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