What are hives?

Hives are the result of an allergic reaction, also known as a hypersensitivity reaction of the immune system. Welts appear throughout the skin.  They occur most commonly on the neck and shoulders, but can be found all over the body, including the legs and head.  Hives can be pea-sized to quarter-sized or larger.  These swellings result when capillaries leak fluid from the blood into the tissue space under the skin.

Hives are usually not itchy or painful.  In some cases, mild itching is observed.  A cold water bath can help to soothe the skin.

Hives can appear in as little as 15 minutes to 24 hours following exposure to an allergen.  Fortunately, they usually resolve quickly. 

What causes hives?

There is a long list of triggers for the allergic reaction that can cause hives in a horse.  Most common causes include biting insects, pasture plants, sprays and soaps.  Sometimes hives are the result of drug reactions caused by antibiotics, vaccines, or dewormers.  Other factors such as cold, heat, or stress can intensify a reaction. 

When are hives an emergency?

In rare cases, severe hives can affect a horse’s ability to breathe.  Whenever hives are seen, it is important to determine if there are any hives on the head and if the horse is having any difficulty breathing.  Call a veterinarian immediately if this is observed. 

How are hives treated?

The best long-term treatment for hives is to eliminate the source of the problem.  Unfortunately, this is rarely accomplished because it is difficult to determine what triggered the reaction.  Any recent changes in pasture, supplements, medications, bedding, or fly spray are important to consider. 

Most hives resolve within 24-48 hours without treatment.  Steroids will help hives disappear more quickly but are usually not necessary, especially if the horse appears comfortable. 

Persistent recurrent cases of hives require additional attention.  The first step should be to rule out an insect allergy by implementing all fly control measures possible: fly spray twice daily, turn out in a fly sheet, and keep inside during morning and evening when insects are at their peak.  Some horses may require a long course of antihistamines or steroids during the summer months in order to be free of hives. 

Allergy testing with skin injections can be performed by a veterinarian to search for specific allergens.  If these are able to be identified, the horse can then benefit from immunotherapy injections.  This involves a custom made allergy vaccine, which helps the horse’s immune system to become more tolerant to the allergen over time.