Bovis ISSN 2398-2993

Mycobacterium tuberculosis

Synonym(s): M. tuberculosis

Contributor(s): Vetstream Ltd, Veronica Fowler

Introduction

Classification

Taxonomy

  • Kingdom: Bacteria.
  • Phylum: Actinobacteria.
  • Order: Actinomycetales.
  • Suborder: Corynebacterineae.
  • Family: Mycobacteriaceae.
  • GenusMycobacterium - closely related to Corynebacterium, Norcadia and Rhodococcus.
  • Species: Mycobacterium tuberculosis (M. tuberculosis).
Not to be confused with the causal agent of bovine tuberculosis (Mycobacterium bovis Mycobacterium bovis).

M. tuberculosis primarily causes disease in humans, non-human primates, dogs and birds, but cases have been reported in cattle.

Etymology

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

Active Forms

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Clinical Effects

Epidemiology

Habitat

  • Reservoir in infected tuberculous individuals.
  • Human beings perpetuate M. tuberculosis.

Lifecycle

  • Multiplication occurs both intracellularly in macrophages and extracellularly.
  • M. tuberculosis is able to evade phagocytic killing inside macrophages due to the large proportion of mycolic acid in the bacterial cell wall.

Transmission

  • Transmission of tuberculosis caused by M. bovis Mycobacterium bovis and M. tuberculosis can occur from human to animals and vice versa.
  • M. tuberculosis does not appear to have an indigenous animal maintenance host.
  • Transmitted through aerosols or fomites - mainly from respiratory discharges from infected animals.

Pathological effects

  • The organism gains access to the body, usually via the respiratory tract, and avoids initial killing by host phagocytes.
  • Gross lesions are not always seen at post-mortem in cattle. The severity of pathology in cattle infected with M. tuberculosis is generally less than the pathology seen in cattle carcasses infected with M. bovis Mycobacterium bovis.
  • The virulence of the organism is due to the high mycolic acid content of the cell wall which protects the bacilli from phagocytosis.
  • Initially, the organism proliferates, and lymphatic spread may occur at this stage. Acute or subacute inflammation occurs with polymorphonuclear infiltration.
  • After delayed hypersensitivity develops, granulomatous inflammation supervenes, and the macrophages become elongated and are concentrically arranged to form a tubercle.
  • Granulomata usually respiratory, gastrointestinal or in regional lymph nodes.
  • Outside these epitheliod cells a fibrous layer builds up and caseous necrosis occurs at the center of the lesion.
  • Liquefaction of the caseous lesion occurs and cavity develops in which further proliferation of the organism takes place.
  • Further spread may occur, via the erosion of bronchi or viscera, to new areas or via the bloodstream.
  • Systemic illness usually only occurs if the host is immunosuppressed.

Control

In countries with eradication programs, it is likely to be illegal to treat affected animals.

  • Antituberculosis chemotherapy of pet animals is discouraged due to the risk of zoonotic infection. Treatment of food-producing animals would be considered irresponsible and inappropriate. However, if treatment was to ever be attempted in a non-food producing animal, then the following would be considered:
    • First-line drugs for tuberculosis Tuberculosis therapy are streptomycin Streptomycin, isoniazid, ethambutol, and rifampicin.
    • Combination of drugs are usually used because resistance often develops under a single-drug regime.
    • Long-term therapy is required to effect a cure and eliminate the organism (9-24 months).
    • Short or incomplete courses of therapy are an important cause of the development of resistant M. tuberculosis.
    • Prophylactic treatment with isoniazid may be considered for pets recently exposed to tuberculosis.

Diagnosis

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Further Reading

Publications

Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Bernardo Villarreal-Ramos et al (2018) Experimental infection of cattle with Mycobacterium tuberculosis isolates shows the attenuation of the human tubercle bacillus for cattle. Sci Rep (1), 894 PubMed.
  • Hlokwe T M, Said H & Gcebe N (2017) Mycobacterium tuberculosis infection in cattle from the Eastern Cape Province of South Africa. BMC Vet Res 13 (1), 299 PubMed.
  • Carpenter J L et al (1998) Tuberculosis in 5 Basset hounds. JAVMA 192 (11), 1563-1586.
  • Aranaz A, Liebana E, Pickering X et al (1996) Use of PCR in the diagnosis of TB in dogs and cats. Vet Rec 138, 276-280.
  • Hart C A, Becking N J & Duerden B I (1996) Tuberculosis into the next century. J Med Microbiol 44, 1-34.
  • Clerex C et al (1992) Tuberculosis in dogs - A case report and review of the literature. JAAHA 28, 207.
  • Tuedten H W et al (1990) Mycobacterium bacteremia in a dog - diagnosis of septicemia by microscopical evaluation of blood. JAAHA 26, 359.

Other sources of information

  • Jordan H L (1995) Canine and Feline Mycobacterial Infections. In: Current Veterinary Therapy XII. Eds: Bonagura J D & Kirk R W. W B Saunders, USA. pp 320-323.

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