Dr. Domenico Santoro is a dermatology resident at the University of Illinois Veterinary Teaching Hospital in Urbana. He explains that urticaria, the medical term for hives, "are flat 'bumps' that can arise very quickly on the body of a horse." They are sometimes itchy, and other times not. In some cases the lumps, which can appear on any part of the body, may also ooze a yellow-tinged fluid.
Many different medications, such as antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs), or topical agents can be the culprit. Other causes range anywhere from a rapid change in temperature, stress, or an autoimmune disease. Dr. Santoro also notes that allergies can play a role as well. He says, "a horse could be having an allergic reaction to food, flies, mosquitoes, or something in its environment." Of course, just as in humans, horses can actually have a seasonal reaction to pollen or molds.
Fortunately, the majority of cases involving hives are not emergencies, but you should call your veterinarian right away. Dr. Santoro also notes that, "if owners notice hives after application of a specific topical product, they should wash the horse immediately." Bathing with shampoo is also a good treatment if the cause is a contact allergy to something in the environment.
Just as some people are highly allergic to certain items (peanut butter or shellfish, for example) and carry an "epi-pen" for emergencies, some horses may also have life threatening allergic reactions which require immediate care. This is why it is critical to call your veterinarian as soon as you notice hives.
In these rare cases, what is of utmost concern is the horse's airway. As you notice bumps on the skin starting to swell, the same type of reaction could be going on in the animal's windpipe, causing difficulty breathing.
With any luck, hives will disappear on their own with no serious complications. But if not, there are several tools your veterinarian can use--though it is important to note that because there are several causes of hives, treatment is highly individualized. For example, "for allergic reactions, the use of antihistamines can be effective to differentiate allergies from possible drug reactions or autoimmune disorders," says Dr. Santoro. Corticosteroids are also used in difficult cases.
Because hives may recur if the cause is not removed from the horse's environment, it is important to find out what the horse is reacting to. In many cases, an inciting agent will not be found. But in recurrent situations, further diagnostics such as a skin biopsy or allergy testing (similar to what is done in human medicine) can be performed by a specialist.
For more information on hives, contact your local veterinarian.