Rain Rot!

By: Lorelei Prichard DVM
About the author

Although the description is fairly disgusting, the phrase “creeping crud” is a well-known term among horsemen. Also described as rain rot, rain scald, dermatophilosis and streptothrichosis, this skin condition is the most common cause of crusting lesions in horses. It can affect cattle, sheep, goats, and on rare occasions, dogs, cats, and humans.

Even if your horse hasn’t experienced the “creeping crud”, chances are you’ve heard of the havoc it can cause to a horse’s coat and skin. Because of its many aliases, I will refer to this skin condition as dermatophilosis throughout the column.

Dermatophilosis is a superficial infection of the skin caused by a bacterium called Dermatophilus congolensis. This infection is limited to the skin, and the bacterium requires trauma and wetting to produce disease. A definitive diagnosis may be confirmed by viewing the organism microscopically in scabs from the lesions or from exudate beneath the scabs, however the characteristic appearance of the lesions can be fairly diagnostic.

Lesions in the form of papules and pustules frequently appear on the rump, saddle area, and face. The classical skin appearance includes multiple, small scabs at the base of matted tufts of hair, sometimes clumped in furrows giving the coat a “paintbrush” effect. The hair is often easy to remove or epilate, and lifting a clump of affected hairs reveals an attached crust with a moist, pink lesion left upon the skin.

Examination of the bottom of the crusted hair clump will often show a moist undersurface with yellowish or greenish exudates. Some horses will find the removal of the crusts uncomfortable, but this condition normally does not otherwise cause itching or distress.

The Dermatophilus bacterium invades a horse’s compromised skin and multiplies within the top layer in the form of branching filaments. The infiltration of the epidermis with these filaments leads to inflammation and oozing of serum, resulting in the formation of a thick scab rich in bacteria.

Have you ever noticed that there are epidemics of dermatophilosis during the rainy season or that some barns tend to have repeated outbreaks? Although the bacteria thrive in an environment void of oxygen, they can survive quite well within the scabs and can persist in their crusty homes for up to 42 months in the environment. When the scabs are moistened, the resident bacteria are activated. This may account for repeated outbreaks in contaminated areas and the increased incidence during the wetter months.

Dermatophilosis is most commonly seen in horses exposed to prolonged wetting, high humidity, and exposure to biting insects. Insects such as flies and ticks can transmit the infective bacteria from one horse to another, and the sharing of contaminated brushes and tack can spread this organism as well. Immune-compromised and debilitated horses are at greatest risk of infection.

Most cases of dermatophilosis are self-limiting, and many horses recover spontaneously in about four weeks without intervention. Uncomplicated cases usually heal without scar formation or hair discoloration, but severe cases may pave the way for secondary bacterial infections. Horses that have been infected are not immune to later infections.

The most important elements of treatment are removal of the predisposing factors (often extreme wetness), protection from biting insects, and crust removal.

A basic protocol for treatment includes:

• Loosen and dispose of infective crusts (bacteria live within and under the scabs); many cases will resolve with daily brushing
• Wearing protective gloves, cleanse the area daily for seven days with a disinfectant such as Dr. Mike’s Medicated Shampo, and then weekly until resolved; towel dry your horse if necessary (remember that the bacteria need a moist environment to thrive)
• Clipping the area may speed up recovery - be sure to disinfect the blades
• Keep horse in a dry environment with adequate fly control
• Disinfect tack, blankets and grooming equipment
• If lesions are extensive or unresponsive to treatment, consult a veterinarian; in severe cases, systemic antibiotics are indicated

When treating your horse for dermatophilosis, remember that human infection, though rare, is possible and that disposable gloves are recommended when removing crusts. Notify your physician if pustules develop on your hands and forearms after contact with infected animals.

Copyright © 2005 Lorelei Prichard DVM