Conditions Contributing to “Rain Rot”

Courtesy of Penn State

Rain rot or rain scald, is caused by bacteria and often is mistaken for a fungal disease. The bacteria live in the outer layer of skin and cause pinpoint to large, crusty scabs to form. When removed, the base of the hairs can be seen sticking through the bottom of the scab. In early or less-severe cases of this disease, simply removing the scabs with shampoos and currying will take care of the problem. More severe cases in which the infection has affected deeper layers of the skin might require antibiotics.

The natural habitat of the bacteria that causes the skin problem is unknown, but many researchers believe that it lives in the soil. However, attempts to isolate it from soil have been unsuccessful. It has been isolated only from the skin of various animals, and it is restricted to the living layers of the epidermis. Infected animals are considered the primary reservoir.

The bacteria that cause this problem can live in dormancy within the skin for some time and become active when the skin is compromised in some way, such as prolonged wetting when left outside, high humidity, high temperature, or attacks by biting insects. When the infective zoospores reach a skin site where the skin’s protection is reduced, rain rot will result. The zoospores germinate and produce hyphae (threadlike tentacles), which penetrate into the living epidermis and then spread in all directions from the initial infection site, resulting is an acute inflammatory skin condition.

If the horse spends most of its time outdoors during the wet season, one should examine the horse periodically for rain rot. During the wet winter months when the horses have full coats of hair, a hands-on examination is needed to see if the horse has a problem. The infected horse usually will have a series of bumps along the back and croup. When rubbed, the “bumps” might rub off in the form of scabs with a small, hairless spot of skin showing.  Unlike other skin problems rain rot is not itchy.

Rain rot is relatively easy to diagnose, a positive diagnosis can be made by your veterinarian by taking a culture and sending it to a laboratory for testing.


  • Treatment of infected animals often involves intra-muscular injections of procaine penicillin and streptomycin.
  • External treatment with disinfectants that contain a cresol or copper salt base can decrease the spread of infection.
  • Remember to read and follow the medical literature written on the label before commencing treatment.
  • Controlling biting insects with insecticides can be effective in preventing the skin breaks that allow bacteria to get started.
  • Remember that a skin problem that goes untreated will get worse.