Mycobacteria spp.

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BACTERIA



Overview

  • Mycobacterial infections are caused by bacteria belonging to the family Mycobacteriaceae, order Actinomycetales
  • Includes obligate pathogens, opportunistic pathogens and saprophytes
  • Cause chronic, progressive, granulomatous infections
  • Cause tuberculosis, Johne's disease and feline leprosy
  • M. bovis, M. tuberculosis and M. avium cause tuberculosis of cattle, tuberculosis of pigs and tuberculosis of dogs respectively
  • The 'classical' tuberculosis lesions are caused by the Mycobacterium tuberculosis complex
  • The Johne's type lesions are caused by the Mycobacterium avium complex
  • Environmental species are found in soil, vegetation and water
  • Mycobacterium leprae and M.lepraemurium cause human, feline/murine leprosy respectively
  • Atypical mycobacteriosis is a localized opportunistic skin and subcutaneous infection caused by saprophytic and rapidly growing atypical mycobacteria
  • Granulomatous lesions in muscle and skin
  • Skin tuberculosis of cattle causes nodules along the lymphatics of the limbs
  • Bovine farcy is thought to be caused by M. senegalense and M. farcinogenes
  • Saprophytic mycobacteria cause granulomatous lesions

Characteristics

  • Aerobic, weakly Gram-positive acid-fast rods
  • Non-motile, non-spore forming
  • Cell walls contain mycolic acid
  • Require egg-based media for growth
  • Slow-growing colonies
  • Resistant to disinfectants and environmental conditions; susceptible to pasteurisation
  • Mycobacteria stain with carbol dyes and resist subsequent decolorization with inorganic acids; this characteristic which is due to the spatial arrangement of mycolic acids within the cell wall makes them acid fast


Identification

  • Identified by Ziehl-Neelson staining
  • Differentiated by culture, biochemical tests, chromatography and molecular techniques
  • Pathogenic species require at least three weeks for growth on egg-based media


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

Avian tuberculosis

  • Caused by members of the M avium complex
  • Depression, loss of condition and lameness in affected birds
  • Granulomatous lesions in liver, spleen, bone marrow and intestines
  • Diagnosis by Ziehl-Neelson staining of smears and post-mortem appearance
  • Tuberculin testing of poultry


Feline leprosy

  • Caused by M. lepraemurium
  • Sporadic infections of cats via bites from infected rodents
  • Subcutaneous nodules form usually on the head or limbs and can ulcerate
  • Smears reveal Ziehl-Neelson-positive rods
  • Diagnosis by histopathology
  • Treatment includes excision of lesions


Johne's Disease (paratuberculosis)

  • Johne's Disease is a chronic, contagious enteritis of ruminants
  • Caused by M avium subsp. paratuberculosis
  • Epidemiology
    • Transmitted to young calves by ingestion of mycobacteria in faeces of infected adults
    • Organisms viable in environment for long periods
    • Long incubation period with clinical signs appearing in cattle over 2 years of age
    • Subclinical carriers can occur, shedding organisms in their faeces
  • Pathogenesis and pathogenicity
    • M avium subsp. paratuberculosis is an intracellular pathogen
    • Mycobacteria are ingested by macrophages in the Peyer's patches
    • Survival and replication of mycobacteria in macrophages initiate an immune-mediated granulomatous reaction
    • Lymphocytes and macrophages accumulate in the lamina propria and submucosa, resulting in marked thickening and folding of the intestinal wall
    • Mesenteric lymph nodes are enlarged
    • A protein-losing enteropathy results, along with failure to absorb nutrients and water
  • Clinical signs
    • Diarrhoea, initially intermittent, and weight loss in cattle
    • Weight loss in sheep and goats
    • Rapidly fatal with weight loss and diarrhoea in some deer
  • Diagnosis
    • All diagnostic procedures have faults but include:
    • Microscopy of rectal biopsies
    • Faecal culture
    • Serology of serum including complement fixation tests, agar-gel immunodiffusion test and an ELISA
    • Histopathology of intestines and lymph nodes
    • Isolation and identification of mycobacteria from faeces and tissues
    • Ziehl-Neelson-positive smears
    • Intradermal tuberculin test
    • DNA probes for detection in faeces
  • Control
    • Slaughter of affected animals
    • Detection and slaughter of subclinical shedders using faecal culture, DNA probes and ELISA
    • Good hygiene to protect young calves
    • Separation and isolation of calves from affected dams
    • Inactivated adjuvanted vaccines are available and reduce shedding of mycobacteria but do not eliminate infection