Book Excerpt: Rainrot

Editor's Note: This is an excerpt from Care and Management of Horses by Heather Smith Thomas. This book is available from

Rainrot (rain scald) is a skin problem that often appears during wet weather. Typical signs include very sensitive skin, clumps of hair coming off, and raw spots or crusty patches on the horse's back. After a rain you may see the horse's hair standing up in an odd pattern on portions of the body that got wet and where water ran down off the horse's sides. As you run your hand over the horse, you might feel heat and his back may be sore. By the next day he may have tight scabs on the sensitive areas. The scabbing may be a light peppering of small bumps, or the whole area may be a painful sheet of crusty, scabby skin.

The scabby bumps and crusts tend to be located in the runoff patterns on the horse's back and body, such as down the flanks and over the shoulders, back, rump, and neck. A heavy rain that wets the whole horse may cause bumpy crusts over most of the body, while a light rain causes only scattered patches. You may first notice the problem when brushing the horse. In early stages it is easier to feel the emerging bumps than to see them, and the bumps may come loose as you rub. They may be hot and tender and the horse may be sensitive when you brush him. As the disease progresses, the bumps become more raised and tufts of hair stand erect.
Rainrot is caused by the bacterium, Dermatophilis congolensis, which seems to have characteristics of both fungi and bacteria. It normally lives in the soil (dirt and mud of a pen or pasture) and is present in dust particles. The combination of water and dirt, such as when a dusty horse gets wet, makes an ideal environment for this opportunistic invader. This microbe can live in a dormant state within the skin for long periods. If the skin is compromised in some way, such as prolonged wetting by rain or high humidity, moisture enables dormant microbes from earlier lesions to establish new infection sites.

Rainrot is rarely a problem in dry weather. Best prevention is to keep horses clean and dry. If rainrot is a recurring problem, regular grooming, with periodic vacuuming to get as much dust and dirt as possible out of the hair, and a bath twice a month with a medicated shampoo can prevent this skin problem.

A common scenario for rainrot development is a period of rain following a dry, dusty spell. The dusty conditions fill the horse's coat with dirt (especially if he rolls), covering the skin with the microbe-laden particles. The rain then provides moisture for the organism to multiply. The problem often continues into winter if weather stays wet. A long, thick winter coat provides ideal conditions for the organism to keep multiplying.

Rainrot does not occur as much in stabled horses as in outdoor horses because they don't get the chance to roll in the dirt and pick up the microbe. The disease also occurs less frequently in horses that are regularly brushed, keeping the skin free of dust and dirt that might harbor the microbe. Sweat, skin secretions, dirt, and manure in the hair of an unbrushed horse can combine with moisture to get the organism started. Some horses seem more susceptible to rainrot infection. If kept on pasture or in a dusty paddock, they tend to develop the problem every year.

Rainrot is not contagious; it is spread by organisms in dirt and dust rather than by direct contact with an infected horse. It may appear in several horses at the same time, however, if they are kept in the same environment. Rainrot can be spread from one horse to another by dust and dirt on grooming tools and saddle pads. Don't use the same brushes or tack on more than one horse. You may want to disinfect grooming tools occasionally with a mixture of one part bleach and three parts water.
Treatment of rainrot consists of cleaning skin and hair with an iodine shampoo (or human dandruff shampoo or an antiseptic pet shampoo). You may have to shampoo the horse daily for seven to ten days (then several times a week) to get rid of bacteria-laden dirt in the hair coat. Massage the skin as you wash it, gently working loose the scabs and crusts. Leave the shampoo on for five to ten minutes before rinsing it off so the iodine has time to do some good. You can follow the shampoo and rinsing with a diluted iodine solution (one part povidone iodine to ten parts water) and leave it on the horse to dry.

After the horse is clean and dry, apply a mixture of equal parts tamed iodine (povidone iodine, such as Betadine, which is not as harsh as tincture of iodine) and mineral oil to affected areas (or an ichthammol salve — ointment made from a coal-tar base). The tamed iodine kills bacteria, and the mineral oil soothes raw spots and softens crusts, making removal of scabs easier next time. Never use iodine undiluted on the horse's skin or it may cause burning and irritation. Most horses tolerate diluted iodine with mineral oil, however. The oil leaves a coating on the hair that lasts several days (until next bathing), keeping the iodine in contact with the affected area longer.

Some veterinarians recommend a course of antibiotics, starting at the beginning of symptoms, to halt the infection before it gets well started. Often this halts the problem before it progresses to hair loss. Some cases of rainrot run their course and heal without treatment, but it takes longer. Serious infections should always be treated to prevent complications and scarring.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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