I have been battling with rain rot in my Thoroughbred mare all summer. What exactly causes “rain rot” and how is it best treated? Could she have skin allergies?
Great question for this time of year! When horses are out in damp, humid weather, conditions are prime for skin conditions. One of the most common is “rain rot” or infectious dermatitis. Infectious dermatitis can be a mixed fungal and bacterial infection; one bacterium in particular, Dermatophilus congolensis, is often implicated. This condition most commonly occurs over the trunk and on the hind legs, and forms deep flat scabs that may have puss underneath. Lesions on the back are often very tender. Some horses experience mild rain rot that is simply an aesthetic annoyance, but it can become severe enough that horses are uncomfortable wearing a saddle and need to be given time off until it heals. There are countless over the counter products and home remedies for rain rot… but I promise you’ll get the infection resolved and the skin healed quicker if you let your veterinarian be involved in treating your horse’s rain rot. Most veterinarians will prescribe some topical treatment such as antifungal/antibacterial shampoo or ointment. With the shampoo, you’ll need to throw in some good old fashioned elbow grease. Most vets agree that the scabs need to be shampooed until they loosen and can be gently picked off. Then, if using a medicated shampoo, you need to let it soak for 5-10 minutes before rinsing thoroughly. In severe cases, I also prescribe oral or injectable antibiotics. Rain rot can be contagious, so be sure to wash saddle pads and brushes after every use until the skin is cleared up.
The evil twin of rain rot is often called “scratches” or “dew poisoning” by horsemen. It is simply infectious dermatitis that occurs on the lower limbs, and is usually worse on white legs. It has the same cause and treatment but looks a little different. Pastern dermatitis usually consists of large raised scabs that are quite painful to pick off. One less common condition that can look very similar to pastern dermatitis is a photosensitivity caused by eating certain plants that contain high levels of photodynamic agents. Some examples of these photodynamic plants are: buckwheat, St. John’s wort, perennial rye grass and burr trefoil. Affected horses will have reddening and dryness that look like sunburn as well as large raised scabs on white skin of the face and lower limbs. Bringing horses out of the sun and off the pasture with the photodynamic plants will usually resolve the condition, although these horses occasionally have complications with liver disease, so you should consult with your vet if you suspect your horse has photosensitivity.
Although it is less common than infectious dermatitis, horses can also develop allergic dermatitis. The most common signs are recurrent hives and itching, but some horses will just have one symptom or the other. If the lesions are concentrated on the topline, your veterinarian will probably discuss insect sensitivity with you. The bugs that cause most insect sensitivities are Culicoides species, known in the south as “No-see-ums”. Horses can also be allergic to things in the environment such as pollens, hay, dust, specific feeds, and even cat dander! Steroids and antihistamines are the mainstay of treating these conditions in the short term, but severe cases may need to be given immunotherapy (allergy shots). The best way to diagnose these allergies is with blood or intradermal allergy testing, so that the specific allergens can be avoided if possible. Also, this allows immunotherapy to be created for that patient’s specific allergens.