As you shed out your horse this spring you may notice small clumps of hair coming out together. These clumps are attached to a scab or crust, and a small amount of purulent material, i.e. pus. The skin underneath these small (1 to 3 mm or 1 to 1 1/2 inches) areas may be irritated, inflamed and sometimes painful. This condition, known commonly as rain rot or rain scald, is a superficial (not deep) infection of the skin caused by an unusual bacteria called Dermatophilus congolensis. This bacteria only causes skin infections. In fact, the infections are never deep like an abscess.
Rain rot is usually encountered in the spring, due to the increased amount of rain, but it can occur any time there is rain and wetness. It affects many horses but particularly horses that are turned out for extended periods of time.
The constant dampness of the skin (usually over the withers, back and rump, where the water tends to pool, as well as on the face and lower limbs) causes damage to the skin, which allows the bacteria's spores ("zoospores" — dormant state) to germinate, much like watering seeds to stimulate growth. The active stage (mycelium) of the bacteria grows readily in the living layers of the wet skin. The inflammation caused by the bacteria colonization results in the purulent material you can see just beneath or attached to the hair as it falls out.
Factors that Promote Infection with Dermatophilus
The bacteria can survive in the crust for several months. It can be transmitted from one horse to another by flies, brushes, blankets or tack. However, certain conditions must be present for infection to occur. Dermatophilus zoospores cannot enter healthy, dry skin. The horse must have some type of damage to the skin as well as some degree of moisture.
If you notice the so-called "paint brush" lesions in horses that are not exposed to the rain, consider other factors that may cause the initial damage to the skin, which allows the bacteria to invade secondarily. For example, biting insects, ectoparasites or improperly fitted tack may cause skin damage, and sweat may be the source of the moisture.
There are other diseases that can cause similar hair loss. If your veterinarian is not sure if rain rot is the problem, the crusts can be examined under the microscope to look for the Dermatophilus organism, or a skin biopsy can be obtained for histology or bacterial culture. Some of the diseases that look like Dermatophilus include ringworm, other bacterial infections (e.g. Staphylococcus, Corynebacterium pseudotuberculosis), skin allergies, sarcoids and migrating parasites. Lesions on the legs might be confused with sporothrocosis (Sporothrix schenkii) or phycomycosis ("swamp cancer" or Florida Horse Leach). The presence of lesions on the back and sides is typical for rain rot, but not for the other diseases.
The first step in the treatment of horses with rain rot is to remove the crusts using an antibacterial shampoo such as betadine (0.5 percent) or chlorhexidine (2 percent). These crusts contain numerous bacteria that spread to other regions, and to tack and the premise.
The crust must be removed and treated as soon as possible. After crust is removed, it is important that the skin is dried as thoroughly as possible, because these bacteria will not grow in dry conditions. Chlorhexidine or lime sulfur topical solutions should be applied daily for approximately one week and then at decreasing intervals depending upon response to treatment. If the horse is severely affected your veterinarian may prescribe oral or injectable antibiotics, such as procaine penicillin G, ampicillin, ceftiafur (Naxcel), potentiated sulfas or tetracyclines.
The key to treating horses with rain rot is allowing the skin to dry in between treatments. Exposure to sunlight is paramount. While your horse is healing it is important that no further damage is done to the skin. This requires rest from any work that requires tack to be placed on affected areas. Finally, remember to separate all tack and brushes from affected animals to minimize spread from one horse to another.
Although rain rot may cause unsightly hair loss and some pain to your horse, once identified it can usually be treated without too much difficulty. The drier you can keep your horse while he or she is recovering the quicker you can get back out to those beautiful spring rides.