TB and other Mycobacteria (McAuliffe) - 11/30/16

Question 1

What causes tuberculosis (TB)?

Two types

Answer 1

Inhalation of aerosolized Mycobacterium tuberculosis

Primary and Secondary

Question 2

Pathogenesis:

How does Primary TB arise and what does it result in?

Symptomatic?

Answer 2

Primary TB arises with initial exposure

• Inhalation → Phagocytosis by alveolar macrophages (macrophages unable to kill organisms)
• Bacterial multiplication occurs intracellularly
• Macrophages themselves spread to regional lymph nodes or to blood
• Immune response results in granuloma formation (containment of infection)
• Cell death in granuloma → caseous necrosis

Result: focal, caseating necrosis in LOWER LOBE OF LUNG and HILAR LYMPH NODES –> undergoes fibrosis and calcification, forming a Ghon complex

Generally asymptomatic but leads to POSITIVE PPD

Question 3

How does Secondary TB arise?

Where does it occur?

What does it form?

Clinical features?

Answer 3

Reactivation of Mycobacterium TB commonly due to AIDS; may also be seen with aging

Occurs at apex of lung (relatively poor lymphatic drainage and high O2 tension)

Forms cavitary foci of caseous necrosis;

May also lead to miliary pulmonary TB (tiny regions of TB scattered across entire lung) or tuberculous bronchopneumonia

Clinical features: fevers, night sweats, cough w hemoptysis, weight loss

Question 4

What would a biopsy of Secondary TB reveal?

Answer 4

Caseating granulomas with central necrosis and Langerhans giant cells;

AFB stain reveals acid-fast bacilli

Question 5

Secondary TB: systemic spread often occurs and can involve any tissue.

What are common sites for TB to spread outside lung? (4)

Answer 5

1. Meninges (meningitis)
2. Cervical lymph nodes
3. Kidneys (sterile pyuria)
4. Lumbar vertebrae (Pott disease)

Question 6

What is the strongest risk factor for developing TB (reactivation of latent TB)?

Answer 6

HIV

Question 7

TST testing

Answer 7

Standard method of determining whether a person is infected with mycobacterium tuberculosis

Skin test should be read within 48-72 hrs after administration.

Measure in millimeters of the induration (do NOT measure erythema - redness)

Question 8

Interferon Gamma Release Assays (IGRAs)

Answer 8

• Whole blood in vitro test:
  
  • Lymphocytes release IFN gamma in presence of TB antigens
• Will be positive in latent or active TB
• Advantages:
  
  • No error in interpretation
  • No follow-up in 48-72 hrs
  • No boosting
  • Not affected by BCG vaccination (false positive on PPD test)
• Disadvantages:
  
  • Must be processed within 12 hrs of collection
  • Indeterminate results
  • May be less reliable in pregnant women, children, immunocompromised
  • Does not distinguish between active and latent TB