The purpose of a male horse's sheath, also known as prepuce, is to protect the penis while retracted. The sheath is made up of folds of skin. The internal layer secretes lubricating oils from the sebaceous glands contained there. These secretions, combined with flaking skin cells and dirt, form a waxy substance called smegma, which accumulates within the folds of skin and can cause irritation, infection and oedema if not removed. Also susceptible is the end or head of the penis, which has a recessed opening around the urethral process (channel from which urine and semen are excreted). Smegma collects within this opening and forms what is known as a bean and requires removal to prevent irritation, difficulty urinating and subsequent infection and swelling. This accumulation of debris in both the penis and sheath are common and likely causes of swelling in the sheath. But if this area is clean of debris, there are other factors to consider.
Insect bites can cause irritation, infection and swelling to the outer genitalia or sheath of the male horse. Either recurrent bites or ensuing scratching or other attempts to relieve the itching cause pain and inflammation. An injury due to a fall, strain or kick from another horse can cause pain and swelling.
A hidden medical condition can cause inflammation in the sheath or belly. Heart disease causes oedema in this area because of the inability to pump blood efficiently, leading to fluid build-up. Equine infectious anaemia, also know as swamp fever, is a viral disease that causes the destruction of red blood cells. It is transmitted by fluids, mainly blood from horse to horse. But the virus itself can be transmitted through non-sterile needles, insects or dirty bits. Some signs of this disease include anaemia, loss of coordination, weight loss, weakness and swelling in the abdomen or sheath. Melanoma or squamous cell carcinoma are two forms of cancer that can occur in the penis or sheath. The growing masses cause swelling or enlargement of the sheath. Weight of horse is another issue. Overweight horses, especially geldings (castrated males), accumulate fat pockets. This can occur in other places, such as the neck, but a gelding may show signs of swelling in the sheath area because of fat deposits.