Difference between revisions of "Mycobacterium bovis"

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(Created page with '===Bovine tuberculosis=== *Epidemiology **World-wide disease caused by ''M. bovis'' **Aerosol transmission between cattle kept in close contact **Transmission to calves via inge…')
 
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Revision as of 10:48, 25 June 2010

Bovine tuberculosis

  • Epidemiology
    • World-wide disease caused by M. bovis
    • Aerosol transmission between cattle kept in close contact
    • Transmission to calves via ingestion od contaminated milk
    • Wildlife reservoirs include badgers and possibly deer in the Europe
  • Pathogenesis and pathogenicity
    • The ability of mycobacteria to survive and multiply within macrophages determines whether disease will occur within the host
    • Survival and multiplication in macrophages at primary site of infection due to prevention of phagosome-lysosome fusion
    • Mycobacteria utilize several virulence factors including cord factor or trehalose dimycolate, surface glycolipid, sulfatides, lipoarabinomannan, heteropolysaccharide, heat shock protein, complement, and tubuloprotein
    • The types of immune responses that are critical in responding to mycobacterial infection are cell-mediated immunity and the delayed hypersensitivity response
    • Pathogenicity of mycobacteria depends on their ability to escape phagocytic killing, mostly imparted by the cell wall consitiutents:
      • Cord factor (trehalose dimycolate) – surface glycolipid responsible for serpentine growth in vitro
      • Suphatides – surface glycolipid containing sulphur which prevents fusion of phagosome with lysosome. cAMP secreted by the bacteria may also facilitate this.
      • LAM – heteropolysaccharide which inhibits macrophage activation by IFNγ and induces macrophages to secrete TNFα which induces fever and IL-10 which suppresses mycobacteria-induced T cell proliferation
      • The wax of the cell wall, peptidoglycans and other glycolipids are responsible for the adjuvant activity – attracts antigen presenting cells
      • Tubuloprotein – important antigen; purified tubuloprotein is the basis of the tuberculin test
    • Mycobacteria are released from macrophages and also migrate within macrophages around the body
    • Waxy cell wall contributes to the host immune response to the mycobacteria and the development of lesions
    • Cell-mediated immune response with activated macrophages and sensitised T cells
    • Delayed-type hypersensitivity response with granuloma formation
    • Lesions contain macrophages, multinucleate giant cells and later a central area of caseous necrosis, giving a cheesy appearance
  • Clinical signs
    • Initially asymptomatic
    • Loss of condition
    • Cough and intermittent pyrexia with lung pathology
    • Tuberculous mastitis with transmission via milk
  • Diagnosis
    • Tuberculin test - comparative intradermal test
    • Avian and bovine tuberculin (purified protein derivative) is injected intradermally into two different clipped sites on the side of the neck
    • Skin thickness at these sites is compared before and 72 hours after the injection of tuberculin with calipers
    • Increases in skin thickness at the bovine PPD site of more than 4mm greater than the avian PPD site are seen as positive (reactor)
    • Blood tests including the gamma interferon assay are being developed
    • Laboratory examination of lesions, lymph nodes and milk
    • Ziehl-Neelson staining of tissues
    • Isolation requires Lowenstein-Jensen medium
  • Control
    • Eradication programs using a test and slaughter policy
    • Reactors positive to the tuberculin test are slaughtered and restrictions applied to the affected herd