Merck Manual

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Pet Owner Version

Dermatophilosis ("Rain Rot") in Horses


Karen A. Moriello

, DVM, DACVD, Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison

Reviewed/Revised Oct 2019 | Modified Nov 2023

This infection of the epidermis (top layer of the skin) is seen worldwide but is more prevalent in tropical environments where rain is frequent and humidity is high. Lameness and loss of performance may occur in horses that are severely affected around the pasterns.

The infection is caused by a species of actinomycete, Dermatophilus congolensis, a microorganism that resembles bacteria and fungi. Factors such as prolonged wetting by rain, high humidity, and high temperature, increase the occurrence of dermatophilosis. The organism can live in the skin quietly until infection is stimulated by climatic conditions. High humidity and moisture increases the release of spores and spreads the infection. Epidemics usually occur during the rainy season.

Dermatophilosis is most common in the young, animals continually exposed to moisture, and animals with weakened immune systems. Most infections subside in 2 to 3 weeks and the wounds heal without scarring. In general, the onset of dry weather speeds healing. In chronic infections, scabs and crusts can spread over a large portion of the body, particularly along the back. Itching is variable. Some wounds may be painful. Wounds on horses with long winter coats develop with matted hair and wounds leading to crust or scab formation with yellow-green pus. With short summer hair, matting and scab formation is uncommon.

Animals with severe generalized infections often lose condition, and movement is difficult if the hooves, lips, and muzzle are severely affected. The condition is painful. Severely infected animals may have to be euthanized.

Diagnosis depends on the appearance of wounds in diseased animals and the finding of the actinomycete in skin smears. The most practical diagnostic test is microscopic examination of scabs or impression smears of the underside of fresh wounds. Thus, your veterinarian will likely take sample scabs or smears for laboratory examination.

Treatment involves appropriate antibiotics. The lesions should be gently soaked and scabs removed. Your veterinarian can provide instructions for this. Topical antibacterial shampoo treatment is often effective and may be prescribed along with other medications. Clipping of the hair coat may be required. Successful treatment requires removing the horse from the damp or wet environment that triggered the infection.

Infected horses should be isolated from other animals to reduce spread of the disease. Careful attention to the cleanliness of living areas, tack, blankets, grooming tools, and other accessories is required and can help control spread of the disease. Insect controls are often recommended for the same reason because insects can carry the disease from an infected horse to a healthy one. The infecting organism can be transmitted to people by direct contact, so affected animals should be handled with gloves, and thorough handwashing is recommended.

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