One of the most common skin conditions I see in horses is called urticaria. It consists of small, one to five-inch raised lesions on the horse's skin. Urticaria is not a disease in itself, but it is a clinical sign. The lesions are typically edematous, meaning the tissue is filled with fluid. It is likely you have seen these lesions on a horse. They seem to appear quickly and when you press on the lesions or bumps, your fingertip will make an indention into the swollen area; this indentation tells you the swelling is due to edema, or fluid, in the tissue. Lesions are more common on the neck and thorax (the part of the body between the neck and the abdomen) and if the condition is severe, multiple smaller lesions can form into a group and make the lesions appear to be quite large. Some of the lesions will even have small amounts of fluid or serum oozing out. Sometimes horses will be rubbing the lesions as if they itch, but most do not.
The most common cause of these lesions in horses is an allergy to flying insects or to another environmental allergy, although ringworm can cause similar reactions in horses that are allergic to the ringworm fungi. There are some other infections of immune conditions that can cause urticaria, but these are uncommon. In cases that do not respond to regular treatment, a biopsy may be required to diagnose some of these other unusual conditions. Because the most common treatment for urticaria is cortisone, it is important to rule out ringworm before using long-term cortisone because ringworm can worsen if cortisone is used. As far as treatment, many cases do not require treatment at all and horses will recover without it. However, if the problem continues or worsens, cortisone treatment is recommended. Allergy testing can be performed in cases that recur to determine the cause of the allergy.