What is the method of diagnosis for Dermatophilus congolensis?

The method of diagnosis for Dermatophilus congolensis, the causative agent of dermatophilosis or rain rot in animals, primarily relies on stained smears and clinical signs. This approach involves examining representative samples of affected skin, which can be stained to reveal the presence of the bacteria. Under a microscope, the characteristic branching filaments or "tram-track" appearance of the bacteria can be identified.

In addition to smear analysis, clinical signs such as scabby lesions and crusting on the skin help to reinforce the diagnosis. Observing these signs in conjunction with laboratory findings allows veterinary professionals to confirm the presence of Dermatophilus congolensis.

While other diagnostic methods exist, such as skin biopsies which can help rule out other skin conditions, they are not the primary mode of diagnosis for dermatophilosis. Blood culture is used for different infectious diseases and is not applicable here. Ultrasound imaging is typically not relevant in the diagnosis of surface infections like those caused by Dermatophilus congolensis. Hence, the combination of stained smears and assessment of clinical signs makes this option the most suitable for diagnosing this specific dermatological infection.