Bacteria 4 (mycobacterium): TB

Question 1

• What bacterium causes TB?

Answer 1

• The three mycobacteria are known to be genetically very similar and hard to distinguish under the microscope so they are collectively referred to as M. tuberculosis (MTB)
• MTB are referred to as acid-fast bacilli (AFB) -> retains the colour duing staining
• They are cultured for between 4 and 12 week on egg-based solid Löwenstein-Jensen medium, producing visible yellow colonies

Question 2

• How are acid-fast stain sputum microscopy samples prepared?

Answer 2

• Ziehl-Nielsen stain
• Normal stains do not penetrate the waxy coat but acid fast can be used -> you use a dye and apply it with some phenol. This stains everything. Then you wash it off with some phenol and it will wash it off from everything except the MTB because the waxy coat protects it
• A second stain (methylene blue) is applied that stains everything except the MTB
• How many sputa?
  
  • WHO recommended: spot-spot sample, i.e. two spot samples in a single clinic appointment as that means patients do not have to travel to clinic twice
• Fluorescent microscopy:
  
  • Fluorescent microscopy another option and has now been made cheap through the use of LED
    
    • Use of rhodamine stain which can go into waxy coat and avoid being washed away
    • The counterstain is non fluorescent to hide all the debris
  • Quicker
  • Easier
  • More sensitive
  • WHO recommended

Question 3

• How many of those exposed to TB develop TB?

Answer 3

Question 4

• Pathophysiology of TB?

Answer 4

1. Droplet nuclei containing tubercle bacilli are inhaled, enter the lungs, and travel to the alveoli
2. Tubercle bacilli multiply in the alveoli.
3. A small number of tubercle bacilli enter the bloodstream and spread throughout the body. The tubercle bacilli may reach any part of the body, including areas where TB disease is more likely to develop (such as the brain, larynx, lymph node, lung, spine, bone, or kidney).
4. Within 2 to 8 weeks, special immune cells called macrophages ingest and surround the tubercle bacilli. The cells form a barrier shell, called a granuloma, that keeps the bacilli contained and under control (LTBI).
5. If the immune system cannot keep the tubercle bacilli under control, the bacilli begin to multiply rapidly (TB disease). This process can occur in different areas in the body, such as the lungs, kidneys, brain, or bone.

The last point is often triggered by reactivation of latent TB or reinfection. The sensitised immune system will cause a delayed type hypersensitivity reaction that leads to tissue destruction at the site of multiplication of bacteria; hence cavitating caseous lesions form, containing large number of multiplying bacteria within a rim of giant cells and granulomata. These are called post-primary lesions and are often found within the lung apices.

Question 5

• What are the different clinical states on the ‘spectrum of TB’?

Answer 5

Question 6

• What different clinical syndromes does TB consist of?

Answer 6

• Main one: Pulmonary
  
  • Cough > 3 weeks should always raise suspicion of TB
  • Haemoptysis, chest pain, breathlessness only in some patients
  • May look ill and wasted but not always
  • Tachycardia and fever common but not septic shock
  • Chest signs may be limited even if grossly abnormal xray

Question 7

• How many people are affected by TB annually?

Answer 7

• Thought to cause one quarter of avoidable deaths in developing countries!!!

Question 8

• How is TB transmitted?

Answer 8

Question 9

• What factors influence infection with TB?

Answer 9

• Two main things:
  
  • How many droplet nuclei accumulate in the atmosphere surrounding a person
    
    • Prolonged contact with infected person
    • Environment of the poor – overcrowded and poorly ventilated
    • No UV light conditions (kills mycobacteria) – prisoners and miners
  • Those with decreased ability to destroy the bacteria in their macrophages
    
    • Corticosteroid therapy
    • HIV infection

Malnutrition, especially vitamin D deficiency, impairs cell-mediated immunity and so constitutes a risk factor for acquiring TB. Immunosuppression from any cause is an added risk factor (HIV, corticosteroids, methotrexate, anti-cancer drugs).

Overcrowding, humid, dark, underventilated and poor living or working conditions also favour its spread.

A hot dry climate especially with a lot of sunshine (UV kills TB) reduces the risk. TB after long-haul flights (>8 h) has been reported but is extremely rare. It is an offence to travel by air if you have open TB.

Question 10

• What are the different diagnostic modalities in TB and what are their advantages and disadvantages?

Answer 10

​Mycobacterial identification by a skilled microscopist is still the gold standard for diagnosis of MTB. However this can take 2–3 weeks and is often haphazard in busy overworked rural hospitals in the tropics. Automated nucleic amplification testing is becoming more available and although expensive has a sensitivity of nearly 100% in smear positive cases and around 70% in smear negative ones. It has a specificity of nearly 100% in both. This process is currently too expensive for widespread use but has a very bright future in primary care clinics as well as in hospital settings.

• CXR - best in low income settings
• ZN stain
  
  • Normal stains do not penetrate the waxy coat but acid fast can be used -> you use a dye and apply it with some phenol. This stains everything. Then you wash it off with some phenol and it will wash it off from everything except the MTB because the waxy coat protects it
  • A second stain (methylene blue) is applied that stains everything except the MTB
• Fluorescent microscopy:
  
  • Fluorescent microscopy another option and has now been made cheap through the use of LED
    
    • Use of rhodamine stain which can go into waxy coat and avoid being washed away
    • The counterstain is non fluorescent to hide all the debris
  • Quicker
  • Easier
  • More sensitive
  • WHO recommended
• Molecular techniques (PCR) are now used as well -> easy to use cartridges are now used for ease of use (GeneXpert MTB/Rif). Also detects rifampicin resistance.
  
  • Same sensitivity as a culture, perfect specificity
• What about tuberculin (PPD) skin test?
  
  • In high burden settings not much predictive value for TB
  • False negatives: immunosuppression, poor technique, severe illness, normal variation
  • False positives: prior BCG, environmental mycobateria
  • Also does not differentiate between latent and active infection
  • preferred test in children under the ages of five
• IFNγ (TB spot) is released by T lymphocytes in response to MTB antigens and this fact is the basis of Interferon Gamma Release Assays (IGRA). Unfortunately this response does not indicate, much less confer, immunity to the disease. Hence the conclusion that a positive response to current tests, based on delayed hypersensitivity, does not indicate protection against MTB. However, the IGRA does not become positive if a person has previously been vaccinated with BCG. Does not differentiate between latend and active infection

Question 11

• What are the different ways that TB can present?

Answer 11

• Pleural TB
  
  • most common cause of straw coloured pleural effusion
  • exudate, high LDH and lymphocytic
  • Smears may be negative but higher yeild with culture and GeneXpert
• Lymph Node TB
  
  • Caseating granuloma
  • Neck and auxilla, often solitary
  • Tender/warm but not exquisitely painful/hot
• TB meningitis
  
  • High morbidity and mortality
  • Sub-acute febrile illness with headache, vomiting and progressive decrease consciousness
  • Meningism
  • LP: high protein, low glucose and a raised number of lymphocytes
• Pericardial TB
  
  • on CXR, ECG, Echo
  • Globular heart on CXR
  • Pulsus paradoxus
• Bone & Joint TB (Pott’s disease)
  
  • Spine: gibbous, skip lesions, sometimes cold abscess
  • single large joint effusion
• Miliary TB
  
  • Disseminated to every part of the body
  • Extremely high mortality
• Genito-urinary TB
  
  • Any part of male and female urinary and reproductive tract
  • Ureteric obstruction, sterile pyuria
  • Pelvic mass
  • Epidydymal swelling
• Paradoxical reactions
  
  • Worsening or development of new lesions while on TB treatment - up to 25% of patients

Question 12

• How do you treat TB?

Answer 12

R = rifampicin

H = isoniazid

Z = pyrazinamide

E = ethambutol

Never use or add a single drug. TB meningitis and bone TB may need higher doses.

DOTS - directly observed therapy

Monitor response

• once at the end of the intensive tx phase
• once during continuation phase
• once at the end of treatment
• 4 months of continuation phase containing HRE if high isoniazid resistance expected
• TB meningitis and TB pericardial adjuvant corticosteroid therapy of 6 – 8 weeks is used as well

Question 13

• How does RIPE work?

Answer 13

R = rifampicin - bactericidal

H = isoniazid - bactericidal

Z = pyrazinamide - bactericidal

E = ethambutol - bacteriostatic

RHZE all ok in pregnancy

Question 14

• What are the side effects of the different TB drugs?

Answer 14

• Isoniazid
  
  • Common: GI upset
  • Moderate: Peripheral neuropathy, drug induced liver injury (DILI)
• Rifampicin
  
  • Common: GI upset, orange discolouration of bodily fluids
  • Moderate: DILI
• Ethambutol
  
  • Retrobulbal neuritis -> loss of acuity and colour vision
• Pyrazinamide
  
  • Common: GI upset, arthralgia
  • Moderate: DILI

Question 15

• How does TB drug resistance develop and how common is it?

Answer 15

• Test using rapid molecular-based methods or conventional methods (culture)
• For example in Moldova, you have 20% MDR-TB incidence so primary resistance is a major problem
• Worldwide general incidence 5 – 6 %
• But in taking a drug history, we tend to talk about drug resistance in terms of “among new and retreatment cases” instead
• Isoniazid usually first one to develop resistance (mono-resistance)
• But more and more you see resistance to both pivotal drugs (rifamp and iso)
• 2009: 250k cases of MDR TB
• 2016: 600k cases of MDR TB
• Only 50% of these are detected
• 20% of those start treatment
• 50% treatment success
• Highest concentration of MDR-TB: Belarus / Moldova
• Greatest number of cases: India, Russia, China
• Diagnosing MDR-TB is difficult
  
  • Slow and difficult -> have to grow organism
  • However, molecular tests may bridge this gap
• It is also difficult in many countries to test for resistance to second line drugs once resistance to isoniazid and rifampicin has been established
• Resistance to second line drugs can be substantial (up to 18%) in some countries

Question 16

• What is the difference between aquired vs primary drug resistance?

Answer 16

Question 17

• How do you classify the different levels of drug resistance in TB?

Answer 17

Question 18

• How do you treat MDR-TB nowadays?

Answer 18

From WHO guidelines:

• In patients with confirmed rifampicin-susceptible and isoniazid-resistant tuberculosis, treatment with rifampicin, ethambutol, pyrazinamide and levofloxacin is recommended for a duration of 6 months
• For MDR-TB (Rifampicin and isoniazid resistant) use 8 months 5 drug intensive treatment with 12 months continuation phase with 4 drugs
• Current preliminary results of STREAM trial show that a 12-month course may be non-inferior
• WHO recommendation: In MDR/RR-TB patients who have not been previously treated for more than 1 month with secondline medicines used in the shorter MDR-TB regimen or in whom resistance to fluoroquinolones and second-line injectable agents has been excluded, a shorter MDR-TB regimen of 9–12 months may be used instead of the longer regimens
• For XDR-TB, you should ideally find sensitivieties and treat accordingly
• In general: greater focus on bequiline and linezolid
  
  • linezolid risky: bone marrow suppression, anaemia, peripheral neuropathy in eyes and limbs
  • Bedaquiline relatively safe: long half life and QTc prolongation

Question 19

• What kind of vaccine is BCG and how effective is it?

Answer 19

• Live attenuated vaccine
• Little evidence that it provides long lasting protection, however, there is evidence that it protects against disseminated forms of TB in children
• Also been shown to be protective against leprosy and Buruli ulcer

Question 20

• How does HIV influence TB epidemiology and clinical presentation?

Answer 20

• For people with pre-existing MTB infection, the lifetime risk of developing tuberculosis goe from 5-10% to 15%
• Higher risk of acquiring MTB infection
• Both MTB and HIV mostly affects adolescents and middle decades of life -> massive loss for economy
• TB not only arises from poverty but it is also a poverty-generating disease
• Clinical:
  
  • Miliary TB if immune system is very compromised
  • The more immunocompromised, the more likely to present with a non-cavitating or disseminated form of the disease

Question 21

• What is DOT and what is DOTS?

Answer 21

• DOT: directly observed therapy
  
  • At least for the intensive phase drugs are taken under the watchful eye of healthcare worker
    
    • Not always easy due to distances travelled etc.
    • Pilot projects in remote areas: family members/shopkeepers observe
• DOTS: 5-point policy package
  
  • see photo

Question 22

What causes Buruli ulcers?

Answer 22