Tuberculosis

Question 1

What is TB? Give a brief description.

Answer 1

Tuberculosis is a chronic, communicable disease caused by mycobacterium tuberculosis (M. TB). Transmission is usually by inhalation of infected droplets. predisposing factors include:

• debilitating illness
• immunosuppresive diseases.

TB is more prevalent in areas of poverty, overcrowding and debilitating disease.

• Cell wall lipids and carbohydrates of M. TB increase its virulence by interfering with phagolysomal fusion.
• Delayed type IV hypersensitivity reaction to the pathogen develops 2-4wks after infection
• Non-specific inflam’y response becomes granulomatous (epithelioid histiocytes, giant cells, peripheral mononuclear cells
• Central caseous necrosis

TB can manifest as either Primary TB or Secondary TB.

Question 2

What is Primary TB?

Answer 2

Primary TB occurs in an individual who has not previously been in contact with M. Tb.

The mycobacterium replicate in macrophages but are controlled by T cells in 95% of cases.

• CD4 and T_Hcells secrete TNF-gamma which activates macrophages to kill intracellular mycobacteria and form epethelioid granulomas.
• CD8 suppressor T cells kill macrophages infected with M. Tb -> caseating granulomas -> calcified scar (Ghon complex) in lung and hilar lymph nodes.

The disease does not usually progress (most are asymptomatic other than flu-like symptoms), but does lead to local scarring and calcification.

Miliary and/or disseminated TB may occur if the bacillus enters the bloodstream.

Question 3

What is Secondary TB?

Answer 3

Secondary TB is active disease in a previously sensitised host.

Usually occurs by reactivation of a dormant bacilli, occasionally can be caused when exogenous source overwhelms immune response.

Usually occurs in the lung apices due higher pO2.

Can progress to miliary, cavitatory TB or tuberculous bronchopneumonia.

Question 4

What is the clinical course of Primary TB?

Answer 4

Usually asymptomatic.

Question 5

What is the clinical course of secondary TB?

Answer 5

Fever, night sweats, weight loss, haemoptysis.

Acid fast bacilli present in sputum cultures and/or lung biopsy.

Question 6

What is systemic miliary TB?

Answer 6

Minute foci of infection - usually in liver, bone marrow, kidney or spleen.

Question 7

What is isolated TB?

Answer 7

TB which spreads via blood to an isolated organ/region. Eg meninges, kidneys, adrenals, fallopian tubes, vertebrae (Potts disease).