Author: Dr. Stephen D. White
School of Veterinary Medicine University of California, Davis, California USA

The allergic skin diseases in the horse that are non-arthropod bite related are atopic dermatitis (atopy), food allergy, and contact allergy. The history of the dermatitis is very important in determining which of these three is most likely in any particular horse. A seasonal pruritus, especially affecting the face and trunk, would be most consistent with atopic dermatitis to pollens; year-round pruritus would be more consistent with an atopic dermatitis as a reaction to molds or barn dust, or a food allergy. Episodes of pruritus that occur after topical treatments of shampoos, dips, etc. would be consistent with a contact allergy. The author finds true food allergy very rare; this may be a reflection of practicing in a primarily referral practice, whereas cases of food allergy (real or presumed) are often diagnosed by the owner and/or the local veterinarian.

Atopic Dermatitis

Atopic dermatitis in horses is becoming increasingly recognized as an important cause of pruritus. The disease may be seasonal or non-seasonal, depending on the allergen(s) involved. Age, breed, and sex predilections have not been extensively reported. In preliminary work at the School of Veterinary Medicine, University of California, Davis (SVM-UCD), the median age at onset was 6.5 years, Thoroughbreds were the most common breed, accounting for 25% of the horses, and males (usually geldings) were almost twice as represented as mares; however, this data is from only a small number of horses, and has not yet been compared to the hospital population at large.

Pruritus, often directed against the face, distal legs or trunk, is the most common clinical sign. Alopecia, erythema, urticaria and papules may all be present . Urticarial lesions may be quite severe, yet non-pruritc. There may be a familial predisposition for urticarial atopic dermatitis in the horse.1 Horses may have a secondary pyoderma, typified by excess scaling, small epidermal collarettes, or encrusted papules (‘miliary dermatitis’). Diagnosis of atopic dermatitis is based on clinical signs and the exclusion of other diagnoses (especially parasite [Culicoides] allergy).

Intradermal tests (IDT) or serum allergy tests may be performed to identify allergens which may be used for hyposensitization. A series of excellent articles investigating IDT in horses with atopic dermatitis, recurrent urticaria, chronic obstructive pulmonary disease (COPD) and healthy horses as controls, have been published from research performed at The Ohio State University.2-5 The interpretive conclusion is that horses with these diseases generally have a higher incidence of positive reactions than healthy horses, but that the diagnosis (as in other species) can not be solely made on the basis of the IDT or serologic test alone; rather, these tests should be interpreted in light of the history of the disease (i.e., a horse with seasonal signs is more likely to have an allergic response to allergens it is exposed to seasonally [pollens in the summer, barn dust in the winter, etc.]). This interpretation thus will increase the clinician’s ability to determine which allergens might be relevant in hyposensitization, should the owners elect to choose that route of treatment.

In a study from France of presumed atopic horses, the most common positive allergens were to Culicoides sp. and the house dust mites.6 A report from Austria compared IDT results between 38 normal horses and 43 horses with ‘summer seasonal recurrent dermatitis’ using 22 allergens (pollens, molds, mites and insects, including Culicoides variipennis). Differences between the two groups of horses were found only in the reactions to deerfly, horsefly, and C variipennis; in the latter differences were only noted at 1:10,000 w/v dilution, and the normal horses had more reactions!7 This may be supportive of another recent report that suggested some arthropod allergens may be being used at either too high or too low a concentration, leading to potential false positive or false negative results.8

What of the blood tests (RAST or ELISA) which are available to diagnose equine atopic dermatitis? In one study, none of the three serologic allergy tests investigated reliably detected allergen hypersensitivity, compared with the IDT; among the three serologic tests, an Fc-epsilon-RIalpha-based ELISA performed significantly better overall than the others.2 In a more recent study, using a monoclonal antibody specific for equine IgE, an ELISA using whole serum and crude allergen preparations only agreed with skin testing for two allergens.8a However, the author has certainly used serologic tests as the basis for determining the allergens to be used in the hyposensitization solution in cases where the owner did not want the horse shaved for the IDT or the horse was recieving antihistamines. Preferentially, an IDT is done at the SVM-UCD on horses with atopic dermatitis with owners who are interested in pursuing hyposensitization. It should be remembered that in regards to food allergy, it is likely that neither serologic tests nor the IDT have any relation to reality.

Corticosteroid treatment is usually effective in the control of pruitus or urticaria resulting from atopic dermatitis. The usual oral medication used is prednisolone, (200-400 mg /500kg q24h). Prednisolone seems to be better absorbed orally in horses than prednisone9. Dexamethasone (0.05-0.1 mg/kg, q 24h) may also be used. The injectable dexamethasone solution may be used orally, although the bioavailability is 60-70% of the injectable route.

Certainly, corticosteroids in horses may cause various adverse effects, including steroid hepatopathy, laminitis, and iatrogenic hyperadrenocorticism.10,11 Therefore, other modalities of treatment may be used, such as the antihistamines hydroxyzine pamoate (200-400 mg /500kg q12h) or cetirizine (0.2 mg/kg q12 h),12 doxepin [a tricyclic anti-depressant with antihistaminic effects] (300-600mg/500kg q12h), or diethylcarbamazine syrup (6-12 mg/kg q24h). Both hydroxyzine and doxepin may cause either drowsiness or nervousness, although these adverse effects are rare. Cetirizine is very expensive, and as it is the active metabolite of hydroxyzine, if the latter is ineffective in an individual horse, the cetirizine probably will be as well. Some clinicians have noted improvement when an essential fatty acid product is added to the feed; Platinum Performance™ (Platinum Performance, Inc.; Buellton, California,USA) is one brand name which has been used successfully in some atopic horses as an adjunctive treatment (W. Rosenkrantz, personal communication, 2004).

Urticaria, secondary to atopy, has been reported as successfully managed with hyposensitization, with horses showing improvement as early as 2 months into treatment.1 In general, hyposensitization for any manifestation of atopic dermatitis in the horse should be evaluated for efficacy for at least a 12 month period. The veterinarian should maintain consistent communication with the client to monitor the progress of the treatment and to encourage the owner to continue with the injections for the full year. If hyposensitization is successful, it is thought that as in other domestic species, most horses will need to be maintained on the injections for life, albeit sometimes at a reduced frequency (1-3 times monthly). Approximately 80% of the owners of atopic horses at SVM-UCD that have had IDT or serologic testing have elected to try hyposensitization. In general, the author finds approximately 70% of atopic horses improve with hyposensitization; other researchers have reported even better results, albeit in a non-controlled report.13