This category includes the clinical presentations of multiple nodules, draining tracts, or severe crusted or exudative lesions. Three categories of disease can cause these lesions: infectious diseases, neoplastic diseases, or sterile diseases.
Most veterinarians are least familiar with the sterile disease category. Some examples of these diseases include reactive histiocytosis, pyogranuloma/granuloma syndrome, and Sweet's syndrome. These diseases are immune-mediated, however, the trigger is rarely identified.
To diagnose a sterile disease, you must rule out infectious and neoplastic causes. This can be challenging because the sterile diseases can have an inflammatory infiltrate which mimics infections or a monocellular infiltrate which mimics neoplasia.
Start with impression smears of exudate or FNA of nodules to assess for obvious infectious organisms or neoplastic cells. Also skin scrape to rule out demodex with severe secondary infection.
If no organisms or neoplastic cells are seen, then skin biopsies are required for diagnosis and appropriate therapy.
Biopsies should be submitted for histopathology with special stains for bacterial, fungal, and mycobacterial organisms.
If organisms or neoplasia is identified, then no further diagnostics are needed. Otherwise, submit tissue for aerobic, anaerobic, mycobacterial, and fungal cultures. This step is necessary to distinguish between infectious and sterile diseases because organisms are not always seen on histopathology. Swab cultures are not recommended because these cultures often reveal only surface contaminants and not the true tissue pathogen.
If both histopathology and tissue cultures are negative for organisms, then a sterile disease process is likely. The pathologist should help you identify the particular syndrome of sterile disease; however some cases do not fit the criteria of a specific named syndrome.
Therapy will vary depending on the diagnosis. Please call for recommendations regarding your patient.