Rain rot is a skin infection caused by the bacterium Dermatophilus congolensis. As long as the horse’s skin is intact, D. congolensis tends to be dormant. But if the skin is constantly wet, as under a damp blanket, the bacterium can get into any small bug bite or scratch and rain rot. Rain rot makes the skin sort of scabby, and the hair rubs off when the scabby areas are removed.
Paler horses, and horses who are aged or in poor condition, are more prone to rain rot.
The best management is to prevent rain rot in the first place, by making sure your horse is kept clean and dry as much as possible. Regular grooming is important and will ensure that you catch any rain rot early. Equally important is to let skin dry out even when the weather is wet for days on end.
Today’s blanket technologies are pretty amazing, but if you’ve taken your horse’s blankets off after a drenching rain, you know that the sheer humidity can make the underside nearly as wet as the “waterproof” surface. If it’s warm out, the horse’s sweat can also accumulate under the rug. Best practice: remove rugs daily to let them and the horse dry out thoroughly.
If your horse does acquire rain rot, work with your veterinarian to address it as soon as you find it. There are topical approaches that work well for minor cases, but if the condition is advanced you may need a prescription.