Rainscald is a common skin disease in horses that is caused by Dermatophilus congolensis

Dermatophilus congolensis

Dermatophilus congolensis is a gram positive bacterium and is the etiologic agent of a disease called Dermatophilosis in animals and humans, a dermatologic condition that manifests itself with the formation of crusty scabs that contain the microorganism. Some people erroneously call it mycotic...

. This is the same organism that causes Mud fever

Mud fever

Mud fever is a group of diseases of horses causing irritation and dermatitis in the lower limbs of horses. Often caused by a mixture of bacteria, typically Dermatophilus congolensis, and Staphylococcus spp, mud fever can also be caused by fungal organisms...

 in horses. This disease is very common in cows, sheep and goats and is also found occasionally in cats, dogs, and humans. D. congolensis is a gram positive bacteria that is thought to originate from the soil. It commonly causes disease in moist, tropical areas, but can also be found in wet northern environments. Moisture and high temperatures facilitate the dispersal and penetration of zoospores into the skin, contributing to the spread of the disease.
Ticks, biting flies, and contact with other infected animals also causes the spread of rainscald. Once in the skin, the bacteria cause inflammation of the skin as well as the typical symptoms associated with rainscald.

Symptoms

There are two different manifestations of rainscald: the winter form, which is more severe due to the longer coat of the horse, and the summer form, which is less severe. Horses are usually affected on the back, head, and neck, where insects commonly bite, and the legs, which are commonly infected if the horse is kept in most footing. Initially, the horse will display a matted coat and bumps which will then progress to crusty scabs and lesions. The animal may also be itchy and display signs of discomfort.

Diagnosis

Diagnosis is most commonly done with the identification of bacteria in the lesions by a microscope observation. Ticks, biting flies, and contact with other infected animals also causes the spread of rainscald. A scab will be taken from the affected animal and stained so that the bacteria are visible under a microscope inspection. A positive diagnosis of rainscald can be confirmed if filamentous bacteria are observed with as well as chains of small, spherical bacteria. If a diagnosis cannot be confirmed with a microscope, blood agar cultures can be grown to confirm the presence of D. congolensis. The resulting colonies have filaments and are yellow in colour.

Treatment

Rainscald normally heals on its own, but scabs are softened with benzoyl peroxide and chlorhexidine and removed in order to speed the healing process. When grooming the horse, be extremely gentle and avoid brushing from affected to unaffected areas, and avoid sharing grooming equipment with other horses. In severe or chronic cases, penicillin

Penicillin

Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....

and streptomycin are injected into the horse to kill the bacteria. Typically the disease is not life threatening, nor does it impact the welfare of the horse, so treatments are more for the owner's sake of mind and cosmetic appeal of the animal.

Prevention

In order to prevent rainscald, it is important to stop the spread of the bacteria. Tick and insect control is an effective way to stop the spread of the bacteria from one animal to another. As well, separating infected animals will help to stop the spread of the bacteria. Keeping the animal in a dry, well-ventilated area out of the rain and wet conditions will stop the bacteria from growing. This dry environment includes a dry ground as well as dry air.