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Medical Index
First Posted: Nov 2006
Jan 5, 2015

Rain Rot, Rain Scald, Streptothricosis, Dermatophilus Congolensis

By Debora Johnson

What is it? What causes it? How can one get rid of it? How can it be prevented? Can it be prevented? These are questions that every horse owner has asked at one time or another. If you are not among the horse owners who have asked these questions, it is just a matter of time!

The answer to these questions depends on who one asks. Is it a bacterium? Is it a fungus? It is called dermatophilus congolenis. Sometimes one may hear the reference to streptothricosis. Apparently it mimics both fungus and bacteria. This culprit is called an actinomycetes. It is often said that this microorganism is found in soil or dirt. There is no hard scientific evidence to support this belief. However, unclean conditions can help in its growth.

The actinomycetes must have a carrier horse that has this microorganism already on it to pass it along. Also, blankets, tack equipment such as brushes, saddles, pads, turnout sheets, rain sheets, halters, leg wraps, etc., that have the microbe on them can be the catalyst for transfer from one horse to another. Even a special rubbing spot on a stall or barn can harbor the organism. Sometimes the carrier horse does not present with rain rot because the horse may have natural immunity; however, it transfers the dermatophilus congolensis to another horse that is more susceptible to the organism. Chestnuts, grays, and horses with a thick coat seem to be more susceptible.

There are a number of conditions that encourage rain rot. The carrier horse has to be present, or as stated above, infected tack can transfer the microorganism. There has to be lots of moisture. Thick coated horses are more susceptible because the coat keeps the moisture close to the skin. There has to be an entry point for the actinomycetes to take hold such as a cut, sore, bite, or open skin of some sort.

Often it is difficult to get rid of rain rot. It can be a real pain for the horse owner! Eventually it will go away; however, to hasten the process it is important to know the culprit. Also, if left unchecked, secondary infections can greatly complicate the situation. This culprit is an aerobe, which means that it dislikes oxygen and loves carbon dioxide. The more oxygen it gets the quicker it dies. It needs carbon dioxide to grow.

What does one do to get rid of rain rot? First it is important to give the horse a bath using a shampoo that is suited to help kill microbes. It is also important to disinfect all tack and equipment to prevent spread. Antibacterial shampoos and rinses containing agents like chlorhexidine are effective for both equipment and animals. It is not advised to use ointments as they tend to hold moisture in while the goal of treatment is to keep these lesions dry. Gently scrape off the scabs. Reduce a heavy coat. I first try an over- the- counter iodine/soap mixture like a betadine scrub. Protect your hands. Use gloves. Keep it out of your eyes and your horse's eyes. Scrub the scabs well. Most horses hate this because taking the scabs off hurts. One can see the horse's skin crawl as the horse anticipates the scrubbing. To say the least, many horses do not like to cooperate. Often this process takes several days or even weeks to complete. A really bad case of rain rot can be awfully stubborn. There are many products on the market to use-betadine, iodine, phenol, nolvasan, and antibiotics. Sometimes ointments are used, but it is important that the air be able to get to the microbe. Ointments often make a barrier. Listerine worked for me with great success. Other people have suggested garden fungicides for roses. I stay away from these because they can cause blistering on the skin (your skin and your horse's skin) and it is difficult to keep powders out of the eyes. Also the powders can be inhaled. A friend uses a mixture of wintergreen rubbing alcohol and baby oil. Always check with your vet. Make sure that your tack is disinfected or the horses may keep being reinfected. To disinfect tack I use 2 tablespoons of Clorox in one gallon of water. Secondary infections may also set in, as they are opportunistic. Staph, strep, and rhodococcus are nasty infections and are usually treated systemically with antibiotics.

Rain rot is usually seen on the horse's back, the rump, and down the legs. There are often clumps of raised hair that have a scab base. Thick patches of scabs can be apparent. There is a crust. When scraped off one often will see ooze. Rain rot is also seen on the fetlock. Sometimes the culprit will be seen on the ear tips, along the horse's mouth, and on the hair near the eyes.

It is important to just do the best possible for the horse. If your horse is prone to rain rot, keep a rain sheet handy during inclement weather or put the horse under some kind of cover. It is impossible to make a perfect environment.

For More Information:

Understanding Rainrot
Horse Skin Problems
Horse Sweet Itch, Queensland Itch, Summer Itch or Culicoides Hypersensitivity, Tail Rubbing
Spring Itch and Your Horse

Medical Index