Mycobacterium spp

From Cow
Electron microscopic view of M. bovis

Mycobacteria are aerobic, non-motile bacteria which cause tuberculosis and leprosy in cattle worldwide. They not only causes significant financial loss but is also a public health hazard[1].

Mycobacteria contain an hydrophobic cell wall which makes them resistant to many antimicrobial drugs.

The main Mycobacterium bacilli identified in cattle are:

  • M. tuberculosis
  • M. bovis
  • M. avium spp. paratuberculosis - Johne's disease (Paratuberculosis)
  • M caprae
  • M. pinnipedii
  • M. microti
  • M. africanum

Infections in cattle occur through inhalation of spores on soil or fomites, resulting in colonisation of the respiratory tract. bacterial growth is slow, often requiring months before clinical disease is noted. A 'tubercle' forms as alveolar pmacrophages attempt to kill the invading bacteria and surrounding epithelioid cells, granulocytes, lymphocytes, and multinucleated giant cells form an isolating walled off nodule around the purulent debris[2]. Calcification results, allowing granulation tissue and a fibrous capsule formation. Nodules may form in the pleura, peritoneum, liver, kidney, skeleton, mammary glands, reproductive tract, and central nervous system[3].

Clinical signs

Affected cattle, often more than 2 years of age, display signs of fever, anorexia and weight loss. Dyspnea and coughing are reliable indicators of active infections. Haemoptysis is usually a terminal sign of advanced disease.

Ausculation of the chest may reveal moist rales indicative of pneumonia. In some cases, the prescapular lymph nodes may be enlarged.

Diagnosis

Preliminary diagnosis can be made on historical evidence of Mycobacterium spp prevalent in the area supported by respiratory symptoms[4]. Intradermal tuberculin testing of herd is usually a definitive diagnostic test in the field. PCR testing of dairy gerd milk is a method of monitoring control methods[5].

Postmortem of euthanased or dead cattle will confirm the presence of tuberculous granulomas in the lungs and surrounding tissue[6].

Isolation of Mycobacterium in the laboratory is diagnostic, though cultures can take up to six weeks to confirm. Molecular techniques, such as restriction fragment length polymorphism and spoligotyping provide useful information in conducting epidemiologic investigations[7].

References

  1. Biet F et al (2005) Zoonotic aspects of Mycobacterium bovis and Mycobacterium avium-intracellulare complex (MAC). Vet Res 36:411–436
  2. McDonough KA et al (1993) Pathogenesis of tuberculosis: interaction of Mycobacterium tuberculosis with macrophages. Infect Immun 61:2763–2773
  3. Whipple DL et al (1996) Distribution of lesions in cattle infected with Mycobacterium bovis. J Vet Diagn Invest 8:351–354
  4. Neill SD et al (1994) Pathogenesis of Mycobacterium bovis infection in cattle. Vet Microbiol 40:41–52
  5. Silaigwana B et al (2012) Molecular Detection and Drug Resistance of Mycobacterium tuberculosis Complex from Cattle at a Dairy Farm in the Nkonkobe Region of South Africa: A Pilot Study. Int J Environ Res Public Health 9(6):2045-2056
  6. Corner LA (1994) Post mortem diagnosis of Mycobacterium bovis infection in cattle. Vet Microbiol 40:53–63
  7. Merck Vet Manual