Lapis ISSN 2398-2969

Mycobacterium spp

Contributor(s): Sarah Pellett, Vicki Baldrey

Introduction

Classification

Taxonomy

  • Family: Mycobacteriaceae.
  • Genus: Mycobacterium.

Etymology

  • Mycobacterium – small fungus-like rod:
    • Gr: myces – a fungus;  bakterion – a small rod.
  • Tuberculum – a small swelling; bovis – of the ox; avium – of birds.

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Clinical Effects

Epidemiology

Habitat

  • Reservoir in affected individuals.
  • Humans perpetuate M. tuberculosis.
  • Soil and water can contain M. avium complex.

Lifecycle

  • Multiplication occurs both intracellularly in macrophages and extracellularly.

Transmission

  • By aerosols or fomites. In the one reported atypical mycobacterial infection confirmed in a juvenile dwarf rabbit the lungs were the only affected organ and the authors suggested infection occurred via inhalation.

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.
  • 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.
  • Granulomas form usually within respiratory system, gastrointestinal system or in regional lymph nodes.
  • Outside these epitheliod cells a fibrous layer builds up and caseous necrosis occurs at the centre 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 sites or via the bloodstream.
  • Systemic illness occurs when the host is immunocompromised.
  • Cytotoxic cellular response or the T helper (Th1) response is vital for the hosts’ immune defence against mycobacterial infection. Columbia Basin pygmy rabbits produce a lower concentration of gamma interferon -> this results in a decrease in macrophage activation -> leads to a lower number of killer T cells making them more susceptible to spontaneous disseminated mycobacteriosis.  Pygmy rabbits also have elevated levels of Th2 cytokines resulting in inhibition of Th1 responses which aid in protecting the host against mycobacteria.

Control

Control via animal

  • Anti-tuberculosis chemotherapy of animals is generally discouraged due to the risk of zoonotic infection.
  • It may be illegal to treat affected animals in certain countries with eradication programs.

Control via chemotherapies

  • First-line drugs for tuberculosis therapy in other species are streptomycin Streptomycin, isoniazid, ethambutol and rifampicin. 
  • Recommendations for treatment for the pygmy rabbit cases were long term medication of a combination of azithromycin Azithromycin, rifabutin and ethambutol.
  • Combinations of drugs are usually used because resistance often develops under a single-drug regime.
  • In other species treatment duration is approximately 9-24 months.
  • Short or incomplete treatment courses contribute to the development of resistant M. tuberculosis.
  • In other species, prophylactic treatment with isoniazid may be considered for animals recently exposed to tuberculosis.

Control via environment

  • For the captive pygmy rabbits, soil used in the enclosures was sterilized.

Diagnosis

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • McClure D E (2012) Mycobacteriosis in the rabbit and rodent. Vet Clin North Am Exot Anim Pract 15 (1), 85-99 PubMed.
  • Harrenstein L A, Finnegan M, Case A et al (2011) Disease issues affecting species recovery of pygmy rabbits. In: Proc 32nd Annual Conference and Expo of the Association of Avian Veterinarians and Association of Exotic Mammal Veterinarians. Seattle. pp 433-438 AvianExoticVetCare.
  • Ludwig E, Reischl U, Janik D et al (2009) Granulomatous pneumonia caused by Mycobacterium genavense in a dwarf rabbit (Oryctolagus cuniculus). Vet Pathol 46 (5), 1000-1002 PubMed.
  • Harrenstein L A, Finnegan M V, Woodford N L et al (2006) Mycobacterium avium in pygmy rabbits (Brachylagus idahoensis): 28 cases. J Zoo & Wildlife Med 37 (4), 498-512 PubMed.
  • Daniels M J, Hutchings M R, Beard P M et al (2003) Do non-ruminant wildlife pose a risk of paratuberculosis to domestic livestock and vice versa in Scotland? J Wildlife Dis 39 (1), 10-15 PubMed.
  • Hines M E 2nd, Kreeger J M, Herron A J (1995) Mycobacterial infections of animals: pathology and pathogenesis. Lab Anim Sci 45 (4), 334-351 PubMed.

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