What are those bumps on your horses’ rump?

November, 2010
Lindsey Wiggins - Multi-County Livestock Agent (Hendry, Glades, Charlotte, Lee, and Collier)


Cause
"Rain rot or rain scald (also known as dermatophilosis) is caused by bacterial infection, and it often is mistaken for a fungal disease," Swinker says. "The bacteria live in the outer layer of skin and cause pinpoint to large, crusty scabs."

Dermatophilus congolensis, the bacterium found to cause this infection, lives dormant within the skin until the skin is compromised in some way, which can happen when there's prolonged wetness, high humidity, high temperature, or attacks by biting insects, according to The Merck Veterinary Manual. Warm temperatures and high humidity can also cause an increase in the number of biting insects (particularly flies and ticks) present that can spread the infection from horse to horse.

Rain rot occurs when the infective zoospores reach a compromised skin site. Swinker says, "The zoospores germinate and produce hyphae (threadlike tentacles), which penetrate into the living epidermis (skin) and spread in all directions, resulting in an acute inflammatory skin condition."

Diagnosis
It's easy to diagnose rain rot, generally on visual confirmation of the skin lesions alone, but a more definite diagnosis can be made by examining a skin scraping for Gram-positive bacteria under a microscope or by culturing the bacteria. Horses with long winter coats will develop paintbrush lesions (raised, matted tufts of hair) along their dorsal surfaces, which include the neck, withers, back, and croup, as well as on the lower limbs. Light-skinned areas are usually more severely affected. As the lesions get larger and join together, they will progress to a crust or scab formation that when removed will expose yellow-green pus between the necrotic and living skin layers. Swinker says, "If rubbed, the 'bumps' might rub off in the form of scabs with a small, hairless spot of skin showing."

Treatment
Most acute cases of rain rot can heal on their own; however, Swinker recommends treating even minor cases because the lesions can spread and worsen. The lesions also interfere with use (especially if they are located along the horse's back or withers) and they can be painful for the horse. She says, "In early or less severe cases, simply removing the scabs (by bathing the horse) with antimicrobial shampoos or rinses, containing agents like chlorhexadine, and currying will take care of the problem." The same antimicrobial shampoos and rinses can also be used to sanitize tack and equipment. However, more severe cases in which the infection has affected deeper skin layers might require your veterinarian to give antibiotic injections (such as procaine penicillin and streptomycin). It is not advised to use ointments for this skin condition because they tend to hold moisture in, while the goal of treatment is to keep the lesions dry. Unlike most skin conditions, rain rot is not usually “itchy”, but it can be painful to the touch. Be cautious when bathing or removing the scabs.

Prevention
Practicing good hygiene, such as daily grooming with clean brushes, along with reducing environmental risk factors (constant wet and/or humid environments and biting insects), are the best ways to prevent your horse from getting rain rot. Because this infection can spread to other horses, it's important to isolate the infected horse to reduce the risk of spreading the infection to pasturemates. Also, be sure to use a separate set of grooming tools and tack (if the infection is light enough to continue working the horse without causing harm), and disinfect these items between each use. Florida horses cannot always avoid temperatures or humidity, but we can utilize insect repellents to decrease the risk of contraction.

Source: Ann Swinker, PhD, extension horse specialist at Penn State University.

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