Giemsa stain or immunofluorescence is used. The characteristic appearance of the lesions and of the organism in smears from typical bovine dermatophilosis makes culture unnecessary in most cases. However, confirmation of the diagnosis may be made by isolating the bacterium. Incubation in air produces similar pinpoint colonies at 24 hours that grow to about 1 mm at 48 hours. The rough colonies are formed by the branching filaments, but continued growth in air stimulates the production of the cocci, which are commonly yellow in colour.
Colonies take on a smooth, often yellowish, appearance.
The cocci are normally vigorously motile when taken from young cultures. The colonies must be differentiated from Nocardia spp. For isolation, material can be streaked out directly from the moist undersurfaces of freshly removed, uncontaminated scabs or from scab emulsions, but the relatively slow-growing D. congolensis is readily overgrown by other bacteria. In most specimens, free cocci, whether motile or not, will be present in emulsions of the material. Small pieces of scab are placed in a bijou bottle containing 1 ml of sterile distilled water and allowed to stand at room temperature for 3-4 hours.