Often, the first signs of rainrot arise just hours after a horse is brought in from the rain: Tufts of hair begin to stand upright in patches over his back, sometimes following the contours of the “drip line” where water runs off his body. Or the tufts might appear on the legs, where he has been wading in deep, wet grass.

The areas with raised hair feel warm to the touch, and the horse may flinch from contact. By the next day painful, tight scabs will have formed over pockets of yellow-green pus. Rainrot is an infection caused by Dermatophilus congolensis, which usually resides on the skin in a dormant state without causing trouble. However, persistent moisture can activate the bacteria, leading to infection. Advanced age, illness, stress, the administration of corticosteroids or other factors that can compromise immune function increase a horse’s susceptibility to rainrot.

Although it’s not the most serious skin infection a horse can contract, rainrot can cause discomfort and hair loss, so it’s good to get a developing case under control as quickly as possible.

Here’s what to do

• Cleanse the coat with an antimicrobial shampoo. A number of products are labeled for use against rainrot, with ingredients including povidone iodine, chlorhexidine, benzoyl peroxide or other antimicrobial agents. Many are available over the counter and can be very effective in mild cases when used as directed. You may need to repeat the treatment over several days.

• Dry out the coat. Bring the horse into a dry environment and rinse off any dirt or mud that may be holding moisture against his skin. Then let his coat dry thoroughly. If the weather permits and it’s not too painful for the horse, clipping longer hair may encourage air to circulate against his skin.

• Apply a topical antimicrobial product. A number of rinses, washes and wipes with antimicrobial ingredients can add an extra bacteria-killing boost to your rainrot treatment. In fact, in very mild or localized cases, a topical rinse or wipe may be all that is needed to control the infection.

• Take sanitary precautions. Remember that D. congolensis can be spread to other horses, so it’s a good idea to wear rubber gloves when working with an infected horse. You’ll also want to avoid using the infected horse’s tack and grooming tools on others. Once the infection has healed, disinfect brushes and other tools in a bleach solution to kill any pathogens they may be carrying, and lay them in the sun to dry. Also wash blankets, coolers and saddle pads that have come in contact with the infected horse and dry them in the sun or in a hot dryer.

To pull or not to pull

You’ll find conflicting opinions on whether it’s better to pull off the scabs and raised tufts of hair typical with rainrot or leave them to fall off on their own. Each approach has benefits and drawbacks.

D. congolensis bacteria are anaerobic, which means they thrive in the low-oxygen environment under the crusts. Gently pulling the scabs exposes the underlying skin to air, which helps to kill the bacteria, and it also allows the antimicrobial products to reach their targets more effectively. On the other hand, pulling on large or tightly bound scabs can cause bleeding and irritation that may lead to complications, and handling them is often painful to the horse, which may cause him to bite, kick or otherwise resist.

Rubbing the crusts with mineral oil or soaking with warm water prior to shampooing may loosen rainrot scabs and help you remove them more easily. The best general rule to follow is to remove crusts that slide off easily, but leave the tight, painful ones until they loosen on their own. It’s not necessary to remove every scab with every treatment.

Call your veterinarian when …

… a horse develops an especially severe, widespread infection. If the rainrot causes painful crusts over wide swaths of your horse’s body, your veterinarian may provide you with a prescription-strength antimicrobial treatment as well as a systemic antibiotic and phenylbutazone or Banamine to address the pain and inflammation. She may also want to examine the horse for an underlying illness, such as Cushing’s disease, that may be compromising his natural immunity.

… rainrot grows worse or does not respond to treatment. Drying out the coat and applying antimicrobial treatments ought to lead to improvement within a few days. If not, you may be dealing with something else. Your veterinarian will want to rule out other skin issues that can look like rainrot, such as ringworm, scratches, allergies, insect hypersensitivities or even certain types of sarcoids—which require different treatments.