by Dr. Amanda Wilson
Lumps, bumps, itching, scratching, and everything in between.
It's that time of year again. Your horse comes in from turnout with a sea of hives on his neck and chest. Or rubs his tail raw on the fence. With the heat and humidity of the season also comes skin problems in our equine friends. These can be frustrating for owners and tough to manage as veterinarians.
How do allergens affect my horse?
The skin, particularly the epidermis, is the first layer of defense against a host of invaders. The respiratory tract also encounters environmental allergens. Various cells in the skin and respiratory tract act upon these invaders to present them to the host's (your horse's) immune system.
Common manifestations of allergies in horses include: hives, itching, redness, hair loss, labored breathing, watery eyes, and coughing. Each of these can represent the body's way of saying "Hey you! There's a problem here!" They are signs of the disease, not the disease itself.
Equine insect bite hypersensitivity is the most common and most understood of the equine allergies. The Culicoides midge is the biggest culprit. Most species of Culicoides feed on the dorsal aspect, or topline of horses. Horses with insect bite hypersensitivity will be itchy and have patches of missing hair and crusted lesions on their face, mane, withers, rump, and tail.
Other possible allergens can be plants, molds, chemicals, drugs.
What types of horses are affected?
Horses typically present with the initial complaint between 1 to 10 years of age, however these signs can begin at any age.
What can owners do?
It may be helpful to keep a daily record of your horse's work, feed, any stabling/environmental changes, any supplements or drugs administered, any topical shampoos or liniments, and any blankets/boots/saddle pads, or other tack that the horse came in contact with. There should also be a recording each day of any changes observed in the horse's skin, eyes, nose, and attitude. This record could be helpful to your veterinarian in determining a potential allergen.
What can your veterinarian do?
There are several methods that your veterinarian may use to determine which allergens are causing your horse's signs. It is important to understand that diagnosing allergic disorders is not always straightforward and in some more complicated cases can require patience and frequent communication on the part of you and your vet to determine the cause of your horse's skin condition.
Serum testing involves a simple blood draw and is designed to detect the IgE antibodies that are produced in response to an allergen. The results of the test help to guide therapy in the form of injections in order to desensitize the horse to its allergens. It is important to remember that these injections are customized to the horse but the test is not always 100% specific to your horse's allergens and treatment failure does occur.
Intradermal skin testing involves injecting drawing a grid on the horse, typically on the neck. Your veterinarian will inject different areas on the grid with different allergens and monitor for a reaction on the skin. This can help determine which allergens your horse is sensitive to and guide desensitization therapy similar to the "allergy shots" that people receive. Just as with serum testing, the test is not always specific and treatment failure can occur.
A more conservative approach to treating an allergic horse is a method called "allergen avoidance." If you are able to keep a detailed and accurate log of your horse's activities and allergic responses as described in the section above, you and your veterinarian may be able to use that to determine a potential cause for your horse's allergic reactions. Once a potential allergen is identified, whether it is a certain grass, type of shavings, fly spray, fabric, etc., you can try eliminating it from your horse's environment and see if the allergic reactions improve.
In some cases, your veterinarian may elect to do a biopsy or skin scraping to rule out other possible causes for your horse's signs.
While allergies are not usually life-threatening to horses, they can be irritating and can cause unnecessary stress to both you and your horse. Your veterinarian can be a valuable tool in getting to the root of the problem and implementing strategies to help keep your horse comfortable.
Pascoe's Principles & Practice of Equine Dermatology, 2e. Knottenbelt, DC.
Update on Equine Allergies. Fadok, VA. Veterinary Clinics of North America, Equine. 29 (2013) 541-550.