Several tests can be performed if a clinician suspects a horse has atopic dermatitis. Skin biopsy shows a superficial-to-deep perivascular dermatitis with eosinophilia. This type of reaction pattern is not specific for atopy and may be seen with other types of allergies.
Other diagnostic tests, including an intradermal allergy test or a serum allergy test, can help to identify potential offending allergens but do not, however, diagnose atopy. This diagnosis must be based on a compatible history, clinical signs, and elimination of other differential diagnoses. Of the two tests, the intradermal allergy test appears to be the more sensitive and better test to identify potential sensitizing allergens in a horse. However, this test is not without its limitations, including false-positive and false-negative results.
Intradermal Allergy Test
Most clinicians sedate horses with xylazine (0.05 mg/kg IV) when they perform intradermal allergy testing. A rectangular area (30 x 15 cm) on the lateral aspect of the neck is clipped with a number 40 blade. Dots spaced approximately 2 to 5 cm apart in permanent marker identify the locations where allergens will be injected. A volume of either 0.5 or 1.0 ml of the diluted aqueous allergen is injected intradermally. Although this author prefers a volume of 1.0 ml, the exact volume does not matter as long as it is the same volume of allergen at each site. Positive and negative controls are injected at the beginning and the end of the skin test. The positive control consists of a commercially prepared solution of histamine diluted to a concentration of 1:100,000 weight to volume. Saline (0.9% NaCl solution) or sterile water serves as the negative control. The reactions are read at set time intervals; the author prefers 15 minutes, 30 minutes, 4 to 6 hours, and 24 hours after allergen injection. The reactions are graded by comparison with the controls; a 4+ reaction is similar to the positive control, whereas a 0 reaction is similar to the negative control. Size, shape, turgidity, and presence of erythema are the subjective determinations used to evaluate skin test reactions. Reactions that are greater than or equal to a 2+ reaction are considered potentially significant. Allergens that produce positive reactions at more than one reading period are probably more significant than allergens that react slightly at only one time.
The history and clinical signs should be correlated with the results of the skin tests. Certain allergens (alfalfa, corn, cornsmut, grain mill dust, grain smut, black ant, mosquitoes, Culicoides organisms, fireant, Rhizopus organisms, Penicillium organisms, sheep wool epithelium, English plantain, red mulberry, black willow, mesquite, and dock sorrel) tend to cause false-positive reactions because they may be irritating when injected intradermally. In these cases a dilution of less than 1:1000 weight to volume or several injections at several different dilutions may need to be performed. Companies that sell allergens are familiar with — and can help the veterinarian to determine — the appropriate dilutions for skin testing. These companies can also help provide a source to obtain information on pollination times for the various allergens.
False-positive reactions have been reported in non-atopic horses with chronic laminitis and musculoskeletal disease. These horses may have hypersensitive immune systems and thus tend to react more to allergens. Normally these positive reactions occur in response to the irritating allergens. If a positive reaction does not correlate well with the clinical history and physical examination, then it probably is not clinically significant.
Some drugs, especially antihistamines and corticosteroids, can interfere with skin test results. Drugs such as acepromazine that affect vasodilation can also affect the test results. If these types of drugs are not withdrawn for an appropriate length of time, false-negative skin test results will occur. This problem is usually obvious by the fact the histamine positive control does not react normally. Usually 0.1 ml of histamine creates a wheal 10 to 15 mm in diameter. Drug withdrawal times for horses should be similar to those used in small animals. Long-acting injectable steroids should not be administered for 3 months before skin testing. Oral steroids or injectable dexamethasone should not be given for at least 1 month before skin testing. Antihistamines should not be administered for a 7- to 10-day period before skin testing. If the horse’s clinical signs are severe, this drug withdrawal period can be difficult to implement, in which case it may be more advisable to skin-test at the end of allergy season. Even so, some owners refuse to take their horses off medications, and others also object to sedation and clipping of their horses. In these cases, intradermal allergy testing is not an option.
Most concentrated solutions of allergens are useable for 6 to 12 months. Prediluted allergen solutions have a shelf life of approximately 1 month. Although purchasing the concentrated solutions and making the dilutions oneself is more cost-effective, the cost of concentrated allergen solutions for most regional allergy screens is several thousand dollars. The shelf life and cost usually make intradermal skin testing cost-prohibitive for the general practitioner. For this reason and because the results of skin testing are not always easy to interpret, a board-certified veterinary dermatologist or a veterinarian who performs multiple skin tests per week should perform the skin tests.
Serum Allergy Testing
Controversy exists regarding the usefulness of the serum allergy tests in horses. Lack of repeatability of test results and sensitivity of serum allergy blood tests are problems. In a recent study, three different serum allergy tests were compared with intradermal allergy test results. These tests were an enzyme-linked immunosorbent assay (ELISA) that uses polyclonal antiequine IgE, a radioimmunosorbent assay (RIA) in which the sample was treated to minimize nonspecific IgE binding, and an ELISA test that uses the Fee receptor immunoglobulin E chain for IgE. None of the three serum allergy tests reliably detected allergen hyper-sensitivity in comparison with intradermal test results.
Despite the questionable efficacy of serum allergy testing in horses, anecdotal reports that some horses are benefited by hyposensitization based on the serum allergy blood tests exist. More studies need to be performed to adequately evaluate this situation.