Dermatophilosis or “Rain Rot”

Dermatophilosis or “rain rot” is a common disease of horses worldwide but is more prevalent in areas where rain is frequent and humidity is high.  It is a skin infection caused by Dermatophilus congolensis – an organism that has some fungal features but is actually a bacteria.  Although the organism is difficult to isolate from the environment, it can live quietly in the skin until infection is promoted.  The two most important factors in promoting infection are moisture and skin damage.  There is a marked increase in the incidence of disease after heavy rainfall.  It is thought that the intensity of the rain is more important than the total amount.  Trauma from biting flies, irritating plants or other injury to the skin can also promote development of the disease.  The primary lesion in dermatophilosis is called a tufted papule (papule is a fancy name for a “bump”).  The lesions can be small or they may spread, resulting in large clumps of hair becoming matted together.  When these “mats” are pulled off the skin, they have a characteristic “paintbrush” appearance that aids in diagnosis.

Dermatophilosis is usually seen on the rump and topline, saddle area, face and neck, and back of pasterns or on the hind cannons.  Lesions can be small dry crusts that are easily removed with grooming or rubbing, or they can be larger crusts with yellow-green or gray colored pus underneath them.  Early in the infection, removal of crusts or touching the affected areas may cause pain.  Individual lesions might appear in a spotty pattern across the horse’s body or they might unite into large areas, leaving large patches of bare skin.  After the scabs and hair fall off, the skin underneath may appear red and raw.  It will take about 7-10 days for new hair to grow in.

Dermatophilosis is contagious and can spread to other animals by contact with the crusts, often on the soil where an infected horse has rolled, or by shared tack and grooming supplies.  The bacteria can live in the crusts for up to 42 months, so control of this disease can be difficult.

There are other skin conditions that can appear similar to dermatophilosis.  Ideally, you should consult your veterinarian about any skin disorder.

Diagnosis of dermatophilosis is usually made by physical examination because the lesions and location are characteristic.  Diagnosis can be confirmed by microscopic examination of either the pus under the crusts or of the crushed crusts themselves.  The bacteria form “packets” of round cells arranged in two to eight parallel rows of branching filaments which have a characteristic “railroad track” appearance.

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