This is a common infection of the skin caused by the bacteria Dermatophilus congolensis. The infectious agent requires skin damage from other causes in order to penetrate the skin and cause infection. Infection results in crusts at the surface of the skin with a characteristic lumpy appearance.
It is most common on the nose, ears and lower legs. In animals with reduced resistance from undernutrition or other stressors, skin infections may become generalised over the body.
The condition predisposes to flystrike, can make shearing difficult, and causes unsightly fleece damage.
It is usually seen in young animals because older animals have become immune from previous infections.
Conditions for spread and establishment of infection are when wet animals are in close contact, such as yarding in heavy rain or high humidity, and during dipping for lice and fly control. It can then spread very quickly through a mob.
Scabs in matted hair and wool on chronically wetted areas of the body should raise suspicions.
Alopecia develops if the scabs are pulled or rubbed off. Lesions occur on parts of body that are wet for prolonged periods. In wet weather commonly affected areas include the dorsal and upper lateral neck and back. Animals grazing in long wet grass may be affected on the nose and pasterns. At sea, it may involve the lower legs and flanks, the areas which are continually wet when standing or lying down in slurry.
Laboratory confirmation is by microscopic identification of organisms from scabs and scrapings submitted chilled, or biopsy specimens submitted in buffered formalin.
Differential diagnoses include ringworm and scald neither of which has matted hair and scab formation before alopecia appears.
Animals will recover spontaneously. Treatment with antibiotics (procaine penicillin, oxytetracycline) is recommended only for severely affected animals, and for sheep that must be shorn and where the shears are unable to pass beneath the scabs. It generally takes 6 weeks after antibiotics are administered for scabs to lift sufficiently and may be longer in stressed sheep in wet conditions.
The spread and severity of the disease can be minimised by avoiding prolonged yarding of wet animals and minimising stressors such as undernutrition.