The list of differential diagnoses in a cat with nonhealing, nodular cutaneous lesions is extensive (Table 1). However, when a cat with chronic, fistulous, draining, ulcerative or nodular dermatitis also has panniculitis, reduce the
primary differential diagnoses to sterile nodular panniculitis, paraneoplastic panniculitis (usually secondary to exocrine
pancreatic neoplasia), vitamin E deficiency, postinjection panniculitis, foreign body panniculitis, and the infectious panniculitides.
Bacterial pseudomycetomas (often caused by Staphylococcus, Proteus, or Pseudomonas species), actinomycosis, nocardiosis, and various tuberculous and atypical mycobacterial organisms are infectious causes of
nodular panniculitis in cats. Depending on the stage of the disease at the time of presentation, nodular panniculitis can
also resemble, and subsequently be misdiagnosed as, deep pyoderma, furunculosis, or cutaneous neoplasia.
Table 1 Differential Diagnoses in Cats with Cutaneous and Subcutaneous Nonhealing Lesions
Diagnostic features and recommendations
The clinicopathologic features of AMP in cats largely reflect a chronic inflammatory disease. The most commonly reported complete
blood count abnormalities include mild to moderate normocytic, normochromic nonregenerative anemia and an inflammatory leukogram.1,2 Hyperglobulinemia (characterized by a polyclonal elevation in the globulin fractions on electrophoresis) is the most frequently
noted serum chemistry profile abnormality, resulting from chronic antigenic stimulation.1 Hypoalbuminemia may also be noted in some cats, which most likely occurs secondary to the chronic exudative skin lesions.
Occasionally, hypercalcemia is also a biochemical feature of feline AMP.10 The hypercalcemia in patients with granulomatous diseases, such as AMP, results from excessive synthesis and secretion of
calcitriol by macrophages infiltrating the inflammatory lesions; the macrophages function independently of normal calcium
homeostatic regulatory mechanisms.11
Cats with nodular, fistulous, draining cutaneous and subcutaneous lesions present a diagnostic challenge to practitioners.
Biopsy without ancillary diagnostic procedures may not identify the cause, as all of the diseases that cause nodular panniculitis
have nearly identical histopathologic features. Practitioners should suspect AMP when examining cats with chronic, nodular,
nonhealing wounds that do not respond to antimicrobial agents that are usually efficacious. However, definitively diagnosing
AMP relies on demonstrating mycobacteria through culture or visual identification of mycobacterial agents in cytologic or
In cases of suspected AMP, definitive diagnosis is greatly expedited by collecting multiple tissue fine-needle aspirates from
nodular lesions and performing deep surgical tissue biopsy, which usually requires general anesthesia. All types of samples
collected from deep lesions, including purulent exudates and tissues, can and should be used for morphologic and microbiologic
evaluation. Cytologic evaluation of acid-fast-stained (modified Fite's method or Ziehl-Neelsen stain) smears from affected
animals can detect the organisms in up to 50% of cats with AMP (Figure 4).1 Cytologic samples from cats with other forms of mycobacterial dermatoses, including feline leprosy and tuberculous infections,
characteristically contain acid-fast bacilli in sufficient numbers to be easily identified; however, bacterial culture to
identify the species is necessary for accurate diagnosis.1
4. Cytologic preparation of a tissue aspirate from an inguinal nodular lesion in a cat with AMP. Note the numerous free and
intracellular mycobacteria, which appear as red-staining (acid-fast) bacilli (modified Fite's method; bar = 25 µm).