When presented with a patient having a severe, crusting to ulcerative dermatosis that is antibiotic non-responsive, involves the face, pinnae, nose and foot pads; and is accompanied by fever and malaise, one should be suspicious that the animal has autoimmune skin disease. However, after proceeding with a diagnostic work-up, it is not uncommon to find the "immune panel" to be negative, and the skin biopsies non-diagnostic. Thus, the definitive diagnosis of autoimmune skin disease can be difficult to make.

Diagnosis of PF is accomplished by having clinical signs and history suggestive of PF, ruling out other disorders (such as demodicosis, dermatophytosis) with simple laboratory tests such as dermatophyte cultures and skin scrapings, lack of response to appropriate antibiotics, and seeing histopathologic evidence of PF. Ideally, when collecting samples for histopathology, always look for intact pustules. Do not scrub the surface of the skin prior to collecting the sample, and be careful not to disrupt the pustule. If you cannot find a pustule, look for papules with erythematous halos, or freshly ruptured, crusted lesions (with erythema around the edges). Another good location is the outside margin of an affected footpad, where biopsying a freshly crusted area is often very rewarding. If using a biopsy punch, use at least a 6 mm punch and collect 2-3 or more representative samples. Place your samples on a small piece of a tongue depressor, dermal side in contact with the wood, prior to placing them in the formalin. (This helps prevent the samples from curling). When submitting your samples, always remember to give the pathologist historical information on the patient, signalment, and a good description of the lesions. This will greatly improve the quality of the report that you receive. Keep in mind that it is sometimes necessary to repeat biopsies for a definitive diagnosis, although this is greatly reduced if multiple samples are collected from carefully selected sites.

After collecting the lesions for histopathology, if any intact pustules remain, rupture the head of the pustule with a 25 g needle and make a smear on a glass slide. Alternatively, remove the crust from a lesion and make an impression smear. Cytology findings of large round epithelial cells (acanthocytes) in conjunction with neutrophils and/or eosinophils, in the absence of bacteria, supports the diagnosis of PF.

Other diagnostics that should be performed include a CBC, biochemical and thyroid profiles, and urinalysis. Elevations in WBC counts with a neutrophilia are expected. The biochemical profile may have elevations in serum globulins and total protein. Some animals may also be hypothyroid and/or have autoimmune thyroiditis. Urinalysis should be normal in most PF cases. Evidence of systemic disease, such as anemia and proteinuria, suggests lupus erythematosus rather than PF. For patients with PF, the ANA, Coombs, and RA factor tests are generally unrewarding and unnecessary.

The final laboratory test to consider is immunoperoxidase skin testing. This is a staining procedure done on formalin-fixed skin specimens, which looks for the presence of antibodies in the intercellular region of the epidermis. Samples submitted for histopathology can be used here, if indicated, although false positive and false negative reactions do occur. If it is unclear that the histologic changes are consistent with PF, immunoperoxidase staining would be a useful supportive test.

[Contributed by Karin M. Beale, DVM, Dipl. ACVD, Houston, TX]