You can’t see into the horse’s immune system—no computer to check or parts to oil—but hives are an allergic skin reaction that let you know something is amiss.
This immune-system response arrives as sometimes-itchy raised bumps under a horse’s skin. It can wreak havoc on a horse in one stall and not even phase his pasturemate across the aisle. With multiple horses under your care, you know that every horse is different—and that’s why an irritant that’s reactive in one horse might not affect any other horse in the barn. Meanwhile, the horse that is affected can quickly advance from a seemingly innocent case of hives into other immune system responses, such as founder, anaphylaxis, and liver failure. Horses with hives need to be monitored closely and, because their immune system is stressed, should not be ridden or hauled needlessly.
Causes of hives include something inhaled, ingested, or contacted directly—pretty much anything and everything in a horse’s environment is suspect. This wide range of immune-system irritants is what makes determining the cause of a particular case of hives so difficult. But you can start with the usual suspects: “Diet, insect bites, and drugs are the main causes,” says Amy Long, DVM, of Jackman’s Animal Clinic in Milroy, Indiana. Stress can also be a factor. “Especially if a horse is stressed, like at a show, his immune system will be more reactive,” and he’ll be more prone to hives, says Mary Brennan, DVM, of Alternative Veterinary Services in Blairsville, Georgia.
Inhaled or ingested, mold spores in hay can trigger an allergic response. This type of hives reaction is most common in late winter and early spring. Mold spores lay dormant during the cold months, but as things heat up—especially in barns with poor ventilation or among tightly packed bales—they start to proliferate.
In terms of hives caused by contact with an irritant, barns that use bulk wood shavings as bedding might see an increased risk, as the wood making up those shavings probably varies between loads. Pine sap and cypress wood are most likely to cause a horse to react. Black walnut, too, will first make a horse break out in hives and may turn to founder. Finding the cause of hives can take much sleuthing. If the cause is a food allergy, the hives usually show up around the rib cage first. If the cause is from something the horse came in contact with, they’re often large, even appearing to be bands around the body. Aside from these scant visual clues, consider what new elements have been introduced to the horse’s environment. Perhaps he’s been turned out in a different pasture, you’ve changed fly sprays, or the owner is trying a new supplement.
“You can use trial-and-error to try and sort out the culprit. To do this, introduce potential causes of the hives one at a time to see what causes the reaction,” suggests Dr. Long. She warns that if the case of hives was severe or accompanied by other symptoms, you should not use this trial-and-error method. “Another way is to have an intradermal skin test performed by a veterinarian. This is the same procedure that is used on humans to detect allergies,” Dr. Long continues. Dr. Brennan, author of “Complete Holistic Care and Healing for Horses,” cautions that a horse with hives already has an over-reactive immune system, so the results of an allergy test may show sensitivity to more than just the offending agent.
Many equine professionals view hives as “just a skin condition.” That can be a mistake. “Hives can be life threatening,” says Dr. Brennan. “We don’t like to think about that, and most people haven’t seen it. I would call a vet and let him know right away. Don’t ever ignore hives.”
While a veterinarian might encourage you to treat the hives on your own, he or she should be notified in case the condition escalates. “When there is no improvement after 24 to 48 hours, if the horse begins acting unusual—like not eating or drinking—if the horse has a fever, has trouble breathing, is breathing very hard, or if the condition worsens, a vet needs to be there,” says Dr. Long.
As far as self-treatment, Dr. Brennan encourages all horse owners to keep an antihistamine such as Benadryl on hand. One entire bottle of children’s Benadryl, given orally, will likely get the situation under control; it will at least tone down a serious situation until the vet arrives. “You never know how long it might take for a vet to get to you,” she stresses.
While she promotes holistic medicine, she also realizes that conventional, chemical treatments have their place in preventing true emergencies. Barn managers often keep dexamethasone powder to administer to horses with hives. While this is useful, one side-effect is founder, which can occur with over-dosing. A veterinarian may treat the horse with an intramuscular shot of a corticosteroid such as prednisone or dexamethasone. You may also get a prescription for an antihistamine like Tri-Hist to put on the horse’s feed if this is a recurring problem. Dr. Brennan also likes to boost the horse’s immune system, as hives is a symptom of a larger immune-system issue. Her favorite product is Scifeed Equine’s Immubiotic+. Once the situation is under control, it’s important to monitor the horse for any further reactions or complications. While hives may seem innocent enough, they’re really a signal that the immune system has something brewing, something that bears watching.