A Hivey Horse

Q: I have a 5-year-old Quarter Horse mare that I've raised since she was a foal. Over the past few months she has suddenly started developing welts. She would have welts everywhere and have huge areas of edema (fluid swelling) under her belly and on her legs. We couldn't figure out what was causing the reactions. She has been on the same bedding and feed her whole life, which is a mixture of alfalfa, Bermuda, and timothy hay. She has also been at the same location since she was born. Our veterinarian administered a corticosteroid shot--she would clear up for a day or two and then be covered in welts again.

We finally opted to have blood work done. It came back reading that she was highly allergic (positive) to Bermuda hay, oats, several weeds, etc., and that she was borderline allergic to alfalfa and timothy hay. We are limited to those three different kinds of hay here in Arizona. I asked my vet what I was supposed to feed my mare, being that she is positive and borderline for the types of hay we have out here, and he recommended alfalfa pellets soaked with water. While this has helped some, the hives haven't gone away completely.

Is it possible for a horse of this age to all of a sudden develop allergies? She has been eating the same feed her whole life. I am desperate to figure out a better way to handle this.

Sarah, via e-mail

A:The history and clinical signs of your mare suggest that she has urticaria, commonly known as hives, and possibly even angioedema. The latter are large swellings, which most commonly involve the muzzle, lower limbs, underbelly, and sometimes eyelids, and they can leak serum or sometimes bleed. These swellings do not usually occur by themselves, but are associated with urticaria. Hives, which may or may not be itchy, have many potential causes including reactions to drugs, vaccines, feedstuffs, plants, insect bites, various infections, as well as contact sensitivity and aeroallergens (airborne substances such as pollen that cause an allergic response). Hives have also been associated with stress, exercise, and cold weather. Many cases remain idiopathic (no cause can be identified).

It is not unusual for a mature horse to suddenly develop hives. Without clear association with a particular etiologic agent or predisposing situation, the cause can be difficult to identify. It can be very helpful for an owner to maintain a journal to ascertain whether the hives can be associated with any weather, environment, feed, or bedding source changes, exercise pattern, a particular activity, or even the estrous cycle. Blood testing is generally not thought to be a valid test for identifying allergies in horses. To my knowledge, there have been no published reports, at least in English, that blood (serum) test results correlate with what is causing clinical allergic disease in horses. Furthermore, horses can have blood test reactions that have no clinical relevance.

Veterinarians often use intradermal skin testing, in which an allergen is injected into the skin, to try to identify potential causes, but a positive skin test reaction does not mean that particular allergen/antigen is the cause of the horse's problems, and false negative reactions can also occur. Veterinarians must interpret reactions to intradermal skin testing in conjunction with the clinical history. In some cases veterinarians combine intradermal skin test reactions with clinical history to formulate a hyposensitization program (e.g., allergy shots) for horses with allergic skin disease.

The most accurate way to identify the cause of your horse's hives is to attempt to identify potential antigens or causes within the horse's environment. This is best done by eliminating what could be potential antigens, observing whether the hives disappear, and then challenging the horse with the previously removed antigen to see whether the hives recur. As hives can wax and wane, their disappearance when a particular antigen is removed from the horse's environment does not prove that antigen is the causative factor. You would need to test that antigen both by elimination and by a challenge test with the horse. The only way you can really tell if your horse has a food allergy is to systematically eliminate each feedstuff and see if it makes a difference. Again, you will need to challenge the horse by reintroducing the suspected culprit to see if the hives reappear.

A pathologist who specializes in skin pathology may help better define the horse's skin problem and potentially help with management by performing microscopic evaluation of a skin biopsy.

Despite the best attempts, some people still are not able to detect the cause of a horse's skin disease. If you exhaust all means of pinpointing the cause of your horse's hives, you and your veterinarian will then have to concentrate on establishing a treatment plan that is most effective in making your horse comfortable with the fewest undesirable side effects.

About the Author

Jill Beech, VMD, Dipl. ACVIM

Jill Beech, VMD, Dipl. ACVIM, is a Professor of Medicine and Reproduction in the Department of Clinical Studies at the University of Pennsylvania's New Bolton Center.
