Antibiotics for rain rot in horses

A bacterial organism known as dermatophilus congolensis, which thrives in high moisture conditions, causes rain rot. Interestingly, while it is classified as a bacterium, it acts like a fungus in many ways. Ultimately, rain rot presents itself as painful scabs that form most often over the horse’s topline. When the scabs are pulled away, they tend to take clumps of hair with them, leaving bald patches over the horses back, hips, face, and other areas. In severe cases, the lesions may become extremely large, and impact multiple layers of skin, which can impair the use of the horse until the condition clears up. It also seems that light colored horses may be more susceptible to rain rot, however, any horse can present with the condition.

When the scabs from a rain rot infection are pulled away, they tend to take clumps of hair with them. Credit http://www.horseandhound.co.uk

Although dermatophilus congolensis is commonly found in the soil, rain rot seems most likely to occur when the skin is softened or compromised, as is the case in high moisture conditions, hence the name. Interestingly, however, young horses (and possibly old horses) with poorly developed or functioning immune systems also seem to be more at risk of rain rot. Further, some horses will carry the rain rot organism without ever showing signs of the actual condition, but they can spread it to other horses.

Curing rain rot can often be as simple as bathing the horse with an antimicrobial shampoo, cleaning or replacing grooming tools, and keeping the horse dry and separated from other horses with active cases when possible. The usual treatment presents a bit of a problem during a Michigan winter, however, in that bathing horses is a challenge for many. Topical antimicrobial products may be useful and in very severe cases of rain rot, the horse may require antibiotic injections. Severe cases that do not respond to treatment may also require a skin biopsy to determine the exact cause of infection. Michigan State University Extension recommends contacting your veterinarian if you suspect severe rain rot.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).

The cause of rain scald is a bacteria known as Dermatophilus congolensis. This bacteria lives on the skin of carrier horses. These horses are prone to developing rain scald, but do not always show clinical signs of it. They act as the source of infection for other horses. As well as there being a carrier animal nearby, two other conditions are required for a horse to be infected: moisture and skin trauma. When the carrier animal’s skin is subject to severe wetting, such as occurs during rain, the bacteria release zoospores, which are the infective stage of the bacteria. Biting and non-biting flies then transmit the zoospores from the carrier horse to a non-infected horse. Brushes, rugs, etc may also be responsible for transmitting the zoospores. If the skin of the non-infected horse is healthy and intact, the bacteria cannot invade and cause rain scald. However, if the skin is damaged, such as occurs with prolonged wetting, fly bites, or other skin disease, the bacteria can invade the skin and cause disease.

The most commonly affected areas are the croup, loins, saddle area, neck and face; when it rains these are the areas of the body that become the wettest. However, in severe cases the disease may become generalised. The infected skin produces discharge which dries and causes the hair to become matted; these mats of hair have been described as resembling small paintbrushes. When these crusts are lifted, the underlying skin is often pink, moist, and occasionally bleeds. Greenish-yellow pus often accumulates on the under surface of the crust. Affected areas can be sore to touch but the disease does not cause the horse to be itchy. This disease can also affect the limbs, and white, non-pigmented skin is more susceptible. Racehorses are particularly prone to this disease occurring, particularly on the hindlimbs, due to very small abrasions of the skin caused by flying debris thrown up by the front hooves during training. When the legs are affected, they can become painful and swollen, causing lameness. Severely affected horses may develop a fever, be depressed, inappetant and lethargic, with swollen lymph nodes. There are usually other factors involved for a horse to become severely affected, such as an underlying disease causing suppression of the immune system.

Q: My horse gets rain rot. What exactly is it and how can I treat and prevent it?

SAMUEL D. HURCOMBE, BSc, BVMS, MS

A: Rain rot is a layman’s term for a common equine bacterial skin disease caused by Dermatophilus congolensis. It is characterized by the formation of crusty scabs, which peel off along with clumps of hair, leaving bare spots on the skin. As its name implies, rain rot appears on the parts of the body exposed to rain: the top of the head, neck and back. It rarely occurs on the legs or belly. We see it most frequently in regions where warm temperatures, high humidity, heavy rainfall and insects create ideal growing conditions for the bacteria—for example, the southern United States in the wintertime.

