Editor’s Note: This excerpt is from Chapter 7 of Care & Management of Horses by Heather Smith Thomas. 

Fall Care

Rainrot

Rainrot (rain scald) is a skin problem that often appears during wet weather. Typical signs include very sensitive skin, clumps of hair coming off, and raw spots or crusty patches on the horse’s back. After a rain you may see the horse’s hair standing up in an odd pattern on portions of the body that got wet and where water ran down off the horse’s sides. As you run your hand over the horse, you might feel heat and his back may be sore. By the next day he may have tight scabs on the sensitive areas. The scabbing may be a light peppering of small bumps, or the whole area may be a painful sheet of crusty, scabby skin.

The scabby bumps and crusts tend to be located in the runoff patterns on the horse’s back and body, such as down the flanks and over the shoulders, back, rump, and neck. A heavy rain that wets the whole horse may cause bumpy crusts over most of the body, while a light rain causes only scattered patches. You may first notice the problem when brushing the horse. In early stages it is easier to feel the emerging bumps than to see them, and the bumps may come loose as you rub. They may be hot and tender and the horse may be sensitive when you brush him. As the disease progresses, the bumps become more raised and tufts of hair stand erect.

Rainrot is caused by the bacterium, Dermatophilis congolensis, which seems to have characteristics of both fungi and bacteria. It normally lives in the soil (dirt and mud of a pen or pasture) and is present in dust particles. The combination of water and dirt, such as when a dusty horse gets wet, makes an ideal environment for this opportunistic invader. This microbe can live in a dormant state within the skin for long periods. If the skin is compromised in some way, such as prolonged wetting by rain or high humidity, moisture enables dormant microbes from earlier lesions to establish new infection sites