This time of year many horse owners are dealing with wet weather, long (or shedding) hair coats, and dirt. Nothing loves this type of environment more than Dermatophilus congolensis, a Gram-positive bacterium that causes a skin infection commonly known as rain rot or rain scald. D. congolensis is commonly believed to live in the soil and has been found on the skin of animals and people. It can spread through contaminated equipment (i.e., brushes, blankets, and shared tack) and biting insects.
Any horse can get rain rot, but horses with compromised immune systems from systemic disease or malnutrition are most likely to be affected. Key environmental factors also must be in place for the bacteria to establish an infection.
Moisture is the main factor in rain rot development. While the disease is termed rain rot, it’s not just the rain that can cause the infection. Horses in the summer heat on stall rest can become infected and show clinical signs—no precipitation required. And in early spring many horses still have long coats and the hot/cold weather fluctuations can lead to more moisture on the skin. Excess moisture compromises the natural barrier the outer layer of the skin (epidermis) provides, making it susceptible to bacterial invasion.
Another factor that can lead to infection is exposure to biting insects, particularly flies and ticks. Warmer weather brings out these pests and, once the skin becomes inflamed and irritated from their bites, the bacteria they’ve introduced can proliferate.
Older horses or those with systemic disease, poor nutritional planes, or allergies that affect skin integrity might be more susceptible to infections. Once infected, these horses might take longer to heal then their younger, healthier counterparts.
Veterinarians usually make a presumptive diagnosis based on the location, appearance, and size of the lesions. They typically appear as small raised or matted tufts of hair, and the skin around the lesion is usually very warm to the touch and might be so sore that your horse flinches at your touch.
The lesions occur primarily on the head, back, and hindquarters but can also be found on the neck or chest. Lesions commonly develop on the pasterns or fetlocks in muddy or wet conditions (typically called mud fever or pastern dermatitis). These lesions start as small pinpoint spots but, if left untreated, they can coalesce into larger mats of hair. The crusts and hair stick together to form paintbrush-looking lesions when they fall off.
Bacteria can dwell on the edges of the crusts or skin lesions. Veterinarians make a definitive diagnosis by removing the crust, mashing it on a microscope slide, applying a special stain, and looking for the bacteria, which tend to form rows like railroad tracts. Other tests include isolation using culture or skin biopsy for more severe infections.
Luckily for us horse owners, rain rot is easy to treat and cure. There are many products to treat rain rot, but most focus on the same concepts: Soften the crusts, remove them, and treat the underlying skin where the bacteria live. A topical antiseptic shampoo can clear up most acute and localized infections. Your veterinarian might recommend products with betadine or chlorhexidine bases as the first line of treatment.
The infections can spread rapidly if not caught and treated early. The most important part of treatment is to remove the crusts. Resist the urge to pick at them, however; always soften them and allow them to loosen on their own. In colder weather, when bathing is not an option, you can work antiseptic sprays and creams into the hair coat to soften the crusty or hard-to-remove lesions and wait 10-15 minutes before removal. Most crusts will come off along with the surrounding hair.
The initial treatment usually includes once-daily shampoos or sprays for three to five days. This will quicken the healing time and reduce bacteria spread. For more chronic or severe infections, the horse might need to be treated with a prescription shampoo and/or systemic antibiotics. Once the infection is resolved, the hair will start to regrow in seven to 10 days.