When Shiloh was 2 she developed a warty looking thing on her inner leg. We thought it was a sarcoid at first. When she turned 3 it started getting smaller and finally just disappeared on its own. I think many skin ailments on young horses are hormonal. I used antibiotic oitment on it when it got scraped or would bleed but otherwise I left it alone.
This is what I found on Thehorse.com
Viral Skin Problems: Warts
The most common warts in horses are those around the muzzle, which are probably caused by the papilloma virus. These are generally seen on young horses, and they spontaneously resolve and disappear after the animal develops immunity to the virus. In some cases (if they interfere with eating or for cosmetic reasons), people try to get rid of them quicker by using an autologous (derived from the horse's own tissue) vaccine made from some of the wart tissue, says Jackson.
Another skin problem caused by the papilloma virus sometimes appear on the inner surface of the ears. These flat, white areas are called aural plaques and are sometimes scaly. "These are spread from horse to horse by fly bites. They don't seem to bother the horse unless they become infected," says Jackson. Insect bites can irritate these and make them worse.
Rees says, "Treatment is usually not effective. I had a mare with these, and no matter what I tried, they didn't go away. It's usually best to just leave them alone. The main thing is to keep flies out of the ears so they are not irritated and don't grow larger. If you are showing the horse and it's a cosmetic issue, you can color them with a marking pen so they are not so obvious."
"Sarcoids are supposedly the most common equine skin tumors, and certain horses seem more predisposed to getting these," says Rees. "There are several types, including a flat, circular, scaly type that looks like ringworm. If you try to biopsy that one, it will become worse."
One type looks nodular (firm, raised above the skin, and looking like a tumor) and is usually 5-20 millimeters wide. Another type resembles ulcerated tissue and is called a fibroblastic sarcoid. Yet another kind looks warty (a verrucous type of sarcoid).
White says the exact cause of sarcoid tumors is not understood, but might have something to do with the papilloma virus that causes warts. Some sarcoids seem to occur at sites of injury, which might indicate a virus as the cause, he says.
"The malevolent type usually has multiple nodules and is very hard and thickened," says Rees. "Some of those can infiltrate the lymph nodes and produce spreading, cordlike lesions. The problem with sarcoids is that when you surgically remove them, they tend to recur; the growth may appear somewhere else on the body later. Surgery is one of the more common treatments, however, and is the treatment of choice for fibroblastic sarcoids. The veterinarian may also freeze the tissue to debulk it and follow up with an injection of bacille Calmette-Guerin (BCG) vaccine, especially if the growth is near the eye."
The BCG vaccine is used in human cancer chemotherapy, but it produces a lot of inflammation, says Rees. "The sarcoid becomes ulcerated and falls off," she explains. "Because of the swelling, the horse is usually put on drugs to help control that secondary inflammation."
Radiation therapy is used on some lesions and is more beneficial for smaller sarcoids that do not penetrate the deeper layers of the skin.
Chemotherapy is also useful in treating equine sarcoids. The drug reported to have the most success is Platinol or cisplatin, but it requires repeat injections and has some handling precautions for humans (gloves, protective clothing, special disposal), so not all veterinarians use it. It should not be used in breeding animals, says Rees. The veterinarian will decide what's best to use in each case.
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