Rainscald (also known as dermatophilosis, rain rot and streptothricosis) is a common skin disease in horses that is caused by the bacterium Dermatophilus congolensis. This is the same organism that causes Mud fever 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 bacterium 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.
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 moist 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 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.
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Rainscald normally heals on its own, however as the condition can spread to involve large areas, prompt treatment is recommended. Although some cases can be severe, most rain scald is minor and can be easily and cheaply treated at home naturally.
First groom the affected parts carefully, to remove any loose hair. Be extremely gentle, the area is very sore itchy and horses will very quickly get fidgety. Next shampoo the area, use warm water and a soft cloth or brush, and massage the lather through the coat as much as the horse will tolerate. Any mild shampoo is fine, Veterinarians often recommend Chlorhexidine gluconate, or Povidone-Iodine, based agents. Remove as much water as possible and dry the horse off as best possible. If not using an antibacterial shampoo, it is recommended to use an antibacterial treatment, this may be a light gel or cream formula although an antibacterial shampoo is the best option. Typically there will be improvement in a few days, and in a week there'll be some sign of new hair growing back. More severe rain scald may take longer.
Persisting rainscald should be referred to a Veterinarian, skin scrapes can be taken and appropriate antibiotics dispensed if needs be. There may be a secondary infection that needs treating.
Typically the condition 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.
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 dry ground as well as dry air.
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