A study at the Michigan State University College of Veterinary Medicine found that the use of corticosteroid and antihistamine treatments for allergies had no effect on blood test results for an antibody commonly associated with allergic reactions.
However, the results of intra-dermal skin tests were affected by the use of the drugs.
Skin tests and a blood sample for Immunoglobulin E (IgE) were performed on normal horses before the start of drug treatment and after treatment for a week with either dexamethasone (20mg injected into muscle once daily) or hydroxyzine (500mg orally twice daily.)
IgE is a class of antibody found only in mammals which plays an important role in allergies, and is especially associated with hypersensitivity.
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 treatment had any effect on the blood test results at any time.
Allergic diseases in horses most commonly affect the skin and the respiratory system. They are often treated symptomatically with corticosteroids or antihistamines. But if the allergen - the substance to which the horse is allergic - can be identified, it may be possible to develop a specific treatment.
The two methods commonly used to try to identify the offending allergen each have their 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 type of hypersensitivity that is involved.
Reactions first seen after 30-60 minutes represent immediate type 1 allergic reactions involving IgE antibodies, while those appearing after 4-10 hours probably involve IgG antibodies. Reactions occurring after 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 confined to referral institutions, and is not widely available in general veterinary practice.
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.
The findings show that treatment with corticosteroids or antihistamines did not affect IgE test results on blood, but did on the "gold standard" intra-dermal test.