Mycobacteria cause a variety of clinical syndromes in the cat, ranging from localised skin conditions to disseminated, often fatal, infections. Mycobacteria are Gram-positive, aerobic, non-spore forming, non-motile members of the Actinomycetales order of bacteria. The high lipid content of the cell wall results in the retention of hot carbolfuschsin stain after treatment with acid and alcohol, rendering the organisms acid-fast when stained with ZN or Fite's stain. Mycolic acid, the major lipid in the cell wall, plus cord factor and wax D are partly responsible for the capacity of the organism to survive within phagocytes, which gives rise to the typical granulomatous immune response by the host.
Conceptually, mycobacteria can be divided into three groups; 1) those species that are obligate pathogens (for example, tuberculous mycobacteria), 2) those that cause leproid/tuberculous granulomata and cannot be cultured by routine laboratory methods (lepromatous mycobacteria) and 3) those that have pathogenic potential but are generally considered to be opportunistic saprophytes (rapidly growing and slowly growing opportunistic mycobacteria).
1. Infectious Caused by Tuberculous Mycobacteria
Given that members of the M. tuberculosis complex (M. tuberculosis, M. bovis, M. microti and M. microti-like) are apparently non-existent in Australia, the author has no personal experience with infections caused by these organisms. As such, they are covered only briefly in this presentation and the reader is directed to the reference provided if further information is sought.1
2. Infections Caused by Lepromatous Mycobacteria
"Feline leprosy" is characterised by the formation of single or multiple dermal or epidermal granulomas (with or without local lymphadenitis) caused by mycobacteria species that cannot be cultured in the laboratory by routine methods.
Previously, it was assumed that the causative agent was exclusively Mycobacterium lepraemurium, the bacterium that causes systemic disease in rats. Genetically, M. lepraemurium is most closely related to M. avium and M. avium subspecies paratuberculosis. Molecular methods have identified the involvement of at least two other, as yet unnamed, species of mycobacteria. One of which has been identified in cats in New South Wales (NSW) and is genetically related to M. leprae, M. haemophilum and M. malmoense.2,3 The other has been identified in a group of cats, predominantly from a restricted geographical area in South Eastern Victoria, which shares similar nucleotide sequences with the M. simiae group.4 Cases of feline leprosy tends to occur in temperate, coastal regions and has been reported in Australia, New Zealand, western Canada, Netherlands, France, Greece, the UK and the USA.
A condition called "feline multisystemic granulomatous mycobacteriosis" has been documented in cats from western Canada and USA (Idaho and Oregon) caused by Mycobacterium visibilis (or M. visibile).5
This condition is characterised by diffuse cutaneous infection and widespread systemic dissemination.
Cats with feline leprosy develop focal, occasionally ulcerated nodules, typically on the head and distal limbs (although lesions can occur anywhere, including the tongue, lips and nasal planum). Clinical course is aggressive and locally recurrent, however cats may develop widespread lesions over several weeks.
The diagnosis of feline leprosy is made via histopathological or cytological documentation of pyogranulomatous inflammation with negatively staining (Romanovsky-stained) or acid fast bacilli (AFB) within macrophages.
Pathologically, feline leprosy can be divided into two forms; the multibacillary lepromatous form and the paucibacillary tuberculous form. Lepromatous leprosy (LL) is characterised by pyogranulomatous dermatitis and panniculitis, immunologically ineffective multinucleate giant cells, absence of necrosis and abundant intrahistiocytic AFB. LL tends to occur in individuals with a suboptimal cell-mediated immunological (CMI) response to the infective organisms. Tuberculous leprosy (TL) is characterised by pyogranulomatous dermatitis and panniculitis with multifocal to coalescing areas of necrosis and moderate to scant intrahistiocytic AFB. Cases with TL have strong CMI. Whether an individual has LL or TL may depend on host factors, such as individual major histocompatibility complex haplotype or acquired defects of CMI (for example, retroviral infection, renal disease, etc), or may be dependent on the organism itself. Interestingly, cats infected with the novel mycobacteria species appear to have a predominance of LL, whereas young cats with M. lepraemurium commonly have TL (although a number of cats with LL caused by M. lepraemurium have also been documented).3,6
Samples should be submitted for culture as occasionally a slowly growing saprophytic or tuberculous species may be the causative agent. PCR is becoming increasingly popular as a diagnostic tool as it offers the advantage of rapid identification of organisms to the species level. Depending on the expertise of the laboratory, PCR can be performed on fresh (frozen) or paraffin embedded tissue.
