Felis ISSN 2398-2950

Mycobacterium spp

Contributor(s): Karen Coyne, Vetstream Ltd, Conor O'Halloran

Introduction

Classification

Taxonomy

  • Order: Actinomycetales
  • Family: Mycobacteriaceae
  • Genus: Mycobacterium; morphologically similar, aerobic, non-spore forming, non-motile bacteria.
  • All mycobacteria are morphologically similar, aerobic, non-spore forming, non-motile bacteria.
  • Mycobacterium are closely related to other intracellular pathogens such as Corynebacterium  CorynebacteriumNocardia  Nocardia spp and Rhodococcus.
  • The various mycobacterial species that have been identified in cats can be grouped into 2 major categories:

1. The Mycobacterium tuberculosis - complex (MTBC)

  • The MTBC consists of several closely related species of mycobacteria capable of causing tuberculosis (TB) in man and other animals.
  • Culture results from the UK have shown that a third of all, and 75%  of culture positive infections were organisms belonging to the MTBC.
  • Of the MTBC pathogens, only Mycobacterium bovis Mycobacterium bovis and Mycobacterium microti have been frequently detected in cats.
  • Mycobacterium tuberculosis Mycobacterium tuberculosis infection, the leading cause of human TB, has only occurred via laboratory infection in the cat and it has been demonstrated that cats possess a high degree of reistance to natural infection with this pathogen though the mechanism for this is not known.

2. Non-tuberculous mycobacteria (NTM)

  • Otherwise referred to as environmental mycobacteria, atypical mycobacteria or mycobacteria other than tuberculosis (MOTT).
  • NTM agents are environmental mycobacteria found in the soil, water, aerosols, protozoa, deep litter and fresh tropical vegetation.
  • More than 140 species have been identified within this group, but not all are capable of causing disease in animals or humans. The species of clinical significance to animals can be divided into:
    • 1. M. leprae complex (MLC): the MLC contains 6 organisms: M. leprae, M. lepromatosis, M. lepraefelis. M. visible. M. lepraemurium and the unnamed causative agent of bovine nodule thelitis. Together these organisms are the causative agents of leprosy in humans, armadillo and red squirrels (M. leprae) and feline leprosy syndrome in cats (M. lepraemurium Mycobacterium lepraemurium, M. lepromatosis and M. visible).
    • 2. M. avium-intracellulare complex (MAC) Mycobacterium avium: MAC infections are significant as the most frequently confirmed NTM infection in companion animals are also (theoretically) potential zoonoses.
    • 3. Slowly growing NTM: these organisms take longer than 7 days to culture in laboratory conditions and are divided into sub-types based on the pigmentation of colonies.
    • 4. Rapidly growing NTM: these organisms (eg M. fortuitum, M. abscessus and M. chelonae) grow in laboratory culture conditions in fewer than 7 days.
  • All these groups of opportunistic pathogens most often infect cats via contamination of open wounds which is reflected in the distribution of lesions on the ventral abdomen.
  • Inoculation of the organism directly into subcutaneous adipose tissue seems to increase the severity of observed disease in cats, to which over-weight cats are predisposed.
  • MAC infections have been successfully cultured most frequently from cats; however the culture data for NTM infections is inevitably skewed by the fact that MLC organisms are not culturable.
  • MAC infections are seen more frequently in Somali Somali, Abyssinian Abyssinian and Siamese Siamese cats, but there is no breed predisposition for the other NTM.
  • Adult males that hunt are most frequently diagnosed and some studies have shown that the prevalence is highest in older cats, especially those with chronic kidney disease and/or FIV infection Feline immunodeficiency virus.
  • Other risk factors that have been identified for NTM disease in cats include FeLV positivity Feline leukemia virus, co-infection with toxoplasmosis Toxoplasmosis, the administration of immunosuppressive drugs such as exogenous corticosteroids, and lymphopenia.

Etymology

  • Gr: myces - a fungus; bakterion - a small rod.

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Clinical Effects

Epidemiology

Transmission

Tuberculous Mycobacteria (M. tuberculosisM. bovis and M. microti)

  • Dogs and cats acquire M. bovis infection when they consume or are bitten/scratched by infected prey, drink contaminated milk or scavenge contaminated carcasses.
  • Aerosolized droplets are the primary transmission mechanism of M. tuberculosis.
  • Cats acquire M. microti infection via hunting of wild rodents, therefore, cats most affected appear to be adult males, and keen hunters. 
  • No current evidence suggests classical immunosuppression increases host susceptibility.

Non tuberculous mycobacteria 

  • Infection most frequently occurs via contamination of open wounds from contaminated soil but can also occur by ingestion of meat or contact with infected soil, water sources or fomites.

Pathological effects

Tuberculous Mycobacteria (M. tuberculosisM. bovis and M. microti)

  • Tubercule bacilli enter via skin penetration, respiratory tract and rarely the gastrointestinal system where they have a chronic incubation period.
  • Local multiplication of the bacillus can develop at the initial site (primary complex), usually in the subcutis, resulting in granuloma (nodule) formation with associated local lymphadenopathy.
  • Cats respond erratically to intradermal tuberculin testing.
  • Immunity largely cell-mediated, but lesions formed in part by the cell-mediated immune response.
  • Antibodies, in the few cases where they are detectable, are markers of infection, not indicators of exposure.
  • The bacilli survive in macrophages and possess a number of mechanisms to subvert the immune response and maintain survivability.

