Q. My horse gets hives every spring and summer. He is on night turnout, and they’re usually worse when I bring him in the barn in the morning. I’ve checked his paddock for harmful plants, and I haven’t found anything concerning. What should I be looking for that might be causing his hives?

A. The challenge with hives is while they are considered one of the more straightforward skin conditions to diagnose, figuring out their exact etiology (cause) is what tends to frustrate both horse owners and veterinarians. Acute, self-resolving episodes of hives can quickly turn into a chronic, recurring ailment over the course of several weeks.

A variety of things can cause hives, including medications (such as antibiotics and anthelmintic drugs, or dewormers), supplements, seasonal plants, molds, and insects. In your horse’s case it is obvious the hives are environmentally induced. I remind owners that even the most well-maintained properties are never free of allergens—they can exist in the soil or even be distributed through the air from miles away.

Another factor I think has the most influence on hives development is the season. Whether it is coincidental or not, hives and most other diseases involving the skin tend to be more common during the warmer and more humid months. Increased moisture from rain and humidity and even from sweat can contribute to making the initial hives reaction worse, even after direct contact with the initial allergen has passed.

Figuring out the underlying cause for allergies in horses at any age can be challenging. I have had patients that have never had allergies previously but present with acute and sometimes severe clinical signs (hives, respiratory distress, and hair loss). The first step is to identify when your horse starts showing signs. Is it seasonal (spring/summer), weather-related (humid or cold and wet), or associated with a change in the horse’s environment (stall bedding versus pasture weeds/environmental allergens)? Ideally, this will help you find the trigger for your horse’s allergies. Hives and allergies are not typically age- or gender-specific.

Most of the time with environmental-allergy-induced hives, you cannot remove the horse completely from the source (i.e., pasture, stall, or bedding) and empirical (based on observation and experience) treatment is the easiest option to provide your horse relief from the clinical signs. You can also use medicated shampoo containing steroids, but bear in mind the effect of the steroid wears off once you rinse the shampoo from your horse’s body.

Systemic anti-inflammatory therapy can provide a quick resolution but often requires repeated dosing. This can include intravenous or oral non-steroidal anti-inflammatory drugs (NSAIDs, such as Banamine), corticosteroids (dexamethasone), antihistamines (cetirizine and diphenhydramine), or a combination of all three. Your horse might need these medications long-term to prevent recurrence of the hives, but they are not without inherent risks or side effects. Talk to your veterinarian to determine if these medications are safe for your horse.

Take-Home Message

Several factors, including medications, supplements, plants, mold, and insects, can trigger hives, with a warm and wet climate often making them worse. Work closely with your veterinarian to develop a management and treatment plan that is safe for your horse. Oftentimes, a change to a cooler season (i.e., summer to fall) is associated with an overall reduction in environmental allergens and complete resolution of hives.