The clinical signs and diagnosis of feline atypical mycobacterial panniculitis


The clinical signs and diagnosis of feline atypical mycobacterial panniculitis

Cases of AMP are infrequent, but can be exasperating. Some affected cats have skin lesions lasting months to years, despite multiple antimicrobial courses. This article tells you when to suspect AMP and how to confirm a diagnosis.

2. Inguinal dermatitis and panniculitis in a cat with AMP characterized by a large subcutaneous nodule (arrowheads) surrounded by adjacent multifocal, punctate ulcers and fistulous tracts. The inset is a close-up view, showing the depressed, purplish areas (arrows) between fistulous tracts that represent areas of thinned epidermis.
In our experience, and that of others,1-4 the most frequently documented progression of disease is as follows: A firm, palpable nodular lesion develops at the injury site and insidiously progresses to involve other regional dermal and subcutaneous tissues. The initial clinical appearance of these lesions can be similar to that of a cat bite abscess (Figure 1) except that empirical abscess management consisting of drainage, cleansing, and short-term systemic antibiotic therapy fails to resolve the lesion. Classically, the disease gradually infiltrates adipose tissue underlying the trauma site, resulting in palpable thickening and a concentric spread within the surrounding skin.1,2 The initial nodular lesion usually becomes alopecic and ulcerated and typically develops multiple, small-diameter, fistulous tracts (Figures 2 & 3). The often serous to seromucous drainage from the fistulous tracts does not resemble the pungent, purulent exudates commonly seen with ruptured cat bite abscesses. Multifocal, purplish lesions often develop adjacent to fistulous tracts and represent areas of thin skin (see inset in Figure 2).2 AMP can have other primary manifestations besides that of erosive, nodular dermatoses, such as a macular or papular eruption. A wide variety of secondary dermal lesions can occur in AMP cases, often including alopecia, ulcers, erosions, and scales.

3. Panniculitis of the left flank region in a cat due to AMP. The multifocal draining tract sinuses and regional alopecia have progressed to involve the flank from an original lesion on the ventral abdomen.
In cats with AMP, the lesions are most frequently distributed over the inguinal and ventral abdominal fat pads. The group IV mycobacteria have a predilection for infecting adipose tissues and obese individuals.1,2 It has been suggested that the triglyceride-rich adipose tissue microenvironment serves as a preferential growth medium for rapidly growing mycobacteria7,8 or may protect the organisms from destruction by the host's immune system.7,8

Lesions in cats with AMP are typically limited to the dermis, subcutaneous adipose tissue, and surrounding deep musculature; systemic signs of illness are uncommon. When present, constitutional signs of illness are nonspecific and include low-grade pyrexia, lethargy, inappetence, localized pain or pruritus, and weight loss.1,2 Dissemination of the infection to internal visceral structures has been uncommonly reported,9 despite the long duration of disease and extensive dermatologic lesions in some cats.