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Tuberculosis (TB)

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What is tuberculosis?

Tuberculosis (TB) is a chronic, communicable infection due to the bacteria Mycobacterium tuberculosis. Tuberculosis most commonly affects the lungs, but can spread to almost any part of the body.

Statistics on tuberculosis

Tuberculosis still kills more adults than any other disease. The global burden of TB still shows a growing trend. The HIV epidemic has fuelled continuous rise tuberculosis cases in Africa, although elsewhere the numbers are stable or falling. Estimates reveal that over 8 million people contract TB yearly, and 2-3 million people die from TB. The number of new cases every year varies from  0-24 for every 100,000 people in developed countries like Australia and Canada, to 300 or more in sub-Saharan Africa. Anyone who is in close contact to a tuberculosis case may catch the disease.

Risk factors for tuberculosis

  • Contact with an infected individual is normally required for primary airborne infection.
  • Any immunosuppressive condition (chemotherapy, corticosteroid use) or debilitating illness will lead to disease reactivation.
  • Positive HIV/AIDS status predisposes individuals to TB infection.
  • The high prevalence of HIV/AIDS in developing countries has been coupled to an increase in the incidence of TB in these countries.

Positive HIV status dramatically increases the risk of infection with Mycobacterium tuberculosis. The risk is twice as high in the first year of seroconversion. It is reported that the annual incidence is as high as 10% in HIV patients from high burden communities.

Multidrug-resistant TB (MDRTB) is estimated to be present in more than 4% of TB patients worldwide (MDRTB is defined as TB resistant to isoniazid and rifampin with or without other drug resistance). Extensively Drug Resistant (XDR) TB also occurs, and is resistant to at least rifampicin and isoniazid, as well as any quinolone and one or more injectable second line agent.

Progression of tuberculosis

The initial infection with the bacteria is often in childhood. It is most commonly spread via inhaled droplets and so the lungs are infected first. Another form of transmission is drinking infected milk.

A nodule forms in the lung, usually in the outer portion of the upper lobes. The body’s defences stop the infection from expanding and the bacteria is trapped inside the nodule. The infection becomes dormant.

Tuberculosis can lie dormant for a number of years until the person’s immune system becomes weak. Diabetes, cancer, steroids, HIV/AIDS or even old age can lead to weakening of the immune system, and the nodule in the lung is reactivated. The nodule and the lymph nodes (Ghon complex) will advance in size and cause damage in the surrounding lung tissue. If the bacteria enter into blood vessels, it can spread to any part of the body.

Miliary tuberculosis is when the bacteria spread and cause small foci of infection throughout the body (“millet seed” appearance, hence the name miliary). This is a common form of tuberculosis in the developing world and amongst immunosuppressed patients.

Infection in isolated organs can produce signs and symptoms in the gut, brain, bones, skin, urinary tract, etc.

Complications of tuberculosis include fluid in the lungs, pus in the lungs, blindness from spread to the eyes, and other complications.

Symptoms of tuberculosis

Tuberculosis does not have specific symptoms. Many people get tuberculosis and do not show any signs. Your body fights against tuberculosis and stops it from spreading and causing disease. When your immune system is weak, your body’s ability to fight infection is diminished. Tuberculosis may then overpower your body’s defenses and cause illness. You might have fever (mild temperature), weight loss, poor appetite, night sweats and general weakness. You may have a cough but not much phlegm.

Clinical examination of tuberculosis

Assessment is similar to that of other respiratory illnesses. Examination of lungs and lymph nodes may be part of the assessment. The physician will listen to your breath sounds, and percussion will test if you have fluid in the chest cavity. Palpating the lymph nodes will detect any swollen nodes.

Prognosis of tuberculosis

Prognosis depends largely on the extent of the disease. Primary, dormant or limited secondary tuberculosis responds very well to chemotherapy. Disseminated tuberculosis is more common in debilitated or immunosuppressed patients, and so prognosis is not as good. Current mortality for all forms of tuberculosis in the UK is 8%. In addition, drug resistance is becoming a problem (more than 10% of new cases in the USA are resistant to standard anti-tuberculous drugs).

How is tuberculosis treated?

Tuberculosis is a notifiable disease in most countries. Contacts need to be traced and screened.

The patient needs to be isolated while infectious. Antibiotic therapy usually follows a two-stage course. The initial phase requires 3-4 agents (rifampicin, isoniazid, pyrazinamide +/- ethambutol if resistance suspected) for two months. The continuation phase requires two drugs (rifampicin and isoniazid) for four months.

Given the long course of treatment, the importance of antibiotic therapy must be stressed. Failure to comply with the regime can lead to the development of drug resistance.

It is important to monitor liver function closely, particularly during the initiation phase, as most anti-tuberculous drugs can cause hepatitis. Ethambutol can cause optic neuritis and so vision (particularly colour vision) should be tested regularly.

Tuberculosis can be vaccinated against with the BCG vaccine. It was originally an oral preparation of M. bovis, but it is now injected intradermally. Although the mode of action is not specifically understood, vaccination prevents non-pulmonary forms of tuberculosis.

References

  1. Bennet DE, Courval JM, Onorato I, Agerton T, Gibson JD, Lambert L, et al. Prevalence of tuberculosis infection in the United States population. American Journal of Respiratory and Critical Medicine. 2007; 177: 348-55.
  2. Isselbacher KJ, Braunwald E, Wilson JD, Martin JB, Fauci AS, Kasper DL, editors. Harrison’s Principles of Internal Medicine Companion Handbook. 13 ed: McGraw Hill Inc; 1994.
  3. Longmore M, Wilkinson I, Torok E, editors. Oxford Handbook of Clinical Medicine. 5 ed. New York: Oxford University Press; 2001.
  4. Maartens G, Wilkinson RJ. Tuberculosis. Lancet. 370(9604): 2030-43.
  5. Orme I, Secrist J, Anathan S, Kwong C, Maddry J, Reynolds R, et al. Search for new drugs for treatment of tuberculosis. Tuberculosis drug screening program. Antimicrobial Agents and Chemotherapy. [Guest Commentary]. 2001; 45(7): 1943-6.
  6. Bagchi S. Promising results for new tuberculosis therapy. Canadian Medical Association Journal. 2008; 178(1): 19. In: Health & Medical Complete [database on the Internet] [cited 2008 Jan 31]. Available from: http://www.proquest.com. Document ID: 1403865521.
  7. Scarparo C, Piccoli P, Rigon A, Ruggiero G, Scagnelli M, Piersimoni C. Comparison of enhanced mycobacterium tuberculosis amplified direct test with COBAS AMPLICOR mycobacterium tuberculosis assay for direct detection of mycobacterium tuberculosis complex in respiratory and extrapulmonary specimens. Journal of Clinical Microbiology. 2000; 38(4): 1559-62.
  8. Cousins DV, Roberts JL. Australia’s campaign to eradicate bovine tuberculosis: The battle for freedom and beyond. Tuberculosis. 2001: 81(1/2): 5-15.
  9. Rubin E, Farber JL, editors. Essential Pathology. New York: J.B. Lippincott company
  10. National Tuberculosis Advisory Committee Multi-Drug Resistant Tuberculosis: Information paper. CDI. 2007; 31(4): 400-9.
  11. Dye C, Williams GB. Criteria for the control of drug resistant tuberculosis. PNAS. 2000: 97(14): 8180-5.
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Dates

Posted On: 26 May, 2003
Modified On: 9 October, 2018
Reviewed On: 8 May, 2008

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