If spring comes, can rainrot be far behind? Anyone with horses has probably had at least passing familiarity with 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, caused by a species of actinomycete (a type of bacterium) called Dermatophilus congolensis. This microorganism normally lives dormant in the skin until awakened by a compromising of the skin’s natural defenses.

This activation can happen any time the skin’s outer layer breaks down due to minor cuts, scratches or abrasions, and is further aggravated by continuous exposure to heat and moisture—such as a horse might experience in daily turnout in wet, warm weather. This triggers a bacterial invasion and an inflammatory response.

In most regions, the spring and summer seasons typically 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—across a single 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 an infection of D. congolensis, your veterinarian can examine the lesions to 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 rainrot usually subsides on its own within two to three weeks if contributing environmental factors are removed. 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 drying up, waiting it out can be a reasonable treatment approach. Stubborn cases, however, may need to be addressed directly.

Avoid home remedies

You might hear about over-the-counter home “remedies” like mouthwash, white vinegar, rubbing alcohol, hydrogen peroxide, essential oils and even antifungal medications for stubborn rainrot. Resist the urge to try these. Not only are most of them ineffective, some can irritate or damage your horse’s skin even further.

“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, as these “can lead to microscopic foreign body reactions” in the skin. 

To treat rainrot

Instead, says Moriello, to treat rainrot do the following:

Environment

• Remove the affected horse from any location that is damp or wet: This doesn’t mean your horse needs to be shut indoors—just ensure he has access to (and uses) shelter or is brought 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.

Skin care

• Gently dislodge the scabs: Soaking the lesions until they soften and the scabs slide free from the coat, together with the administration of appropriate antibiotics such as penicillin by your veterinarian. 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.”

Resist 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, and this can be facilitated by using a ‘showerhead’ garden watering tool. This can help loosen the debris and rinse it from the skin surface.”

Thwart bacteria

• Take measures to kill the bacteria: Clipping thick hair around the lesions and the use of a prescription antibac-terial shampoo, such as a chlorhexidine product to 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 ends up 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. But that’s not always easy to do.

Because the causal organism can be transmitted to people, anyone handling a horse with rainrot needs to wear disposable gloves and/or wash their hands afterward. Additional biosecurity measures worth taking include cleaning, disinfecting and drying any tack, grooming tools and other gear that have come into contact with the infected horse. Also avoid sharing these items used on infected horses with others.

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 easily transmit the organism responsible for rainrot to others. 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. But those who are continually under the elements are more prone to the conditions that cause chronic rainrot. What can be done for 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.   

Ensure that your horses have access to sunlight, which will dry the haircoat.

Maintain good nutrition.

Pay particular attention to the 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 seem to be more severely affected than others and require special care.

Rainrot is common in the spring, but it doesn’t have to be a seasonal nightmare. Precautionary measures and proper care, you can prevent many cases and control the minor ones that do arise. And that will leave you and your horse more time to enjoy the season.