Autoimmune and immune-mediated diseases can be difficult to identify at times due to their similar appearance of bacterial infections of the skin (i.e., pustules, papules, crusts, and ulcerations). Cytology can be of significant value in helping to rule out a secondary bacterial infection.
If bacteria is seen on cytology, you have to quantify the bacteria to determine if it is indeed the cause of the lesions. If there are only rare bacteria present and the skin is highly inflamed with numerous crusts and ulcerations, you have to continue searching for an underlying cause. If bacteria are absent, antibiotics are unnecessary as they can cause mutations of the normal flora or even cause an adverse drug reaction (two topics discussed in this newsletter). Further diagnostics are then needed to determine the underlying cause of the cutaneous lesions.
Biopsies can give you valuable information and potentially a diagnosis, but you must know when the best time is to biopsy and which lesions should be sampled. You should treat the secondary bacterial infection first in order to submit the highest quality sample to the pathologist, without the secondary inflammation from the bacteria. You should also try to maximize the number of biopsy samples that you are allowed to take for the same charge on histopathology. We always take a minimum of three samples unless we run out of tissue to sample. There have been multiple instances where the identifiable inflammatory pattern or key diagnostic finding on histopathology is only found in one of the three samples.
—Jason Pieper, DVM, DACVD