Tuberculosis: Pathogen, Pathology, Treatment Flashcards

1
Q

Describe the incidence of TB over time.

A

Massive decline in incidence of TB in Europe in 19th century prior to any diagnostic test to identify disease or any drug to treat disease
Due to improvement in living standards, reduction in poverty, reduction in overcrowding . Further decline as a result of BCG, streptomycin.
Resurgence of TB in recent years.

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2
Q

Explain the reasons for the recent resurgence of TB.

A

1) Poverty: Failure to deal with extreme poverty in some of the poorest part of the world
2) Health systems: e.g. Soviet Union
During it, TB was managed by strong but authoritarian programs so when it collapsed, programmes collapsed too (particularly in prison populations).
3) HIV (esp. in Africa): HIV predisposes to “overt TB in people also infected with M. TB”.

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3
Q

Mycobacterium-tuberculosis:

  • Structure
  • Generation time
  • Special features
A
  • Structure: Acid-fast, fungus-like, gram positive bacterium with waxy cell wall.
  • Generation time: Slow (17 h)
  • Special features: Myobacterium TB has complex metabolic responses in latent/persistent state.
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4
Q

Explain the implications of the structure of TB on diagnosis and treatment.

A

Outer cell wall very thick and waxy (difficult to penetrate).

  • Good from diagnostic POV (on miscroscope slide, dyes cannot penetrate very well so can use dyes to identify M. TB).
  • Bad for treatment (drugs cannot get in very well so cannot kill TB cell)
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5
Q

Define generation time.

A

Time taken by the bacteria to double in number during a specified time period

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6
Q

What is the implication of the generation time of M. TB on diagnosis and treatment.

A

♦ Diagnosis: If using growth of TB cells to confirm is patient has infection, have to wait several days, sometimes several weeks to get confirmation.

♦ Treatment: Cellular processes drugs are trying to interrupt are very very slow (so
need to treat patients for long time in order to get results)

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7
Q

Summarise the pathogenesis of tuberculosis.

A

1) Transmission
2) Primary Infection
3) Latent Infection (majority never get to 4)
4) Active Disease (does not always occur, may not be sick for entire lifetime)

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8
Q

Identify factors which increase likelihood of developing active disease once the infection is latent.

A

♠ HIV
♠ Cancer chemotherapy (in general, depressed immune system)
♠ anti-TNFα treatment (in general, depressed immune system)
♠ low IFNγ (in general, depressed immune system)
♠ vitamin D deficiency
♠ Immune immaturity/senescence

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9
Q

How is TB transmitted ?

A

person inhales droplet nuclei containing tuberculosis bacteria (e.g. from someone sneezing or coughing, i.e. aerosol).

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10
Q

How many TB organisms does someone with TB approximately have per mL sputum ? What is the size of their droplet nuclei ? How many bacteria are there per droplet ?

A

10^8 organisms/mL sputum.

Droplet nuclei <5μm
Each with ~6 bacilli

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11
Q

What are the main factors influencing infection to TB ?

A

1) Number of infecting bacili (i.e. bacterial load)
2) Duration of Exposure
3) Strength of immune system (probably the most important factor once infected, in determining whether you ever actively fall sick)

High bacterial load, long exposure and a weak immune system would all contribute to a high probability of developing TB.
A weak immune system would further increase the probability of actually displaying the active illness.

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12
Q

Describe the timeline of TB.

A

1) Primary Infection
2) Primary disease (if get active disease soon after infection, especially in children). This is non-infectious
3) Secondary disease (years later, after TB rests in lymph nodes in the lungs. Due to endogenous reactivation following depression of the immune system, usually infectious especially if smear positive i.e. can see bacteria you are coughing out
4) Can potentially be cured, but if so can get re-infected later
5) If re-infection following cure, can get secondary disease due to exogenous re infection, which is usually infectious especially if smear positive.
6) Potentially death

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13
Q

How is TB infection diagnosed ?

A

1) Tuberculin Skin Test (TST) (= Mantoux test): inject protein from TB under skin. If you have encountered it before, localised inflammation, arm becomes red. Otherwise ignore that small protein load and mark goes away.
2) Interferon Gamma Release Assays (IGRA): take blood sample, which you stimulate using TB antigens and if blood able to produce high quantity of interferon soon after, evidence of reprimed immunological response.

These two^ can diagnose asymptomatic TB.

