Infection: Mycobacterium Tuberculosis

Question 1

How is tuberculosis diagnosed?

Answer 1

Cant be gram stained so use acid fast staining instead - it is a bacillus
Tuberculin skin test
Chest xrays
Culture sputum

Question 2

Does mycobacterium tuberculosis grown fast or slow?

Answer 2

Very slow growing - the generation time is 15-20hrs - therefore culture can take 2 - 12 weeks

Question 3

What is the mode of transmission of mycobacterium tuberculosis?
How contagious is TB?

Answer 3

Via respiratory droplets - coughing, sneezing, speaking. Casual contact is not enough for transmission - need prolonged exposure eg family members, homeless, prisons

Question 4

Why is mycobacterium tuberculosis not phagocytosed by alveolar macrophages?

Answer 4

It has a cell wall with a high lipid content which prevents fusion of the phagosome to the lysosome.
It can even multiply inside alveolar macrophages

Question 5

What is the treatment for TB?

Answer 5

R ifampicin (makes urine orangey red)
I troniazid
P yrazinamide
E thambutol

Because TB is slow growing it is a prolonged course (6months - 2 years) therefore need to closely monitor compliance. If taken correctly 90% cure rate

Question 6

What is latent TB and how can it progress?

Answer 6

Latent infection is where there are live bacilli but they are dormant - not multiplying.
From latency 95% of cases will self cure and the other 5% will become post primary TB (the bacilli start multiplying).

Question 7

What are the main differences between latent TB infection and TB disease?

Answer 7

Latent TB is not a case of TB - there are no symptoms, normal xray, sputum negative. However there is a strong immune response.
TB disease is infectious, has abnormal xray and there are symptoms.

Question 8

What are the risk factors for latent TB being reactivated to post primary TB infection?

Answer 8

• Infection with HIV is the highest risk (therefore all cases of TB are tested for HIV)
• immunosuppressive therapy
• diabetes mellitis
• malignancy

Question 9

How can pulmonary TB spread to become miliary TB? (whole body)

Answer 9

Can spread directly, through the blood stream or via lymphatics

Question 10

What are the common symptoms for pulmonary TB?

Answer 10

Fever, weight loss, night sweats, malaise

Cough (most common) may be heamoptysis and breathlessness if pleural effusion

Question 11

What is the appearance of the chest Xray in someone with pulmonary TB?

Answer 11

ill-defined patchy consolidation
Can get cavitations
May have pleural effusion

Question 12

What is the tuberculin sensitivity test?

Answer 12

Used to diagnose latent TB
Tuberculin is injected intradermally and read 48-72 hours later - the swelling should be measured (not the redness)
Looking for a strong immune response

Question 13

TB is a notifiable disease, what does this mean?

Answer 13

The doctor making or suspecting a diagnosis of TB is legally responsible to notify public health so they can begin screening of close contacts.

Question 14

How can TB be prevented?

Answer 14

BCG vaccine - a live vaccine therefore cant be given to HIV pts.
Given to babies in high prevalence communities - 70-80% effective at preventing severe childhood TB.

Question 15

What is extra-pulmonary TB and how can this present?

Answer 15

TB affects other parts of the body eg

• TB meningitis (chronic headache, fevers)
• lymph node TB (abcesses, mainly affects cervical LNs)
• orthapaedic TB (spreads to bones and joints via blood, spinal TB is most common)