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Rain rot (also called rain scald) is an anaerobic infection that requires the absence of oxygen to live and spread. When a horse catches rain rot he will develop a series of crusty scabs and/or matted/raised tufts of hair that, when pulled off, will most often exhibit pink skin laced with some pus. Rain rot is contagious, generally transmitted by shared horse tack (such as blankets), mutual rubbing posts such as a fence, etc. Although contagious, there are some factors that place the odds against rain rot:
Given the way this infection works, the most obvious form of prevention is keeping your horse dry during periods of intense rainfall. Climates that are dry such as Arizona don't see much occurrences of rain rot, while very humid areas like Florida will be a far easier breeding ground for these bacteria. In addition groom your horse often, particularly during the early spring months. Not only will removing the winter coat make your partner more elegant, it will make it far more difficult for rain rot to take root.
What should you do if your horse does develop an unsightly case of rain rot? First, don't let it stress you out – studies have indicated a horse feels no pain, itching or discomfort from the infection itself. It is also superficial and relatively harmless. Ugly, yes – but harmless. That being said, rain rot should be addressed as soon as possible to prevent the appearance of secondary bacterial infections that also enjoy the presence of moisture.
Treatment consists of bathing your horse and lathering him in antimicrobal shampoo. As you do this try to pick off the scabs, but beware – this can cause discomfort and pain to the horse (remember when I said the cure hurts more than the infection?), so take it slow. The task might seem distasteful, but it's necessary because otherwise the infection can continue to thrive underneath the scabs. By removing the scabs you are exposing the skin to air (and by default oxygen), which will cause the infection to dry up and heal. The baths should take place for approximately seven days.
Also important is to make sure the horse remains in a dry and ventilated area during the treatment. Adequate protection against bug bites should also be provided.
Finally, there are other bacterial infections, skin allergies and parasites that can create symptoms that appear similar to horse rain rot, so if you're uncertain of your horse's affliction you should get assistance from your veterinarian.