Mycobacteria spp.

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 and murine leprosy
• Atypical mycobacteriosis is a localized opportunistic skin and subcutaneous infection caused by saprophytic and rapidly growing atypical mycobacteria
• Granulomatous lesions in muscle and skin

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 4cm 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