An allergy is a condition in which the body reacts adversely (locally or systemically) to a certain substance (allergen). Allergic reactions can be triggered in horses by many things–environmental allergens such as dust, pollen, and mold; insect bites; substances in feeds; things put on or touched by the horse; or injections. Reactions can be localized in the skin and appear as swelling and redness (sometimes itching) at the site of allergic contact, or show up as hives all over the body. More severe reactions might involve additional body systems such as the respiratory and circulatory systems; these conditions can become life-threatening unless reversed.

Sometimes a reaction's cause is fairly obvious–the horse has just received an injection or medication, a new type of bedding was put into his stall, or the owner started using a new fly spray that caused a skin reaction. At other times, it can be hard to pinpoint the triggering factor.

Causes and Types

Some horses are more sensitive to certain drugs, vaccines, pollens, etc., just as humans are.

Vaccination reactions are fairly common in horses. Some of these reactions are allergic and some are merely local irritation (heat and swelling at the injection site) that disappears within a day or two. New vaccines are extensively tested to make sure the vaccine is effective in preventing disease and to make sure horses don't react adversely to the antigen or carrier. Most vaccines are 99% safe, which means there is a small percentage of horses that could have a reaction, explains David Cross, DVM, PhD, clinical assistant professor at the University of Missouri. A horse with