When your gelding's penis is pulled up near his body, it is covered by folds of skin called a sheath. Glandular secretions from the sheath create smegma when mixed with dirt from the environment. If you notice that your horse's sheath is swollen, it may need to be cleaned -- but swelling may be a symptom of a serious illness.
If your horse has a swollen sheath and hesitates to lower his penis to urinate, or has smegma visible on the the penis or hind legs, cleaning his sheath will likely resolve the swelling. Some horses also develop beans, or small lumps of debris that gather at the tip of the penis. Beans may inhibit urination and become painful.
Use mineral oil or a mild soap to clean the sheath. Wear latex gloves and use paper towels or cotton to wash your horse's penis and the inside folds of the sheath. Use your finger to remove any beans from the tip of his penis. Rinse the sheath thoroughly removing the smegma with clean water and towels. Some horses may allow you to rinse the sheath with a hose, but not all will tolerate this. Any soap residue may cause further irritation to the sheath, so rinse thoroughly.
Trauma and Infections
Trauma from a kick, laceration or bruising may cause the sheath to swell. Swelling from trauma usually accompanies pain and heat in the area of the injury. If the skin is broken or scratched, the area may become infected.
Contact your vet if your horse is injured or shows signs of infection. The vet will prescribe antibiotics to clear up any bacterial infections.
Parasites can cause sheath swelling. For example, habronemiasis is a parasitic infection caused by fly larvae; swelling and itching are symptoms. Some horses may injure themselves attempting to relieve the itch. It is usually treated with a topical anti-parasitic. In severe cases, swelling from the parasite may block the urethral opening and require surgery.
If your horse has fluid buildup or edema in other parts of the body, the fluid naturally falls to the low point of the horse due to gravity. The sheath is a natural area where these fluids might accumulate. Exercise generally resolves swelling in these cases. However, treatment may be necessary for the conditions causing the edema, such as liver disease or low blood protein.
Equine Metabolic Syndrome
If your gelding has a swollen sheath along with symptoms of laminitis, or founder, it may be a sign of equine metabolic syndrome, a chronic form of insulin resistance. Symptoms of laminitis include lameness and divergent hoof rings. Horses with equine metabolic syndrome also tend to have fat deposited along the crest of the neck. Horses with the condition tend to be obese and between 5 and 15 years of age. Pony breeds, Paso Finos and Morgans are the breeds most commonly affected.
Insulin resistance is treated by reducing the horse's access to pasture, managing sugar and starch intake from feed, and regularly exercising the horse. Your vet may prescribe a medication such as levothyroxine sodium to help your horse lose weight and increase his insulin sensitivity.