Mycobacteria
Phenotypic characteristics
Aerobic
Non-spore forming & non-motile
Rod shaped / bacilli
Acid-fast
Require complex egg-enriched media for culture growth
Acid-fast organism
mycolic acid
Same cell wall structure as gram +ve but stain poorly
Cell wall contains peptidoglycan, lipids and mycolic acid within layer
High lipid and mycolic acid protects from acids, alkalis and digestion in macrophages. Impedes entry of molecules, slow growing, responsible for inflam response
Typical habitats
Lipid rich walls --> hydrophobic, resistance to adverse environments
Environmental found in soil, vegetation and water sources
'Obligate pathogen' varieties shed by infected animals
Mycobacterial disease = chronic + progressive
High degree of host specificity
Important species and host/diseases
M. tuberculosis (MTBC) - Human tuberculosis
M. bovis (MTBC) - Cattle tuberculosis
M. africanum (MTBC) - Human tuberculosis (Africa)
M. avium subsp avium - Avian tuberculosis
M. microti (MTBC) - Voles tuberculosis
M. marinum - Fish tuberculosis
M. leprae - Man leprosy
M. lepraemuriam - Rat+feline leprosy
M. avium subsp. paratuberculosis - Cattle, sheep, goats, deer paratuberculosis (Johne's disease)
MTBC - Mycobacterium tuberculosis complex
Typing
Ziel-Neelsen (ZN) method used to stain, stain red
Differentiation uses: Cultural characteristics, biochemical tests, animal innoculation and molecular techniques
Phylogenetics and taxonomy
2 groups cluster v. closely on 16S rRNA gene
Bovine and human TB caused by members of a group of closely phylogenetically related bacteria called MTBC
Avian TB and Johne's disease caused by emembers of mycobacterium avium complex (MAC) which are closely related
Mycobacteria host adaptation
M.bovis used to be significant cause of human TB - Raw milk
M. bovis infections decreased rapidly following pasteurization and meat-control practices
M. bovis does not easily transmit between humans. (similarly M. tuberculosis does not easily travel from animal to animal). Therefore adapted for host
Mycobacterium bovis
Pathogenesis
Clinical signs
Bovine TB
Zoonotic + economic implications
Eradication programs worldwide, some successful
Transmission through aerosols created from cattle
Risk factors for transmission: Possums/badgers and housing
Contaminated milk - Can infect calves or humans
Macrophages accumulate at infection site
Granuloma formation = inflam response in lungs, recognizable tubercle
Able to survive+multiply in host macrophages
Only present in advances disease
In advanced pulmonary TB animals develop cough/intermittent pyrexia
Mammary tissue infection --> Lymph node enlargement + mastitis --> increases spread
Diagnosis
Tuberculin test (standard ante-mortem test)
Reactive 30-50 days after infection. Delayed hyper-sensitivity to M. bovis tuberculoprotein. Purified protein derivative injected intradermally, PPD injected and site checked a few days later.
Current control strategies
Treatment/vaccination inappropriate
Most countries worldwide = tuberculin testing of cattle followed by isolation and slaughter or reactors
Routine meat inspection
Culling of wildlife reservoirs
Control in UK considerations (bovine TB)
One of larger studies shows badgers involved in transmission
Middle of cull = Reduction in bovine TB
Edge of cull = Increase in TB. uncertain why
Considerations: Animal conservation, financial: Farmer and national
Research towards cattle vaccine (bovine TB)
Shows efficacy, but no way of differentiation infected vs vaccinated
Mycobacterium bovis BCG most suitable vaccine. Reduces progression, severity and excretion of bacteria
Badger vaccination. Oral vaccine being developed
Mycobacterium tuberculosis (human TB)
Similar to cattle TB
Easily transmitted by resp route - Coughing/speaking
Tuberculin test used
Antibiotic (isoniazid) v. effective - interacts with synth of mycolic acid
Not used in animal for fear of resistance
Vaccination with M. bovis strain: bacillus Calmette-Guerin (BCG) strain
M. lepraemurium
Feline leprosy
Cutaneous disease
Worldwide, endemic in wild rodents in some parts of world
Low prevalence in UK/USA
Transmission through bites from infected rodents
Not zoonotic
DIagnosis on ho=istopathalogical features
Treatment: Surgical excision of lesions and antibiotics for min 2. Months
Mycobacterium avium
Avian tuberculosis = important (although rare in UK)
Chronic and progressive wasting/weakeness. Diarrhoea common
No vaccine available
Control: Entire flock depopulated and repopulated on non-infected soil/fresh litter
M. avium, subsp. Paratuberculosis (MAP)
Johne's disease/paratuberculosis
Chronic contagious enteritis = fatal
Young animals more susceptible, ingestion of contaminated milk
Transmission also possible through faecal shedding
Immune mediated granulomatous reaction
2 when first symptoms
Death 1 year after detection
Worldwide
Endemic in europe and USA