Rain Rot in Horses

Dermatophilosis is a skin infection caused by the bacterium Dermatophilus congolensis. Skin lesions of dermatophilosis are usually crusty and resemble scabs. Areas of affected skin and hair can usually be pulled off easily, and the underlying skin appears moist, sometimes with small accumulations of pus.

The areas of the horse most commonly affected are the back, croup, face, neck, and legs. Most horses show no clinical signs other than crusty dermatitis and/or hair loss, but some horses with severe infections can become painful. Dermatophytosis, or ringworm, is another common cause of dermatitis in horses and can appear similar to dermatophilosis, but it is a fungal infection.

The common name “rain rot” comes from the association of dermatophilosis with wet conditions. Minor skin trauma in combination with chronically wet skin and coat sets the stage for infection. The bacteria are difficult to culture and are believed to be spread by carrier animals rather than originating from the environment.  Dermatophilosis has a slight chance of affecting humans. Therefore, those handling horses suspected of having dermatophilosis or tack and blankets from those horses should wear gloves and practice effective hand hygiene to prevent self-infection or spread to other horses.

The diagnosis of dermatophilosis is usually straightforward. Representative crusts from affected horses are removed, minced in saline onto a microscope slide, and stained. The bacteria have a characteristic microscopic appearance and occur in long, paired chains (railroad track appearance).  Bacterial culture is usually unrewarding because the bacterium is difficult to grow in a laboratory.

Treatment of dermatophilosis involves topical antiseptic soaps such as povidone-iodine or chlorhexidine. Horses that have more painful, extensive infections can require sedation to remove crusts and allow bathing. The horse should be scrubbed and the antiseptic soap should be left on the horse for 10-15 minutes before a complete rinse. Dermatophilosis infections rarely require treatment with systemic antibiotics such as penicillin or potentiated sulfonamides. Affected horses should also be kept where they can stay clean and dry.

If wet weather is forecasted, horse owners might want to keep susceptible horses stabled or outfitted with a lightweight, waterproof sheet or blanket. The sheet should be well-fitted and put on before precipitation begins. If it is placed on after the horse is wet, the environment under the sheet can keep the skin moist and will be even more conducive for a flare-up of dematophilosis.