied on to differentiate between normal and disease horse. But it can be used to support a diagnosis – or to identify allergens that should be avoided or used to produce a vaccine.

For more details see:

Effects of dexamethasone and hydroxyzine treatment on intradermal testing and allergen-specific IgE serum testing in normal horses.
A Petersen and HC Schott II
Vet Dermatology (2009) 19 (Supp 1) 25