Non tuberculous mycobacteria 

  • Organism enters an open wound from the environment.
  • Engulfed by phagocytic cells (dendritic cells and macrophages).
  • There is an unknown but presumed chronic incubation period.
  • Granulomas form in an attempt to contain the organism.
  • Spread to adjacent tissues or throughout the body via lymphatic or hematogenous dissemination.
  • Primary clinical presentation of disseminated M.avium include enlarged lymph nodes, tonsillary inflammation, inappetence and/or anorexia.
  • Submandibular, cervical and mesenteric nodes may also be affected
  • Other reported symptoms have included fever, vomiting, bloody feces, breathing difficulty due to compression of lungs by enlarged nodes, and lameness.
  • Infection disseminates throughout other tissues, including spleen, liver, and bone marrow.
  • Progression of the disease depends on the ability of macrophages to inhibit intracellular growth of the organisms
  • M. avium granulomatous lesions are indistinguishable from tubercular lesions of M. tuberculosis and M.bovis.

Control

Control via chemotherapies

Tuberculous Mycobacteria (M. tuberculosisM. bovis and M. microti)

  • Treatment can be challenging and prognosis is dependent on the species of mycobacteria present.
  • Treatment of M.tuberculosis infections (almost exclusively seen in canines) should not be attempted due to the public health risks attendant with this infection.
  • Surgical excision of lesions can be beneficial but is rarely curative and medical follow up is usually needed.
  • Mycobacteria are resistant to most antimicrobials because of the high lipid content and complexity of their cell walls, together with their ability to reside within macrophages.
  • A wide range of drugs have been used successfully: for MTBC infections the best outcomes are seen with a combination of rifampicin Rifampicin, azithromycin Azithromycin/clarithromycin Clarithromycin and pradofloxacin/marbofloxacin for a minimum of 3 months and for 2 months beyond the resolution of clinical signs.
  • Long-term therapy is required to effect a cure and eliminate the organism (3-6 months or more).

Non tuberculous Mycobacteria

  • Treatment can be challenging and prognosis is dependent on the species of mycobacteria present.
  • Surgical exicion of lesions can be beneficial but is rarely curative and medical follow up is usually needed.
  • Mycobacteria are resistant to most antimicrobials because of the high lipid content and complexity of their cell walls, together with their ability to reside within macrophages.
  • A wide range of drugs have been used successfully: for NTM infections include combinations of rifampicin Rifampicin with clofazime Clofazimine , doxycycline Doxycycline, clarithromycin Clarithromycin and ethambutol.
  • Long-term therapy is required to effect a cure and eliminate the organism (9-24 months).
  • Treatment for NTM infections is usually prolonged and resolution of signs takes, on average, a year.

Vaccination

  • No vaccine available.

Diagnosis

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Gunn-Moore D A, Gaunt C, Shaw D J (2013) Incidence of mycobacterial infections in cats in Great Britain: estimate from feline samples submitted to diagnostic laboratories. Transbound Emerg Dis 60 (4), 338-344 PubMed.
  • Gunn-Moore D A, McFarland S E, Brewer J I et al (2011) Mycobacterial disease in cats in Great Britain: I. Culture results, geographical distribution and clincial presentation of 339 cases. J Fel Med Surg 13 (12), 934-944 PubMed
  • Rivière D, Pingret J L, Etievant M et al (2011) Disseminated Mycobacterium avium subspecies infection in a cat. J Feline Med Surg 13 (2), 125-128 PubMed
  • Baral R M, Metcalfe S S, Krockenberger M B et al (2006) Disseminated Mycobacterium avium infection in young cats: overrepresentation of Abyssinian cats. J Fel Med Surg (1), 23-44 PubMed.
  • Barry M, Taylor J & Woods J P (2002) Disseminated Mycobacterium avium infection in a cat. Can Vet J 43 (5), 369-371 PubMed.
  • Arvand M, Mielke M E, Weinke T et al (1998) Primary isolation of Mycobacterium tuberculosis on blood agar during the diagnostic process for cat scratch disease. Infection 26 (4), 254 PubMed.
  • Gunn-Moore D A & Shaw S (1997) Mycobacterial disease in the cat. In Practice 19 (9), 493-499, 501 VetMedResource.
  • Hughes M S, Ball N W, Beck L A et al (1997) Determination of the etiology of presumptive feline leprosy by 16S rRNA gene analysis. J Clin Microbiol 35 (10), 2464-2471 PubMed
  • Aranaz A, Liébana E, Pickering X et al (1996) Use of polymerase chain reaction​ in the diagnosis of tuberculosis in dogs and cats. Vet Rec 138 (12), 276-280 PubMed.
  • Gunn-Moore D A, Jenkins P A & Lucke V M (1996) Feline tuberculosis; a literature review and discussion of 19 cases caused by an unusual mycobacterial variant. Vet Rec 138 (3), 53-58 PubMed
  • Hart C A, Beeching N J & Duerden B I (1996) Tuberculosis into the next century. Proceedings of a symposium held on 4 February 1995 at the Liverpool School of Medicine.​ J Med Microbiol 44 (1), 1-34 PubMed.
  • Jordan H L, Cohn L A, Armstrong P J (1994) Disseminated Mycobacterium avium complex infection in three Siamese cats. JAVMA 204 (1), 90-93 PubMed
  • Isaac J, Whitehead J, Adams J W et al (1983) An outbreak of Mycobacterium bovis in cats in an animal house. Aus Vet J 60 (8), 243-245 PubMed.
  • Orr C M, Kelly D F, Lucke V M (1980) Tuberculosis in cats: a report of two cases. JSAP 21 (4), 247-253 PubMed

Other sources of information

  • Greene C E (2006) Mycobacterial infections. In: Infectious diseases of the Dog and Cat. 2nd Edn. Ed. Greene C E. W B Saunders Co. pp 462-488.

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