3) Sputum smear microscopy (a little insensitive, requires high amount of pathogens)
4) Sputum culture (gold standard, can use liquid or solid media)
5) Molecular Testing (“GeneXpert”): take sputum sample, put it into cartridge, place into machine, runs PCR to detect nucleic acids of TB cells from sputum sample.

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14
Q

What is the difference between pulmonary TB and clinical (i.e. active illness) TB ?

A

85% of clinical TB are pulmonary TB

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15
Q

What type of TB patient contributes to the majority of TB transmission ?

A

Sputum smear positive pulmonary TB patients contribute to most of the community transmission of TB.

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16
Q

Why is being smear positive so significant in transmission ?

A

Because smear positive means sufficient bacteria in the lungs that they actually cough them out and can see them on microscope.
Important because can more quickly transmit disease + can be more severe TB .

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17
Q

Identify the main symptoms of pulmonary TB.

A
  • Sputum production (± haemoptysis)
  • Fever
  • Night sweats
  • Weight loss
  • Cough of ≥ 2-3 weeks, not responding to antibiotics
18
Q

What are the different kinds of sputum cultures we can use in TB testing ?

A

Solid and liquid sputum samples

19
Q

What are the pros and cons of sputum culture testing as a diagnostic tool for TB ?

A

PROS

  • Gives an additional 20-30% diagnostic yield
  • Allows us to perform susceptibility testing (i.e. regrow bacteria and use anti TB drugs to see if they work, cannot do that with microscopy)

CONS

  • Expensive
  • Slow (can take up to weeks)
  • Biohazard
20
Q

What are the pros and cons of molecular testing (GeneXpert) for TB diagnosis ?

A

PROS:

  • As sensitive as culture, more sensitive than smear microscopy
  • Quick
  • Can detect a bit of DNA to say if TB cells in sputum and if organism is likely to be resistant or susceptible to rifampicin

CONS
-Expensive

21
Q

Describe the X-ray appearance of primary TB.

A

Ghon Complex, made up of small (often calcified) focus of pulmonary infection, and associated lymphadenopathy

22
Q

Identify the main types of secondary TB and describe their X-ray appearance.

A
  • Cavitating TB: Almost always secondary disease, in which case obvious destruction of large segment of lung (cavitations), but contained.
  • Miliary TB: Can be primary or post-primary disease, in which immune system very quick and bacterial burden very high so disease is disseminated to a lot of the lung field (from transmission POV, not so bad). Includes 1-3 mm diameter nodules, which are uniform in size and uniformly distributed.

Refer to slide 21 in lecture on TB.

23
Q

Given that pulmonary TB forms only 85% of clinical TB cases, what is the rest made of?

A

Extra-pulmonary TB, such as TB meningitis, TB lymphadenitis.

24
Q

What was the first drug discovered for the treatment of TB ?

A

Streptomycin

25
Q

What is one problem with Streptomycin treatment ?

A

If only one drug is used, resistance emerges very quickly so bacterial load may decrease initially but will end up increasing again once resistance arises.

26
Q

What is the first-line treatment of TB (for drug sensitive TB) ?

A
Intensive phase (2 months):
Rifampicin
Isoniazid 
Pyrazinamide
Ethambutol 

Continuous phase (4 months):
Rifampicin
Isoniazid

Also remember to monitor liver function closely (most of these drugs cause hepatotoxicity)

27
Q

RIFAMPICIN:

  • Bacterial targets
  • Action
  • Dose
  • Side effects
A
  • Bacterial targets: Highly bactericidal against rapidly replicating and non-replicating bacteria
  • Action: Inhibits bacterial DNA-dependent RNA polymerase
  • Dose: Single daily oral dose (10 mg/kg), but this may be too low
  • Side effects: Hepatitis + itch and rash and GI upset + discoloration of urine and sweat and tears + drug interactions (e.g. anti-retroviral therapy interactions) and induces liver enzymes (CYP450) increasing clearance of drugs (e.g. warfarin, steroids such as OCP, opiates such as methadone)
28
Q

ISONIAZID:

  • Bacterial targets
  • Action
  • Dose
  • Side effects and Resistance
A
  • Bacterial targets: highly bactericidal against rapidly replicating bacteria
  • Action: Prodrug, activated by KatG enzyme, then inhibits mycolic acid biosynthesis in cell wall.
  • Dose: Single daily oral dose (5 mg/kg), easily tolerated
  • Side effects: Hepatitis + peripheral neuropathy (can be minimised by using pyridoxine i.e. vitamin B6)
  • Resistance: Overall 10%, varies by population
29
Q

