Skin diseases are fairly common in horses. There are a number of different types of problems, with varying causes—everything from parasites to viruses, fungi, bacteria and allergies. They can also vary in severity. Some are more likely to occur during wet weather or muddy conditions, which often occur in fall and winter. Two of the most common include “scratches” and “rain rot”.
SCRATCHES
Scratches is the common name for areas of lumpy, weepy or crusty skin on the lower legs, especially around the heel and pastern and sometimes up the cannon. Dr. Stephen White, Professor of Dermatology, School of Veterinary Medicine, at the University of California-Davis says this condition is also called greasy heel or mud fever, referring to any kind of skin lesion on the pastern and fetlock areas. “There can be many reasons and causes; cases that keep recurring are often due to vasculitis (inflammation of the blood vessels under the skin),” he said.
“The lesions are most common in unpigmented (pink) skin but can also occur in colored skin. The cause is not always understood. On rare occasion it may be due to a drug that was given to the horse, causing the reaction, or due to photosensitization, which is seen most frequently in unpigmented skin.”
“When confronted with scratches, the veterinarian tries to determine whether the cause is bacterial, fungal or a parasite, such as certain mites that cause leg mange. The mange can now be treated with medications like ivermectin that kill the mites. Thus it is essential to have a correct diagnosis, to know how to treat a skin problem successfully,” said White. If the lumpy crusty skin is caused by a bacterial and/or fungal infection, medications can be applied or administered that will often resolve those infections.
The cause is not always known, but the problem occurs most frequently in wet conditions. There is likely some trauma to the skin—maybe a nick or abrasion. In winter the skin might be broken by ice or crusted snow. Most infections are a mixed bag of bacteria and fungi that are present in the mud and fecal material that the horses are walking and standing in. The disease starts as a wet rash on the leg/pastern/heel area and then it becomes a crusted scab.
The most appropriate treatment will depend on the cause, and if your veterinarian thinks the infection might be a mix of bacteria and fungi, there are some medications that can be applied to the affected areas to help combat the infection. Keeping the feet and legs clean and dry can also help.
RAIN ROT
This is a common term for dermatophilosis. The organism causing this skin disease is the spore-forming bacterium Dermatophilus congolensis. It needs moisture to emerge from its dormant form and become infective. “We call the disease rain rot because it usually occurs in rainy seasons. The skin lesions often appear along the back because the horse is standing in the rain, or may appear on the lower legs if they have been wet. This may be one of the differentials when diagnosing causes of scratches — occurring after the horse has been standing in water, or has been sprayed with water in the stall when people clean out the stall with a high-pressure hose while the horse is in the stall,” said White.
“Three things are needed for rain rot to occur—water, the organism and some sort of micro-trauma to the skin to open the way for these bacteria. This could be anything from fly bites, to another horse biting the animal, or little scrapes and abrasions that happen while rubbing or rolling, or the water impact from a high-pressure hose.”
Rain rot is usually easy to diagnose, and also easy to treat. “This infection is susceptible to most antibiotics. But you might treat the horse and the skin problem clears up—and he goes out in the rain again and starts a new infection. Along with treatment you may need to do some environmental or management changes,” he said.
For treatment, most veterinarians prefer a two-pronged approach—topical and systemic. “A gentle shampoo works well, with removal of the scabs and crusts caused by this organism, but the crusts are quite painful to pull off. Many people use a betadine shampoo, but actually the best thing is systemic antibiotics. Penicillin often works very well. The trimethoprim sulfa (TMS) drugs also work and this is an oral product—which is nice for folks who don’t want to have to give their horse an injection every day.”