Canis ISSN: 2398-2942

Mycobacterium tuberculosis

Synonym(s): M. tuberculosis

Contributor(s): Richard Walker

Introduction

Classification

Taxonomy

  • Family: Mycobacteriaceae.
  • Genus: Mycobacterium- closely related to CorynebacteriumNocardia Nocardia spp and Rhodococcus.

Etymology

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

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Clinical Effects

Epidemiology

Habitat

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

Lifecycle

  • Multiplication occurs both intracellularly in macrophages and extracellularly.

Transmission

  • Infection in dogs usually contracted from humans.
  • By 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.
  • The subsequent lesions produced are in part due to the cell-mediated immune response which is generated after the infection has become established.
  • Tuberculosis Pulmonary tuberculosis.
  • The virulence of the organism is due to the lipids of the cell wall which protect 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 a 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 host immunosuppressed.

Control

Control via animal

  • Antituberculosis chemotherapy of animals is discouraged due to the risk of zoonotic infection.
  • In countries with eradication programs, it may be illegal to treat affected animals.

Control via chemotherapies

  • First-line drugs for tuberculosis therapy are streptomycin Streptomycin , isoniazid, ethambutol, and rifampicin.
  • Combinations 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 VetMedResource and PubMed.
  • Aranaz A, Liebana E, Pickering X, Novoa C, Mateos L & Dominnguez L (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.
  • Carpenter J L et al (1998) Tuberculosis in 5 Basset hounds. JAVMA 192(11), 1563-1586.
  • 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, Philadelphia: WB Saunders. pp 320-323.

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