If you see crusty, coin-sized patches of skin on your horse, it could be ringworm. Despite its name, it has nothing to do with worms, but it can spread easily and can irritate you and your horse. Ringworm is treatable but you have to be diligent and act quickly (See References 1 and 2).
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Ringworm is caused by fungi that love dead skin and hair, known as dermatophytes. You will notice dry, crusty, fairly round spots on your horse, that may be clustered together (References 1 and 2). Your vet can confirm that it is ringworm by scraping the skin cells and examining them under a microscope, but he may start you on a course of treatment before he has the results (Reference 3).
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Several easy-to-use products are readily available without a veterinarian prescription, such as iodine shampoos or over-the-counter fungal treatments for human conditions such as athlete's foot. Your veterinarian may recommend a combination, and have you clean and scrub the affected area with a shampoo or wash, and then apply a topical anti-fungal cream or powder after the lesions dry (References 1 and 3). When you wash and scrub the patchy spots, remove the scabs before treating so that the topical application can get directly to the fungi (Reference 3).
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There is no need to panic if you suspect your horse has ringworm, but there are a few important things to keep in mind. First, by the time you see any indication of ringworm it can already have spread to other surfaces, including fences, other horses, your tack and you (Reference 3). Second, if left untreated, it can spread quickly on your horse, becoming a major source of irritation and discomfort for him. It can also spread to other horses and animals, and to you, so make sure you keep your horse in a separate stall or pen away from other horses (References 1 and 3).
Containment and Prevention
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The fungi that cause ringworm can live on surfaces for long periods, so disinfecting your horse's environment as much as possible is important to contain and eradicate ringworm. Also clean halters, tack, and any blankets or other items your horse wore. Ringworm is characterised as zoophilic, meaning it can spread from animals to humans, so don't forget to wear disposable gloves when treating your horse or when handling his items.
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Not all healthy horses exposed to ringworm will get an infection, and once your horse has ringworm he often will build an immunity to it that can last for years. This means that younger horses are generally more susceptible than older ones who may have already been exposed, but older horses or those with compromised immune systems can also be affected more easily.