If spring comes, can rainrot be far behind? If you’ve got horses, you’ll want to be prepared for this relentless, all-too-common, crusty skin affliction.

By the end of winter, your horse’s skin, whether insulated by a long, shaggy haircoat or bundled under a blanket, has become a hotbed for rainrot. Then, when the weather warms up and spring rains move in, the trouble begins.

The telltale bumps of rainrot typically appear on your horse’s back first. But they can easily pop up anywhere and everywhere on his body, including his face, rump and legs. These crusts, lesions and scabs can be itchy or painful and often contain pus. You’ll notice bald patches, with hair coming off in clumps. Rainrot can be frustratingly persistent and lead to other, more debilitating infections in vulnerable horses.

So, while not every horse will get rainrot, it makes sense to do what you can to prevent it. As with any battle, it helps to understand your foe and choose your weapons wisely.

Causes and contributing factors

Rainrot is an infection of the skin’s top layer. It is caused by a species of bacteria called Dermatophilus congolensis. This microorganism normally lives dormant in the skin until awakened by a compromising of the skin’s natural defenses.

This can occur any time the skin’s outer layer breaks down due to minor cuts, scratches or abrasions. Activation is boosted by continuous exposure to heat and moisture—exactly what daily turnout in wet, warm weather might bring. This triggers a bacterial invasion and an inflammatory response.

In most regions, the spring and summer seasons provide the perfect conditions for rainrot because of frequent rain, high humidity and warmer temperatures. These conditions allow the bacteria to thrive. If you live in a tropical zone, rainrot is more likely to be a year-round problem.

Because rainrot spreads easily—on a horse’s body and from horse to horse via shared equipment—you can quickly go from one case to several in a short period of time. Three factors increase susceptibility to rainrot:

 • youth;

• continuous exposure to wet conditions; and

• a weakened immune system (most often seen in old horses and those recovering from illness).

Diagnosis and treatment

Rainrot is common enough that most horse keepers recognize the signs and initiate treatment accordingly. And, for the most part, this is safe to do.

However, rainrot can sometimes resemble other skin disorders, and vice versa. Rainrot that resists treatment might not be rainrot at all. To definitively diagnose a D. congolensis infection, your veterinarian will rule out other conditions like ringworm or a staph infection. This could involve taking sample scabs or skin smears to examine under a microscope for the presence of the offending actinomycete.

In healthy horses, mild to moderate cases of rainrot usually subside on their own within two to three weeks. But you’ll need to address contributing environmental factors—like continual exposure to moisture. The scabs and hair will fall off. But the raw skin should heal and new hair growth should emerge within about 10 days. If your horse has only a small patch of rainrot and the weather is dry, waiting it out may be an option. However, you’ll want to address more extensive or stubborn cases directly.

You might hear about over-the-counter home “remedies” for rainrot like mineral oil, white vinegar, rubbing alcohol, hydrogen peroxide or essential oils. Resist the urge to try these. Not only are most ineffective, some can actually irritate or damage your horse’s skin even further or cause additional problems.

“I would avoid applying home-remedy oils or lotions to the area unless directed by a veterinarian,” says Karen Moriello, DVM, DACVD, clinical professor of dermatology at the University of Wisconsin-Madison’s School of Veterinary Medicine. Also avoid “drying powders” of any kind, she adds. These “can lead to microscopic foreign body reactions” in the skin, she says. 

Instead, says Moriello, treatment of rainrot calls for:

Removing the affected horse from any location that is damp or wet.

This doesn’t mean keeping your horse indoors—just ensure he has access to (and uses) shelter or bring him in when it starts raining. A light, breathable sheet can help protect him from rain without trapping moisture against the skin, which would make matters worse.

Gently dislodging the scabs

Soak the lesions until they soften and the scabs slide free from the coat. Moriello recommends using plain warm water for the first few soaks to loosen and remove the largest amount of crusts and scurf. Because the affected area can be very painful, “… it cannot be emphasized enough how gentle one needs be,” she notes. “Soaking the lesions will facilitate removal of the matted hairs and exudate. It is very important not to cause further microtrauma to the skin, so the owner will want to avoid using brushes.”

Resisting the urge to “pick” the scabs off the skin.

“Scabs should be soaked thoroughly so that they just rinse off or are able to be removed with very gentle traction,” says Moriello. “It may take several soakings of the affected area. This can be facilitated by using a ‘showerhead’ garden watering tool. This can help loosen the debris and rinse it from the skin surface.”    

Taking measures to kill the bacteria

Clipping thick hair around the lesions and using a prescription antibacterial shampoo, such as a chlorhexidine product, will help kill the bacteria. “What is important here is to not apply the antibacterial soap directly to the skin,” Moriello stresses. “It is hard to rinse off thoroughly and can cause an allergic or irritant reaction.”

Instead, Moriello recommends diluting the antibacterial soap in warm water and then applying the thoroughly mixed concentration to the skin. Then rinse the skin thoroughly. “Care should be taken to rinse off the sudsy solution from the ‘downstream’ part of the sudsy solution,” adds Moriello. “Then pat the skin dry; do not rub, as this can cause microtrauma.”

If your horse is left with widespread bald patches, it might be necessary to protect the bare skin with a thick antibacterial ointment, Moriello says. (Cream formulations are too thin, so be sure to use an ointment.)

Preventing the spread of rainrot

Isolation of the affected individual is the most effective way to prevent the spread of rainrot to other horses.

Also wear disposable gloves and wash your hands after handling a horse with rainrot. Additional biosecurity measures worth taking include cleaning, disinfecting and drying any tack, grooming tools and other gear that come into contact with the affected horse. Of course, the repeated use of these items on a horse with rainrot or sharing them with other horses is a bad idea.

When rainrot coincides with fly season, it’s also time to step up your insect control program. Flies or mosquitoes that land on an infected horse’s coat can carry the rainrot organisms to healthy horses. Therefore, it’s smart to apply insecticides frequently and use barrier clothing such as fly masks, sheets and boots.

Tips for protecting pasture dwellers

Outdoor living is generally very healthy for horses, living under the elements can increase susceptibility to conditions like rainrot. What can be done to protect horses living outdoors 24/7? Moriello offers the following suggestions:

• Provide some shelter for your horses to get out of the rain from the hooves up. “Horses are pretty smart and will want to stay someplace dry (or drier) if it feels safe,” she says.   

• Make sure your horses have access to sunlight, which will dry the haircoat.

• Maintain good nutrition.

• Pay particular attention to the health of skin under blankets or sheets.

• Practice good insect control. Rainrot often starts with microtrauma to the skin, and insects are the biggest offenders. But anything that causes nicks, cuts or scratches can lead to the infection, including contact with thorny plants or stiff brushes.

• Seek better accommodations for susceptible horses. Some individuals require special care.

Rainrot is common in the spring, but it doesn’t have to be a seasonal nightmare. With precautionary measures, early diagnosis and proper care, you can prevent many cases and control the minor ones that do arise, leaving you and your horse to enjoy the season.