The signs of rain rot are unmistakable: A few hours after coming in from the rain, your horse's coat begins to stand up in a peculiar pattern, either bordered by the "drip line" of rain runoff or in patches. As you begin to smooth it down, you feel a radiating heat and your horse flinches from your touch. By the next day, sensitive, tight scabs have appeared where the rain was concentrated on his back and sides, making your horse a miserable mess. The culprit? Rain rot.
Rain rot is caused by Dermatophilus spp., bacteria that normally live without consequence in the equine coat. However, a rain followed by slow-drying, humid conditions enables the organism to multiply, which irritates the hair follicles and skin of afflicted horses.
The scabbing, which may range in severity from a light "peppering" to a continuous painful sheet, follows the run-off pattern of water over the horse's back and rump. Rarely do subsequent rains initiate new areas of infection.
The earlier you detect rain rot, the easier it will be to spare your horse the discomfort and cosmetic problems associated with it. A brief course of penicillin injections started at the initial signs of a raised coat will solve the problem without hair loss. If you begin penicillin after the scabs appear, the treatment will still be effective, but healing will take longer. Without treatment, rain rot runs its course in one to four weeks, depending on the extent and severity of the scabbing.
As healing progresses, resist the urge to pick the scabs, as they are very painful and can bleed. Instead, soften them with mineral oil and let them work themselves loose. Then apply a mild medicated shampoo and try to gently rub the scabs free with your fingertips. If the scabs don't slip off easily, oil them again.
Preventing rain rot is a matter of good grooming. A dirty coat naturally contains more organisms and more skin debris to feed them. Twice-weekly or more frequent brushing or vacuuming usually limits the incidence and severity of rain rot. Some horses, however, are particularly susceptible and may need to be protected from wet weather to prevent future attacks in ensuing months. To prevent the spread of rainrot, don't share brushes, tack and blankets belonging to afflicted horses.