Rain rot (or rain scald) is a bacterial skin disease that often behaves like a fungus. It causes an infection under the horse’s skin that makes pustules of pus, usually on the back, shoulders and flanks where rain runs down the horse, hence the name. The skin over top of these pustules will eventually slough off, taking off some hair. These small pieces of scab attached to hair are described as “paintbrush lesions” because they resemble a fine paintbrush.
If the disease progresses, the horse may have areas of alopecia (hair loss) on his shoulders, back and rump. Rain rot may also be found on a horse’s face and under his mane. Severe conditions can be painful and lead to secondary infections.
A Perfect Storm of Causes
The main bacterial pathogen is Dermatophilus congolensis, but because it can be difficult to find the actual bacterium by examining a skin smear or using PCR (polymerase chain reaction) testing, some researchers suspect that other bacteria are involved. Veterinary researchers aren’t sure if the bacteria “live” on an affected horse, or if they are picked up in the horse’s environment. Some horses may also be considered as carriers of D. congolensis. It’s certainly possible that one carrier is infecting others in the herd, but the disease is probably more of a reflection of the horses’ environment and how they are being managed rather than the infection being spread from horse-to-horse.
While rain rot is mainly found in tropical locations, cases can also develop in more temperate climates such as in the western provinces and southern Ontario. For example, while I was interning in Florida, I saw multiple cases of rain rot because it rains a great deal there. There are also many biting insects that cause breaks in the horse’s skin, and that’s how the bacterium enters the horse’s system.
In western Canada, veterinarians see cases of rain rot during the winter and early spring when horses have thick coats. When there’s wet snow, horses can get a blanket of crusty snow on their backs. If owners don’t brush off the snow, it will slowly melt and keep the horses’ skin damp – making them more susceptible to rain rot.
If not used properly, blankets may lead to rain rot. For example, if you blanket a horse while he’s still sweaty from a workout, the damp environment under the blanket could potentially promote the condition. Another situation is when owners blanket horses that have full winter coats – the animals get overheated and sweat. In these cases, rain rot can develop in the horses’ armpit and groin areas.
Timely Treatment & Preventative Measures
If you suspect your horse has rain rot, call your vet and decide on treatment based on the severity of his case. The first step is to shield the affected area from exposure to moisture and allow oxygen to reach the skin. Remove wet sheets and blankets and let your horse dry before putting on dry ones.
You will then use a chlorhexidine (antibacterial) soap to wash the specific areas showing signs of rain rot, and then dry these areas very well. Once the skin is soft, you can gently scrape off some of the scabs. I recommend repeating this process every day or two for about a week, and then once a week until the pustules disappear. Be sure to wear gloves because D. congolensis bacteria is transmittable to humans. During treatment, try to keep your horse out of the rain as much as possible. It’s also a good idea to use only one set of brushes, blankets and tack per horse. In most cases, antibiotics aren’t used as treatment, but if it’s a severe case, you may need to give systemic antibiotic drugs to help the horse get over the acute phase.
Groom your horse regularly so you can keep an eye on the condition of his skin. Spend extra time during the wet seasons, when his coat is thick.
In addition to treating the disease, your vet will discuss making changes to your horse’s environment (ensuring that he has access to adequate shelter) and management practices (not blanketing unless it’s necessary). If the problem persists, there may be other reasons that make the horse vulnerable. Rain rot can develop in elderly horses or immunocompromised animals. Neglect can also lead to rain rot. Malnourished horses aren’t healthy enough to fight off the bacterial infection.
If your horse is turned out all the time, give him a place to go where he can completely get out of the rain. If the climate is very humid, give him a shady place to get out of the sun. You can even set up fans so horses can dry themselves off. Regular grooming to check your horse’s skin condition, particularly if blanketed, is key. Let horses dry out completely after untacking before putting a blanket on and don’t over-blanket so that your horse is sweaty underneath. Use repellants or flysheets and masks to minimize insect bites. If your horse’s skin is hypersensitive to insect bites, your vet can give intramuscular doses of long-acting steroids to reduce the urge to scratch and damage skin.
If the condition is a one-time event, then the disease is straightforward to treat. But if the horse has underlying issues that make it more susceptible to rain rot, then you need to work with your vet to identify and address the reason your horse is more prone to the disease. If a horse is immunocompromised or malnourished, you will need to address those issues. If your horse gets rain rot because he doesn’t have a place to get out of the weather and nothing is done to change that situation, he will likely get the disease again. Treating and managing rain rot can be a drawn-out process, so prevention is a much better option.
Dr. Suzanne Mund, who graduated from the Western College of Veterinary Medicine (WCVM) in 2013, is a resident in large animal surgery and a Master of Science graduate student in the WCVM Department of Large Animal Clinical Sciences.