Last Updated on February 25, 2022 by Allison Price
Atopic dermatitis is an allergic reaction to allergens such as molds, pollens, and barn dust. It is likely to be more common in horses than previously reported. It can occur in either a seasonal or a year-round manner depending on the climate.
There are three main types of pruritus: urticaria (“hives”) with pruritus (itching), or urticaria alone. Pruritus is most common on the trunk and face. The most consistent signs are those that occur during the season. However, year-round symptoms would be consistent with allergies to molds and barn dust. Horses with urticaria secondary to atopic skin disease could have a family problem.
Atopic dermatitis can be diagnosed based on clinical signs, exclusion of any other diagnoses, especially Culicoides [‘no-see ums] allergy]. Secondary bacterial skin infections (pyoderma) can occur in horses. This is characterized by excessive scale, especially in the form of a circle (epidermal neckties).
Atopic Dermatitis and USEF Drugs & Medications Rules
The United States Equestrian Federation (USEF), Equine Drugs and Medications Rules prohibits the use of most commonly prescribed medications for horses suffering from atopic dermatitis.
Stephen D. White DVM, Dipl. These usually include corticosteroids and antihistamines. Equine Drugs and Medications Rules, in addition to limiting prohibited substances’ use, allow legitimate use with proper documentation and withdrawal. The only conditions for therapeutic administrations are that the medication is used to treat an injury or illness. Horses must also be removed from competition within 24 hours of administration.
A medication report must also be filed with the show steward, technical delegate or in the competition office within an hour of the administration. You can also file medication reports online.
Dexamethasone is a corticosteroid that is not restricted in quantity. A medication report form is not required. To avoid excessive drug levels in horses during competition, there are specific recommendations. These guidelines can be found online.
Hyposensitization for atopic dermatitis is allowed under USEF rules, provided that no injections are made within 12 hours of competing. No medication report forms are required. For further information regarding the use of medications while remaining compliant with the Equine Drugs and Medications rules, please visit https://www.usef.org/_IFrames/Drugs/Default.aspx or call 800/633-2472.
–Stephen Schumacher DVM, Chief Administrator of the USEF Equine Drugs and Medications Program
To identify allergens that could be hyposensitized (allergy shots or oral drops), intradermal or serum allergy tests can both be done. Horses suffering from atopic dermatitis or recurrent hives have a higher rate of positive reactions than horses in healthy condition. However, the IDT and serologic testing cannot be used to diagnose the problem. All testing should be done in conjunction with the owner’s history to determine if hyposensitization is possible.
Medical therapy may include corticosteroids. Prednisolone and dexamethasone, the most common, are used. Prednisone is more easily absorbed orally by horses than prednisone. Although the injectable dexamethasone can be administered orally, the bioavailability of the solution is only 60%-70% that of the injectable.
Antihistamines–including hydroxyzine pamoate, cetirizine, or doxepin (a tricyclic anti-depressant with antihistaminic effects)–can be quite effective. Although antihistamines are rarely associated with drowsiness and excitability in horses (although the author has observed these adverse effects in less that 5% of horses who have received them), It has been demonstrated that the commonly used antihistamine, pyrilamine maleate, is not bioavailable when administered orally to horses. However, it can be detected in urine for up to one week following a single dose. This could pose problems with drug withdrawal times. As an adjunctive treatment, some clinicians have seen improvement in horses when essential fatty-acid products are added to their feed.
Hyposensitization has been successful in managing atopic dermatitis. Horses have shown improvement as soon as two months after treatment. Hyposensitization must be continued for 12 months before a final evaluation can be done. Veterinarians should communicate with clients to keep them informed about the progress and encourage owners to continue the injections for the full one year. Although it is common to assume that all patients in domestic animals will require the injections for their entire lives, horses may be different. A small percentage of horses have experienced remissions after the injections were stopped. Hyposensitization is generally effective in reducing the incidence of atopic horses.