Mycobacteria spp.
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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