pathogenesis of TB

Question 1

what are clinically relevant mycobacteria?

Answer 1

M. tuberculosis= Tuberculosis
M. bovis = Bovine tuberculosis- in vac

M. leprae= Leprosy
M. ulcerans= Buruli ulcer

Question 2

who gets NTM?

Answer 2

• immunosuppressed e.g. HIV

- cystic fibrosis

Question 3

what stain is used for TB?

Answer 3

Ziehl–Neelsen
Auramine fluorescence
“Acid-Fast Bacilli” (AFBs)

Question 4

what is the clinical presentation of pulmonary TB?

Answer 4

Cough (>3wks)
Weight loss
Fatigue
Fever
Night sweats
Hemoptysis (1/3rd)
Difficulty breathing

but some patients may just have mild feelings of non specific malaise

Question 5

who should be medically evaluated for TB?

Answer 5

Anyone with TB symptoms or positive TST or IGRA result should be medically evaluated for TB disease

Question 6

what are the components of a TB medical evaluation?

Answer 6

Components of medical evaluation:

1. Medical/family history (household contacts)
2. Physical examination
3. Test for TB infection
4. Chest x-ray
5. Bacteriological examination

Question 7

how is TB diagnosed

Answer 7

- Culture is standard but lengthy
Liquid culture (MGIT) and MODS both culture based but quicker and can do resistance testing (days-weeks). Time to positivity.
Solid media (3-4 weeks)

• Smear positive (AFB microscopy)
  Molecular methods including PCR increasingly used (GeneXpert MTB/RIF)
• Need sputum

Question 8

what is the TST result for active vs latent TB?

Answer 8

both are positive

Question 9

what is the treatment for TB?

Answer 9

4,2,4

(rifampicin, isoniazid, ethambutol and pyrazinamide) for 2 months,

then
Isoniazid and rifampicin continued for 4 months

However, bone, joint, miliary and TB meningitis are treated for 1 year

Question 10

why has there been an increase in NTM?

Answer 10

1. Increased identification
   Better clinical recognition
2. Improvement in other treatments unmasking infection, or providing niche (M.abscessus in CF patients)
3. Improved laboratory techniques for isolation and identification

Question 11

which group is TB most common in?

Answer 11

Commonest between 6 months and 4 years

Rapid progression in younger children

40% will have abnormal CXR

Brain stem is the most common site Cranial nerves frequently involved

Question 12

what is the cause of GI TB?

Answer 12

1. Primary infection due to unpasteurized milk
2. Secondary infection:
   - 1° complex elsewhere with reinfection
   • ingestion of expectorated, infected sputum
   • contiguous spread from organs

Question 13

what is the 3rd most common mycobacterial infection?

Answer 13

Mycobacterium ulcerans

Mainly in West Africa

Necrotizing ulceration (toxin-mediated - mycolactone) following water contact

Surgical treatment

Question 14

what is the second most common cause of mycobacterial infection?

Answer 14

Mycobacterium leprae

Effects peripheral nerves

> 50% of cases in India
Transmitted by respiratory droplets

Not very contagious

Leprosy is curable with drug treatment

Question 15

why is TB till a problem?

Answer 15

Difficult to diagnose

BCG vaccination efficacy 0%-80%

Long treatment: 4 drugs for 6 months (need healthcare systems)

Poor patient compliance, need for Directly Observed Therapy (DOT)

Drug resistance (MDR/XDR-TB)

Latent infection (10% lifetime risk of reactivation of TB disease)