The clinical signs and diagnosis of feline atypical mycobacterial panniculitis - Veterinary Medicine
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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.


VETERINARY MEDICINE



5A. A photomicrograph of skin obtained by deep surgical biopsy from a cat with AMP. The inflammation associated with diffuse pyogranulomatous panniculitis extends into the superficial adipose tissue. The arrow indicates an extracellular lipid vacuole (lipid lake) (hematoxylin-eosin; bar = 100 m).
Because of the nonspecific histologic appearances of panniculitis, it is important to communicate your suspicion of AMP to the pathologist examining the samples. Histologic examination of deep tissue biopsy samples from cats with AMP reveal nodular to diffuse pyogranulomatous panniculitis and dermatitis (Figures 5A & 5B).1-3 Inflammation commonly extends into the superficial dermis and adjacent skeletal muscle. In samples routinely stained with hematoxylin-eosin (Figure 5A), it is not uncommon for a pathologist to report nonstaining "ghosts," which are presumed to represent mycobacteria that stain poorly. Although the mycobacteria that cause AMP are not usually found in high numbers or density, requesting acid-fast staining can greatly facilitate identification of intracellular, acid-fast bacilli within macrophages or sometimes within extracellular lipid vacuoles (lipid lakes) (Figures 5A & 5B).1,2,5 Although clinicians and pathologists can suspect AMP, only microbiologists can identify the specific etiologic agent.


5B. A photomicrograph of skin from the cat in Figure 5A showing acid-fast bacilli within an extracellular lipid vacuole (Ziehl-Neelsen; bar = 10 m).
To identify AMP, it is also important to reserve samples of aseptically collected deep tissue samples for culture.12 Collecting superficial samples with swabs frequently results in culturing environmental contaminants. Although mycobacteria will grow on blood agar media, they typically do so only after three to seven days under ideal conditions.12,13 Since microbiology laboratories do not hold most negative cultures for that length of time, these samples may be prematurely discarded unless the laboratory has been alerted to a possible atypical mycobacterial infection. A variety of specialized culture media and environmental conditions can be used to facilitate growth of mycobacteria. These vary among diagnostic laboratories. When aseptically obtained deep tissue samples from cats with clinical AMP are cultured in appropriate conditions, mycobacterial growth is usually heavy.2 If sparse growth of mycobacteria is reported, it may represent colonization or contamination of the lesion, not infection. In these instances, submitting additional culture samples is recommended to confirm a diagnosis of AMP. It should be possible to repeatedly isolate the infecting organism in cases of clinical disease.

Previous reports in the veterinary literature state that M. fortuitum, M. smegmatis, and M. chelonae of the Runyon group IV organisms appear to be the principal infecting pathogens in cases of AMP.1,2 However, other closely related organisms have also been implicated in veterinary and human disease.3 Further subspecies identification and serotyping of Runyon group IV mycobacteria have been described.2,3 Precise identification of the infecting strain to the subspecies level is seldom necessary for managing individual clinical cases but is useful for microbiologic epidemiologic investigations into this uncommon disease. Thus, to select the best treatment the most crucial information to obtain from the microbiology laboratory is confirmation that the organism is a rapidly growing mycobacterial species and documentation of its antibiotic susceptibility.

Mycobacterial antimicrobial susceptibility profiles can be determined at specialized regional diagnostic facilities* on isolates obtained from local microbiology laboratories. Susceptibility is most commonly tested by using a disk diffusion technique, an agar disk elution modification of the proportion method, or both.

*Two such facilities are the University of Florida Veterinary Medical Teaching Hospital Laboratory, Gainesville, Fla., and The National Jewish Medical and Research Center, Denver, Colo.


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