PYRAZINAMIDE:

  • Bacterial targets
  • Action
  • Dose
  • Side effects and Resistance
A
  • Bacterial targets: bacteriostatic, but bactericidal at pH 2 (e.g. inside cells)
  • Action: Exact mechanism unknown but inhibits fatty acid synthetase 1. Also, in combination therapy, accelerates the sterilising effect of rifampicin and isoniazid, allowing for 6 month treatment
  • Dose: Single oral daily dose (15-35 mg/kg)
  • Side effects: Hepatitis + Hyperuricemia (lab test) which can exacerbate gout (i.e. joint pain due to increase urate)
30
Q

ETHAMBUTOL

  • Bacterial targets
  • Action
  • Dose
  • Side effects
A
  • Bacterial targets: Bacteriostatic
  • Action: Poorly understand but inhibits arabinosyn transferase. Used to help prevent resistance to other drugs.
  • Dose: Single daily oral dose (15 mg/kg), usually well tolerated
  • Side effects: optic neuritis (i.e. ocular toxicity) (uncommon at dose < 15 mg/kg)
31
Q

Can we shorten TB treatment to less than 6 months ? Mention any recent research.

A

No, attempted to use 8-methoxyfluoroquinolones to reduce treatment duration but failed due to high rate of post-treatment relapse in experimental arm.

32
Q

How does antibiotic resistance occur ?

A

Naturally occurring M. Tuberculosis mutations exist, including 1:10^7 cells with INH resistance and 1:10^9 cells with RIF resistance.
These resistant organisms are selected for by antibiotic therapy.
Resistance to Isoniazid usually precedes resistance to Rifampicin.

33
Q

Which specific mutations account for the main resistant bacterial strains ?

A

KatG mutations account for most of INH resistant TB strains.
inhA Mutations account for a third of INH resistant TB strains.

rpoB mutations account for most of RIF resistant TB strains.

34
Q

Identify the main resistant TB strains, and explain what each is resistant to.

A

MDR-TB: Resistant to RIF and INH
Pre-XDR-TB: (resistance to first line drugs +) Additional second line drug resistance to: any injectable second line drug OR fluoroquinolone
XDR-TB: (resistance to first line drugs +) Additional second line drug resistance to: injectable second line drug and fluoroquinolone

35
Q

What proportion of MDR-TBs are XDR-TBs ?

A

About 10%

36
Q

Describe the global distribution of MDR-TB.

A

45% originate in India, China, Russia.

In general high incidence in Eastern Europe and Central Asia.

37
Q

What is the second line treatment of TB ? When is it used ?

A

Used for MDR-TB.

1) Intensive Phase (8 months)
At least 5 drugs, may include injectable ones

2) Continuous Phase (12 months)
At least 4 drugs, all oral

38
Q

What are the challenged of MDR-TB treatment (i.e. second line treatment) ?

A
  • Less effective: Patients are infectious for longer, and cured more slowly
  • More toxic- more side effects and may require regimen changes during therapy
  • Not well-studied: Best doses and combinations are incompletely understood
39
Q

What are the pros of TB first-line treatment ?

A
  • Regimen standardised internationally
  • Strong clinical trial evidence
  • Only moderate toxicity
40
Q

Describe any quicker treatment alternatives to drug-resistant TB.

A

Bangladesh Regimen

  • 9 months instead of 24
  • Intensive phase (4 months) includes 4th generation fluoroquinolone (i.e. Gatifloxacin) and clofazimine
  • Continuous phase (5 months, or until sputum smear conversion if not smear negative at 4 months) also includes ^
  • Follow up under routine conditions
  • Low cost per patient
  • May be used for patients who have not been previously treated with second-line drugs and in whom resistance to fluoroquinolone and second-line injectable agents has been excluded or considered highly unlikely.
41
Q

Describe the results of the current trial comparing ‘WHO standard’ MDR-TB therapy vs ’Bangladesh style’ regimen.

A

Show non inferiority to the conventional regimen

42
Q

Identify any “new” TB drugs.

A

Nix-TB regimen (still under trial), potential for shorter, all-oral, and affordable treatment for XDR-TB