The clinical signs and diagnosis of feline atypical mycobacterial panniculitis

THE POPULARITY of pet cats and their importance to our society are clearly demonstrated by the growing number of private practitioners who exclusively treat cats, the increasing availability of postgraduate feline medicine training programs offered at veterinary teaching institutions, and the formation of multi-disciplinary, nationally recognized committees and task forces charged with investigating important emerging feline diseases. One disease that has arisen from this increased interest in feline disease is feline atypical mycobacterial panniculitis (AMP).

Feline AMP (also called opportunistic mycobacterial granuloma) is an uncommon infectious disease of dermal tissues and subcutaneous fat. Although the incidence of AMP is low, the disease often presents diagnostic and therapeutic challenges and can cause severe morbidity in cats.

In general, feline mycobacterial infections can be divided into three forms: classic tuberculosis (caused by Mycobacterium bovis, Mycobacterium tuberculosis, Mycobacterium microti, and a recently described new variant species), feline leprosy (caused by Mycobacterium lepraemurium), and the atypical mycobacterioses, which cause AMP.1 Feline AMP is caused by infection with rapidly growing Runyon group IV mycobacteria. This group includes Mycobacterium smegmatis, Mycobacterium phlei, Mycobacterium fortuitum, and Mycobacterium chelonae, which are ubiquitous saprophytic organisms.1,2 Runyon group IV species share morphologic features with other mycobacteria in that they are gram-positive, acid-fast rods capable of intracellular propagation within phagocytic cells.1,3

In this article, we focus on the clinical and clinicopathologic signs associated with feline AMP and the recommended methods for diagnosis. We discuss the treatment of AMP in the next article.

Patient history and clinical signs

Feline AMP typically causes chronic ulcerative or nodular draining subcutaneous and dermal infections. It primarily affects young to middle-aged female cats, usually without concurrent immunosuppressive diseases (e.g. feline immunodeficiency virus infection, feline leukemia virus infection).1,2,4 Feline AMP is considered an opportunistic infection that most often occurs in immunocompetent cats. It is not unusual for a cat to have a several-month to multiple-year history of a nonhealing skin wound that has demonstrated no response or an incomplete response to multiple courses of antimicrobial therapy. A history of skin trauma, usually resulting from bite or scratch wounds, is frequently incriminated as the inciting cause.1,2 Infection results from inoculation or contamination of the wounds with soil containing mycobacteria2 ; it can also occur as a result of penetrating missile and vehicular injuries. Although most affected cats initially described in the literature were from tropical and subtropical climates, group IV mycobacteria have been isolated from water and soil samples from multiple temperate regions, and investigators have reported infected cats with a wide global geographic distribution.5,6

1. A large cat bite abscess.

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.

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 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

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.

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.

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 histologic samples.

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).

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.

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 µm).

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.

5B. A photomicrograph of skin from the cat in Figure 5A showing acid-fast bacilli within an extracellular lipid vacuole (Ziehl-Neelsen; bar = 10 µm).

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.

New adjunct tests

Several new diagnostic techniques allow for more accurate diagnosis of specific variants of mycobacteria infecting cats with AMP. These techniques can be performed on biopsy samples and can be used as adjuncts to the routine diagnostic procedures previously recommended since many mycobacterial species are not easily differentiated by existing morphologic and microbiologic methods. Genetic tests, such as sequencing procedures in which the genetic code of a targeted mycobacterial gene (i.e. 16S rDNA) is identified and compared with a known database, allow identification of the specific mycobacterial species known to cause AMP.3,14 In addition, since the mycobacterial species causing AMP are difficult to identify by histologic examination, procedures such as polymerase chain reaction can demonstrate and amplify mycobacterial nucleic acid in tissue samples, which can then be used for further diagnostic testing, including sequence analysis.3,14 In one study, immunostaining with polyclonal anti-Mycobacterium bovis antibodies in skin biopsy samples from animals suspected of having AMP facilitated identification of mycobacteria when compared with conventional histochemical stains alone.15 Practitioners should inquire about the availability and cost of these new ancillary procedures from their pathology service.

Conclusion

Cats with chronic nodular dermatitis and panniculitis pose unique diagnostic problems depending on the ultimate diagnosis, but the following guidelines should prove useful for identifying most cases of AMP.

1. Determine that the cat's illness is consistent with one or more of the historical, clinical, or clinicopathologic syndromes characteristic of atypical mycobacterial infections.

2. Exclude more common causes of dermatologic disease by using routine diagnostic methods.

3. Obtain multiple deep tissue biopsy samples early in the clinical course of disease for cytologic, histologic, and microbiologic examination.

4. Alert the diagnostic laboratory receiving aspirate and biopsy samples to the possibility of an atypical mycobacterial infection, so the laboratory can apply techniques to facilitate mycobacterial species identification.

5. Request antimicrobial susceptibility testing on all successfully cultured and suspected pathogenic mycobacteria.

6. Perform serial biopsies when necessary to confirm a diagnosis of AMP.

John H. Rossmeisl Jr., DVM, MS, DACVIM (internal medicine and neurology)

Thomas O. Manning, DVM, MS, DACVD

Department of Small Animal Clinical Sciences

Virginia-Maryland Regional College of Veterinary Medicine

Virginia Tech

Blacksburg, VA 24061

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