Rain rot, also known as rain scald, mud fever, dew poisoning, dermatophilosis and cutaneous actinomycosis, is a skin infection caused by a bacterial pathogen called Dermatophilus congolensis. These bacteria are often present on healthy animals and generally cause problems only when conditions are favorable for infection.
Jenny Wilkinson, DVM, of the University of Vermont, said that horse owners need to know this is contagious. “People originally thought it was caused by a fungus that lived in the soil. Research showed it is caused by bacteria that live on the skin. Horses may carry it and never develop a problem, but other horses they come into contact with might be susceptible to infection. We’re not sure why some horses get rain rot and others (in the same environment) do not, but it may be their immune system,” said Wilkinson.
“We usually see rain rot when we get a lot of rain, or during a muddy season. We see it over the top of the horse (where the body has been wet from rain) and sometimes infections in the back of the pastern,” she said. In these instances the infection could be confused with scratches.
“It can be difficult to differentiate between the two types of infections. The only way would be to do a skin scraping and look at it under a microscope–to see the bacteria. Dermatophilus congolensis has a distinct appearance; they line up next to one another and form a pattern that looks like railroad tracks,” she explained.
To treat rain rot, she clips the hair off affected areas, which exposes bacteria to air. Long hair holds moisture next to the skin. “If you can get the hair away and get the scabs off, treatment will be more affective. I use betadine or chlorhexadine (Nolvasan) and then make sure the area is dried,” says Wilkinson.
“Some veterinarians mix up a scratches paste, and it works nicely for scratches or rain rot on the lower legs. It’s a combination of furazone, penicillin and gentocin—very broad-spectrum. The horse may start out with rain rot and is then susceptible to secondary bacterial infections. We use a broad-spectrum ointment to make sure we cover everything that might be there,” she says.
“Another treatment that works is White Lightning (containing chlorine dioxide), used for thrush and white line disease in the hoof. It works well for scratches or rain rot and is not abrasive. It permeates the area and gets in under the crusts. When you squeeze it out of the container, it creates a chemical reaction that produces a gas that allows it to penetrate better. It is anti-bacterial and anti-fungal,” she said.
It’s important to keep the horse in a clean, dry environment. “If he lives outside, put him in a clean, dry stall until you get the rain rot or scratches cleared up. If there are crusts along the back and withers, which we often see in wet weather, clean it up to allow air to get to it. I don’t use ointments on the back, because they hold moisture. Use betadine or chlorhexadine and dry it,” she said.
“I’ve seen some cases on the pastern get so bad that they result in cellulitis that goes up the leg. Bacteria under the skin cause swelling, pain and lameness. In that case you’ll need to give the horse systemic antibiotics,” said Wilkinson.
She usually recommends TMS tablets (trimethoprim sulfa) orally. “If that drug doesn’t work, we switch to a combination of IM penicillin and IV genticyn. If that doesn’t work, we’ll use a stronger IV medication. Most rain rot cases can be treated locally, without systemic antibiotics, however, if you catch it early.”