Hematomas occur in horses that suffer blunt trauma over areas of large muscle mass. The term translates from Latin as a mass (toma) made up of blood (hem).
The most common scenario of hematoma in the horse comes with horses that run together. The most common site on the body is the chest or the hindquarters at the level of the stifle. This history of course suggests the most common cause as getting kicked by another horse. In my observation, this condition is also age-related, as you see most hematomas in weanlings or yearlings that run together.
In observing young horses interacting, it is not unlike adolescents “rough housing” and somebody gets bruised up. The trauma of the kick doesn’t tear or cut the skin, but does damage or rupture underlying blood vessels. The size of the resulting hematoma depends on the size of the blood vessel damaged.
These swellings show up “overnight,” so to speak, because of the spontaneity of the event. This fact, plus the consistency of the swelling, helps differentiate a hematoma from an abscess. The immediate treatment is basically nothing. You may want to put the horse in a small pen to minimize activity that may cause further extravasation of blood from the damaged vessels. One shouldn’t be anxious to open and drain these swellings too soon, as you’re relying on pressure to stop the leaking of blood.
With hematomas smaller than roughly the size of a football, I think one can leave them alone. In time, the clot will organize and reabsorb. With large, pendulous hematomas, it is often indicated to drain them as one would do with an abscess. By the time a hematoma is ready to open, it has basically become a seroma, which is a swelling full of serum. This change is due to the blood clot contracting and the cellular factions becoming part of a chicken-fat clot, leaving serum as the liquid component that comes out when the lesion is opened.
The opening should be at the most ventral aspect and large enough to remove the fibrin (chicken-fat) clot. The opening has to be maintained patent long enough for the cavity to heal together, obliterating the space caused by the hematoma. Sometimes antibiotics are indicated if infection seems a threat. Most of these lesions heal with no long-term effects, but it may take several weeks for all the swelling to be resolved.