As an Amazon Associate, Practical Horseman may earn an affiliate commission when you buy through links on our site. Product links are selected by Practical Horseman editors.

The bacterial spores that produce this disease cannot penetrate a healthy horse’s natural skin barrier unless the skin experiences trauma (such as a wound, scrape or abrasion), an insect bites the horse or excess moisture on the skin strips away its normal layer of protective oils. Any of these events can allow the spores to invade the outer skin layers, where they then spread. In response to this invasion, the horse’s immune system produces extra white blood cells and proteins, which accumulate in small, pus-filled bumps on the skin, called pustules. If you run your hand along your horse’s coat, you will feel these lumps.

As each pustule matures, the skin layer beneath it dies off. The resulting dead skin cells clump together with surrounding hairs, forming scabby tufts (sometimes called “paintbrush lesions” because they resemble tiny paintbrushes), that can be easily pulled off. During this period, your horse’s skin may be sensitive and/or itchy. If he scratches the area, he may traumatize the skin even further, thus spreading the bacteria.

Individual lesions may appear in a spotty pattern across your horse’s body or they may coalesce into more expansive areas, leaving broad patches of bare skin. After the scabs and hair fall off, the skin underneath will appear red and raw. If the predisposing environmental factors are removed, the skin quickly re-epithelializes (heals)—usually within seven to 10 days—and new hair begins to grow in.

On the other hand, if you do not address the environmental factors—for example, your horse continues to stand outside in the rain without protection—rain rot can recur all winter long. Left untreated, it may lead to secondary infections, such as Staphylococcal folliculitis. In very rare situations, it can progress to a point at which the bacterial population overwhelms the horse’s immune system. These extreme cases may exhibit fever, pain, loss of appetite and other signs of discomfort in addition to large areas of hair loss.

Rain rot is often mistaken for fungal diseases, such as ringworm, which are far less common. Antifungal medications have no effect on the bacteria, so it is very important to diagnosis the condition correctly. Your veterinarian can perform a definitive diagnosis by culturing a lesion, although most simply identify the disease clinically.

The vast majority of healthy horses heal spontaneously from rain rot without any treatment necessary, so long as the environmental factors are removed. If your horse is turned out frequently in rainy conditions, be sure he has access to shelter—a shed or lean-to. A light, breathable sheet may help to protect him from rain, but avoid heavier blankets that trap moisture against his skin. Clean and wash all of his tack and equipment thoroughly and take a break from riding if he develops lesions in the saddle area (the saddle will trap sweat against his skin and may cause discomfort while the lesions are active). When you bathe him, scrape excess water off his body and keep him inside until his coat dries thoroughly. During the buggy season, apply insect repellent regularly.

These measures will solve the vast majority of rain-rot cases. If your horse’s symptoms persist, ask your veterinarian to recommend a shampoo containing a keratolytic agent, such as benzoyl peroxide. Such products strip away dead layers of skin, along with crusty scabs and bacteria, and help to treat secondary conditions. I would not recommend treating a topical disease like rain rot with systemic antibiotics. This would not only disrupt the natural micro-ecology of your horse’s gastrointestinal tract unnecessarily but also contribute to the broader problem of antibacterial resistance created by the overuse of antibiotics.

With a few simple, preventive measures and extra diligence during the rainiest times of the year, you should be able to cure your horse’s rain rot for good.

A native of Australia, Samuel Hurcombe, BSc, BVMS, MS, studied veterinary medicine at Murdoch University in Perth before completing his internship in Sydney. He moved to the U.S. in 2005 and did a three-year internal-medicine residency followed by a two-year critical-care fellowship at The Ohio State University. He served as an assistant professor of equine emergency and critical care at Ohio State from 2010 to 2014. Last July, he joined the faculty at Cornell University, where he focuses his research and clinical work on gastrointestinal disease, specifically medical and surgical colic treatment. Dr. Hurcombe is a diplomate of the American College of Veterinary Internal Medicine and the American College of Veterinary Emergency and Critical Care.

What is the fastest way to cure rain rot?

How do you get rid of rain rot on horses fast?

What is the best treatment for Rain Scald in horses?

Is Vetericyn good for rain rot?