Horses have rain rot issues, also known as rain scald or dermatophilosis, caused by bacteria that thrives in humid and wet conditions. The bacteria infects the outer layer of skin, causing scab-like sores and lesions of raised and matted hair along the horse's coat.
A horse that spends too much time in very humid, hot conditions can contract a bacterial infection that thrives just under the top layer of skin. The bacterium usually already lives within the skin but stays dormant until the weather conditions spur it into reproduction. It then creates zoospores or single-celled reproductive units that anchor into the skin and spread using threadlike tentacles. The horse's hide appears lumpy at first, then develops into scabs or hairless patches. A horse with longer hair develops raised bumps of matted hair. These skin lesions are usually filled with pus.
Although rain rot can resolve itself in time, it can spread along the skin or transfer to other horses. For mild cases, an owner can bathe the horse in antimicrobial shampoos and curry the horse to remove any scabs. However, skin rot that has moved into the deeper tissue layers requires antibiotic injections to kill the bacteria. A horse owner can prevent or decrease the risk of skin rot by keeping his horse well groomed.