Cause Rain scald is caused by an unusual bacterium, Dermatophilus congolensis , with funguslike properties. Chronically affected horses are the main source of infection. During wet weather, the dampened lesions release zoospores, the infective stage of the organism. The infection is then spread from horse to horse by direct contact or by flies. Normal healthy skin is quite resistant to dermatophilus infection, so predisposing factors decreasing skin resistance are necessary for infection.
Heavy rainfall damaging the skin and prolonged wetting of the skin are common factors.
Signs Rain scalds are highly contagious and outbreaks can involve all ages. The rump, back and neck are the most commonly affected areas but the lesions may be spread by rain to the legs, chest and belly. In the early stages of the disease, the changes are often easier to feel than see. Thick crusts can be felt under the hair. If the crusts are removed, the hair comes with them exposing a moist reddened area.