Every horse owner has experienced or knows someone whose horse has experienced Rain Rot. Those painful, oozing, crusty scabs that can seemingly appear overnight and seem to spread at the speed of light.

What is Rain Rot?

Dermatophilosis, more commonly referred to as “Rain Rot” is an infection of the epidermis of the skin by Dermatophilus congolensis. D. congolensis is a gram positive, non-acid-fast anaerobic actinomycete. Actinomycetes are organisms with characteristics common to both bacteria and fungi but yet possessing distinctive features to delimit them into a distinct category. Like a bacteria, they are unicellular, but like fungi, they produce hyphae (long branching filaments).

The natural habitat of D. congolensis is unknown, but as most actinomycetes are soil dwellers, it probably can reside there, although attempts to isolate it from soil have been unsuccessful to date. It has been recovered only from infected or carrier hosts and these are thought to be the main reservoir of the disease.

The condition usually appears during wet, humid times of the year and often when horses are blanketed. The zoospores require moisture and warm temperatures in order to be released. Blankets help to provide a warm and moist environment over the horse’s skin, especially if the horse sweats under the blanket of the blanket does not get changed after it gets wet from rain or snow.

In order for an infection to be established, there must be some break in the skin such as an abrasion, a bug bite or other breach of the protective barrier. Low concentrations of carbon dioxide emitted from the skin attract the motile zoospores to susceptible areas on the skin surface. Zoospores germinate to produce hyphae, which penetrate into the living epidermis and subsequently spread in all directions from the initial focus. Hyphal penetration causes an acute inflammatory reaction. This inflammatory reaction then culminates with the epidermis cornifying, separating, and forming the characteristic scab.

The lesions usually start out with the hair matted together as “paintbrush lesions,” then progress to curst or scabs, and finally become accumulations of cutaneous keratinized wart-like lesions. A thick yellow-green pus is usually present under the crusts. Lesions can occur anywhere on the body are most common over the dorsal surface (back), upper and lateral areas of the neck, face, and chest. White haired areas seem to be particularly susceptible.

Dermatophilosis is seen in all age, sex, and breeds. However, younger animals, those housed in very moist, humid conditions, and immunocompromised animals are more susceptible. Animals will display various degrees of pruritus (itchiness). Most affected animals recover spontaneously within 3 weeks of the initial infection (provided chronic maceration of the skin does not occur). In general, the onset of dry weather speeds healing. Uncomplicated skin lesions heal without scar formation. Lesions on the lower limbs can be painful enough to affect a horse’s performance. And certainly, if the lesions cover a large area of the body, especially over the back, it may not be possible to saddle your horse.

Horses become infected through contact with carrier or overtly infected animals. Fomites such as blankets, brushes, saddle pads, and tack can also transmit the disease. Biting insects such as flies and ticks can also serve as vectors.

Diagnosis is usually made based on clinical signs and the typical appearance of lesions. A smear of the crusts or pus can be made to identify D. congolensis. A definitive diagnosis is made by demonstrating the organism in cytologic preparations, isolation via culture, and/or via skin biopsy. An indirect fluorescent antibody technique and a single dilution ELISA test have been developed for large serologic and epidemiologic surveys. The most practical diagnostic test is cytologic examination of fresh crusts and/or impression smears of the underside of freshly avulsed lesions. Fresh crusts are minced on a glass microscope slide with a sterile scalpel blade in several drops of sterile saline.

How Do I Treat Rain Rot?

Horses are treated using topical antibacterial shampoos that contain chlorhexidine, povidone-iodine, or benzoyl peroxide. The horse should be lathered up, the shampoo left to soak for 10 minutes and then rinsed. Any loose scabs should be gently removed. Any adherent crusts can be treated with a povidone-iodine ointment to help to soften them for later removal. Severely affected horses may require parenteral antibiotics. However, these should be reserved for only the most severely affected and must be used in combination with topical treatments. Topical treatment with povidone-iodine has been found to be superior to parenteral oxytetracycline alone (100% to 66% effective, respectively).

Affected horses should be isolated and all equipment that has been in contact with them should be disinfected. Crusts that are removed from horses should be disposed of in the trash and not thrown on the floor in order to prevent creating a source of reinfection. Preventing chronic maceration of the skin and keeping your horse dry are important. Make sure that wet blankets are removed and horses have shelter to get out of the rain if they choose.

Finally, it is very important to keep in mind that dermatophilosis can be transmitted to humans. Contact with an infected animal can lead to infection on exposed skin of the handler. You should wear gloves when handling infected animals and always thoroughly wash your hands with soap after contact with the horse or their equipment.

Registered 2013 by Equestrian Collections
Author: Sallie S. Hyman, VMD, DACVIM, CVA

Information in this article is for educational purposes only and is not a substitute for evaluation by an equine professional. In particular, all horse owners should seek advice and treatment from a licensed veterinarian, such as TEVA, for their horses' medical care.

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