Definitive treatment guidelines for each of the causative agents are yet to be established. Treatment with clarithromycin (62.5 mg/cat PO q 12hr) plus rifampicin (10-15 mg/kg PO q 24hr) and/or clofazimine (25-50 mg/cat PO q 24-48hr) for at least 2 months after clinical resolution seems to give the best chance of resolution. The concurrent use of multiple anti-mycobacterial agents is recommended because of the high incidence of acquired drug resistance. Wide surgical excision, especially in the case of early, localised infections may be helpful (this should be combined with adjunct medical therapy as lesions tend to be locally recurrent).
3. Infections Caused by Opportunistic Mycobacteria
Rapidly Growing Mycobacterial (RGM) Infections
Also known as "atypical mycobacteria", the causative agents are ubiquitous saprophytes, able to grow on synthetic culture media within 7 days at 24o to 45o C. RGM include members of the M. smegmatis, M. fortuitum group, M. chelonae/abscessus group, M. phlei, M. flavescens and M. thermoresistible. Initially thought to be more common in tropical and subtropical areas (for example, South Eastern USA), many cases have now been documented in temperate regions, including Australia, Canada, Finland and Germany. In Australia, most infections in cats are caused by M. smegmatis group (followed by M. fortuitum),6 whereas in the USA infections with M. fortuitum group followed by M chelonae/abscessus group appear more common,7 (at least in New South Wales and California, respectively).
Localised infection of the skin and subcutis (mycobacterial panniculitis) is the most common clinical presentation. This localised infection tends to occur in immunocompetent individuals, where the organisms presumably gain entry into host tissues via a breach in the integument (usually via inoculation during a cat fight or other penetrative injury).
Typically, early lesions are found in the inguinal area (although infections can start in the axilla), then spread to adjacent areas such as the perineum and the lateral abdominal and thoracic body walls. Adipose tissue aids in the survival and replication of organisms because it provides triglycerides for growth and/or physical protection from phagocytes, and as such, obese individuals may be predisposed.
Lesions are characterised by the presence of draining tracts, with purple-blue skin indentations ("pepper-pot" appearance) and patchy alopecia. Palpation of the affected skin and subcutaneous tissue reveals variable fluctuant and firm, "ropey" areas, with the skin adherent to underlying tissues. The exudate from the fistulae is typically watery, although secondary bacterial infections may cause this to become purulent. Lesions are not overly painful and affected cats are not usually systemically unwell, but some may be depressed, pyrexic and have weight loss.
Pneumonia caused by RGM is reported in cats, however this is rare. Predisposing factors were not identified in most cases, except one where the infection was thought to be secondary to aspiration of liquid paraffin. Disseminated systemic RGM infections are also rare, and are usually thought to arise as a result of impaired CMI.
The cyto- and histopathology of lesions reveals pyogranulomatous inflammation with low numbers of AFB. The diagnosis can be readily obtained by culture of fluid aspirated through intact disinfected skin (in an attempt to avoid culturing surface contaminants), with or without the assistance of ultrasound guidance. Clinicians need to notify the laboratory of a clinical suspicion of RGM so that appropriate culture methods can be used. Culture of deep tissue samples can also be attempted. PCR to identify organisms can be achieved from either fresh or paraffin embedded samples.
Until susceptibility data is known, empiric therapy with doxycycline (5-10 mg/kg PO q 12hr) and/or a fluoroquinolone is reasonable. M. smegmatis is usually susceptible to doxycycline and fluoroquinolones, but tends to be resistant to clarithromycin. M. fortuitum tends to show higher levels of resistance and M. chelonae tends to be resistant to all commonly used drugs except clarithromycin. A cure may be achieved with long-term administration of drugs but occasionally recalcitrant lesions require en bloc resection, with appropriate reconstructive techniques (e.g., skin-fold advancement flaps). The total duration of therapy is usually 3-12 months (ideally 1-2 months past clinical resolution of signs).
