Sooner or later, you’re going to encounter a horse with rain rot. It’s found most often on horses maintained outside in times of high rainfall, especially if they have thick, winter coats. Download a PDF of this article here.
If untreated, these spots will spread and often coalesce. Any horse can develop this infection, but it is most likely to occur in horses with compromised immune systems from malnutrition, old age or diseases like Cushing’s.
The organism thrives in the warm, moist conditions against the skin. However, it can also occur as a result of bug bites, too. The same organism (Dermatophilus) also can cause severe heel dermatitis (scratches).
The lesions begin as multiple small bumps/scabs, which can spread and grow together to form large patches if not detected and treated early. The scabs are tightly adherent to the skin and painful to remove. They come off with the hair and leave open sores. Once the scabs are off and medications can get to the skin level itself, healing is usually rapid (see below for help removing scabs).
A variety of treatments can be used. Iodine-based shampoos and other medicated or herbal shampoos with tea-tree oil may work on mild, early cases, but when crusting is heavy the most they can usually do is slow the appearance of new lesions.
We’ve round the combination of EQyss Micro-Tek shampoo and Gel or the Absorbine Medicated Twin Pack Shampoo and Spray are both good for moderate cases, although they may not penetrate heavy areas of scab.
For areas of thick scabbing, we like the application of a tea-tree-oil based sheath cleaner, like Excalibur from Farnam. Wet the area and leave it on five to 10 minutes then rinse. Crusts/scabs may come off after a single treatment. The animal can then be bathed with a tea-tree-oil shampoo and open areas protected with a tea-tree-oil-based cream. Note: Some horses are sensitive to tea-tree-oil, so do a spot treatment and wait 24 hours to see if there’s a reaction before using the treatment.
A very highly effective is the antifungal dust, captan. Bathe the horse to remove surface dirt and any loose scabs then rinse a solution of ½ oz. Captan to 150 oz. of water over the infected areas. Pour it on or apply with a sponge. Scrape away the excess, then let it dry on the coat.
Some vets advise to just sprinkle the dry powder on the horse while still wet from the shampooing. Captan is an EPA-registered fungicide, intended for garden use, so be sure to follow safety instructions on the package. Captan should never be used around the horse’s eyes, as it is a severe eye irritant. Wear protective gloves and eye goggles while applying it, as it is very dusty.
Sprayed-on original Listerine is a popular treatment for mild to moderate cases of rain rot. After spraying, just leave the Listerine on the horse. Remember, though, that it can be painful on open skin, if there are open areas under those crusty lesions. Once the initial sting is over, though, its mild anesthetic and anti-itch properties kick in.
In the fungal category, horses can get ringworm, a highly infectious fungus that can also be transmitted to people. Ringworm infections are circular lesions that start as an inflamed and scaly area, very itchy, which grows out from the center. Because of how easily it is spread, ringworm is best treated with antifungal medications.
Captan also works well on ringworm, or you can use a topical antifungal cream from your vet or a human product made for fungal foot or vaginal infections.
The most severe form of ringworm is caused by the fungus Trichophytum, which can spread to humans. Trichophytum infections usually cause itching, while milder infections with other fungal strains will not. If your horse has an infection that looks like ringworm and is also itchy, call your veterinarian. You want the heavy guns (prescription remedies) to deal with this, and you don't want to risk it spreading from horse to horse to you.
Very young, debilitated/malnourished and immunocompromised horses are at greatest risk for ringworm. Bacteria as the primary cause of skin problems is rare, but secondary infection can occur in any area with broken skin.
Other fungal species can also cause problems, especially on the face and at the tail base. They typically do not cause as intense an itching. The lesions are hairless patches with varying degrees of scaling but are not as angry looking as true ringworm and are slow to spread or enlarge.
Medicated shampoos aren't everyday shampoos, so save them for when the horse obviously has a skin problem that calls for antimicrobial effects. If you need a daily shampoo, are favorites are Corona, Orvus, Lucky Braids and Absorbine. On the herbal end, again, tea-tree oil is a good ingredient choice for both antibacterial and antifungal effects. Aloe is a common inclusion and helps by soothing and encouraging healing rather than antimicrobial effects.
Some shampoos still include iodine but their concentrations are only half of what you find in iodine-based skin scrubs. Iodine’s drying and irritating effects can only be prevented by skin conditioners and aloe.
Phenol (carbolic acid) was the first disinfectant used for surgeries, back in the late 1800s. It’s also an effective local anesthetic in open wounds, but concern about potential for tissue damage has led to use of more modern disinfectants.
Benzalkonium chloride is the active ingredient in hand sanitizers, diaper wipes and some dandruff shampoos. Triclosan is a widely used antibacterial with antifungal activity as well. Another common ingredient is PCMX (chloroxylenol), which is a rapid acting antibacterial and antifungal with very low toxicity.
Others have had success with human dandruff shampoos, and that can work in many cases as well. We're just not fans of using human shampoos on horses because of the difference in pH. And, we feel many rain rot cases need a double-barrel shampoo-spray treatment.
The best antimicrobial in the world won’t do any good if it can’t get at the organism. The ability to effectively soften and release dense buildups is therefore important.
For treatment of difficult problems like rain rot and heavily scabbed scratches, we got the best results combining a medicated shampoo with a medicated topical spray or cream to use between baths. We’ve found that combining products with different types of active ingredients runs the risk of creating additional problems, such as chemical burns. And, as much as we like Captan for really difficult cases, it isn't our first choice.
For that reason, our top choices include a number of spray/shampoo combination products that we have had success with over the years: