Mycobacterium Tuberculosis – Part 1 – Diagnosis of Pulmonary Tuberculosis (TB), Mantoux Test

• Tuberculosis is the world’s most spreading disease and developing drug resistance.

Important Facts of M.Tuberculosis

1. Mycobacterium tuberculosis is the causative agent.
2. It is estimated that 20 to 43 % of the world population is suffering from TB.
3. In the USA 15 million people are infected (Old statics).
4. TB occurs in :
   1. Poor community, considered to be the disease of poor people.
   2. Malnourished people.
   3. Homeless.
   4. Overcrowded community.
   5. Substandard housing.
5. Mode of spread: This is an airborne disease.
   1. Primary TB = Clinically and radiologically is silent.
   2. Latent TB = Do not have active disease and can not spread the disease to others.
   3. Active TB = 10% of the latent TB develop active TB when not given treatment.
   4. Progressive primary TB = 5 % of the primary active TB with signs and symptoms.
6. This is thought that 90% of the disease is a reactivation of latent TB.

Microbiology of Mycobacterium Tuberculosis:

1. These are acid-fast bacilli.
   1. These are rods shape and grow in cords.
   2. The growth is very slow on special media.
2. They get gram stain but is very week, and these are gram stain positive.
3. These are non-motile, obligatory aerobes, and intracellular organisms.
4. Humans are the only reservoir.

Pathogenesis:

1. Mycobacterium tubercle bacilli cause damage by invading the macrophagic cells by Type IV hypersensitivity reaction.
2. This bacteria leads to caseating necrosis and granuloma formation.

3. There are multinucleated giant cells, Langhans’ type cells.
4. TB  bacteria consists of slightly curved or straight rods.
   1. It cannot be stained by the gram’s stain but are acid-fast.
   2. These are nonmotile and without spores.
   3. Pathogenic bacteria are slow-growing may take 4 to 6 weeks.
5. The common types are :
   1. Mycobacterium tuberculosis.
   2. Mycobacterium bovis.
   3. others are Runyon group 1 to IV.

Sign and Symptoms

1. The patient will have :
   1. Malaise.
   2. Anorexia.
   3. Weight loss.
   4. Fever.
   5. Night sweating.
   6. A chronic cough is a common presentation of pulmonary TB.
   7. Blood streaked sputum is common.
   8. The patient may have hemoptysis.
   9. Rarely patients are asymptomatic.
   10. In advanced disease:
       1. There may be clubbing of nails.
       2. Enlarged lymph nodes in the neck.
       3. The patient may develop pleural effusion.

11. Clinically the possibilities of tuberculosis are:
    1. TT (Manteaux test) positive cases and these cases may be inactive asymptomatic people.
    2. Primary tuberculosis shows the Ghon complex.
       1. There is a lesion in the lung and involvement of the lymph nodes.
    3. Secondary tuberculosis involves the upper lobe of the lung because of the higher oxygen concentration.
       1. This usually seen in impaired immunity.
       2. There may be cavity formation in the lung.
       3. Sputum smears are AFB positive.
       4. The disease is contagious.
    4. Miliary tuberculosis is a wide-spread disease.
       1. It involves the lungs, CNS, kidneys, GI tract.
       2. It may involve any organs including the bones.
    5. Extra-pulmonary may involve CNS and leads to chronic meningitis.
       1. There may be the formation of tuberculoma in the brain.
       2. There may be the involvement of the skin.
    6. TB is very common in the AID’s patients.
       1. In AID’s patients, TT may be negative due to compromised immune systems.

Laboratory diagnosis

1. Definite diagnosis depends upon the demonstration of T.Bacilli by:
   1. Culture.
   2. Culture on solid media needs 12 weeks.
   3. Culture on liquid media needs several days.
   4. PCR by DNA or RNA amplification method.
      

      TB bacilli

2. Sputum, three consecutive samples is recommended for:
   1. Fluorochrome staining with rhodamine-auramine.
   2. AFB stain or Ziehl-Neelsen stain.
   3. An early morning specimen is recommended.
3. Bronchoscopy is advised for bronchial washing in case of negative sputum.
4. Transbronchial lung biopsy increases the diagnostic yield.
5. Gastric aspiration. An early morning sample is an alternative to bronchoscopy.
6. Blood culture, 15% of the case may give a positive culture to T.bacilli.
7. The sensitivity should be done once the culture is positive.
   1. The sensitivity should be done if the sputum culture is positive after the treatment for 2 months.
8. Needle biopsy of the pleura shows the granulomas in 60 % of the cases.
9. Pleural fluid cultures are positive in < 25 % of the cases.
10. Radiology,  X-ray chest shows small homogenous opacity.
11. Mantoux test or Tuberculin test (TT).
    12. TT will not distinguish between latent or active TB.
    13. 0.1 ml (5 tuberculin units) of PPD should be injected intradermally.
    14. The best site is the volar surface of the arm.
    15. Injected with 27 G needle.
    16. Read after 48 to 72 hours for induration (thickening of the injected area).
    17. It takes 2 to 10 weeks to develop an immune response to PPD after the infection.

12. Other specimens can also be used are:
    1. Urine. The first-morning clean catch is collected for three consecutive days.
    2. Stool. This should be collected in a clean sterile container.
    3. Blood. Lysed centrifuged blood is used for culture.
13. Niacin test. Mycobacterium produces Niacin. This can be tested by commercially available kits.

Preventive measures

1. This disease can be prevented in health workers.
2. If your TB test is positive it means you have contact with the patient and may not have active disease.
3. Vaccination like BCG is helpful to prevent the disease.
   1. BCG is live attenuated bacteria.
   2. It is not available in the USA.

Treatment

1. The following drugs are used in patients with tuberculosis:
   1. Isoniazid (INH).
      1. This is advised in TT positive people.
      2. This is also given prophylactically in AIDs patients.
   2. Combination of drugs like:
      1. Isoniazid.
      2. Rifampicin.
      3. Pyrazinamide.
      4. Ethambutol.
   3. Isolation is also important to stop the spread of the disease.
   4. Steroids are contraindicated in these patients, because there may be reactivation of tuberculosis.

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