Slowly Growing Opportunistic Mycobacterial Infections
Slowly growing, non-tuberculous mycobacteria are ubiquitous in soil and water. Species include M. avium-intracellulare complex (MAC), M. genavense, M. terrae complex, M. simiae, M. xenopi. Usually, disseminated infection is observed in individuals with disturbances of CMI. Such conditions may be inherent, such as the breed related susceptibility seen in Siamese and Abyssinians9 or acquired (illness, immunosuppressive drugs etc). Occasionally, these organisms cause localised disease in apparently immunocompetent individuals, presumable due to introduction of organisms via a breach in the integument.
Generally, clinical signs of disease are referable to the areas in which the infection and subsequent granulomatous inflammation occurs. As such, affected cats may develop signs referable to disseminated disease (generally involving the respiratory and/or intestinal tracts), with or without cutaneous, often ulcerated, granulomata and lymphadenitis (either peripheral or intracavitary). Cats may become anorexic and pyrexic. Chronic weight loss may be a feature, particularly if the intestinal tract is involved.
As with other mycobacterial infections, documentation of AFB in cyto- or histopathological specimens, plus identification of the organism via culture and/or molecular methods are the keys to successful diagnosis.
Treatment of these infections is often difficult and protracted, but successfully treated cases have been reported.9 Clarithromycin forms the cornerstone of therapy, but this agent should be combined with clofazimine, rifampicin and/or enrofloxacin (5mg/kg PO q 24hr) to reduce the chance of antimicrobial resistance. Surgical excision of granulomatous tissue, if feasible, may be beneficial.
1. Gunn-Moore DA, Greene CE. Infections caused by slow-growing mycobacteria. In: Infectious diseases of the dog and cat. Ed. Greene CE. 3rd Edn. Saunders Elsevier, St Louis. 2006 pp 462-477.
2. Hughes MS, James G, Taylor MJ, McCarroll J, Neill SD, Chen SCA, Mitchell DH, Love DN, Malik R. PCR studies of feline leprosy cases. Journal of Feline Medicine and Surgery 2004;6:235-243.
3. Malik R, Hughes MS, James G, Martin P, Wigney DI, Canfield PJ, Chen SCA, Mitchell DH, Love DN. Feline leprosy: two different clinical syndromes. Journal of Feline Medicine and Surgery
4. Fyfe J, McCowan C, O'Brien CR, Globan M, Birch C, Revill P, Barrs VRD, Wayne J, Hughes MS, Holloway SA, Malik R. Molecular characterisation of a novel fastidious mycobacterium causing lepromatous lesions of the skin, subcutis, cornea and conjunctiva of cats living in Victoria, Australia. In preparation 2007.
5. Appleyard GD, Clark EG. Histologic and genotypic characterization of a novel Mycobacterium species found in three cats. Journal of Clinical Microbiology 2002;40:2425-2430.
6. Davies JL, Sibley JA, Myers S, Clark EG, Appleyard GD. Histological and genotypical characterization of feline cutaneous mycobacteriosis: a retrospective study of formalin-fixed paraffin-embedded tissues. Veterinary Dermatology 2006;17:155-162.
7. Malik R, Wigney DI, Dawson D, Martin P, Hunt GB, Love DN. Infection of the subcutis and skin of cats with rapidly growing mycobacteria: a review of microbiological and clinical findings. Journal of Feline Medicine and Surgery 2000;2:35-48.
8. Jang SS, Hirsch DC. Members of the genus mycobacterium affecting dogs and cats. Journal of the American Animal Hospital Association 2002;38: 217-220
9. Baral RM, Metcalfe SS, Krockenberger MB, Catt MJ, Barrs VR, McWhirter C, Hutson CA, Wigney DI, Martin P, Chen SCA, Mitchell DH, Malik R. Disseminated Mycobacterium avium infection in young cats: overrepresentation of Abyssinian cats. Journal of Feline Medicine and Surgery 2005;8:23-44.