Tuberculosis (TB) is the 8th most common cause of death worldwide, accounting for 1.34 million deaths per year. One third of the global population are thought to carry latent TB and the majority of cases occur in the developing world. The UK incidence of TB is 12/100,000. TB is considered to be a re-emerging disease and immigration (from high prevalence areas), HIV (particularly in Africa) and emerging drug resistance are thought to be contributing to this. Figure 1 lists risk factors for contracting TB.
The bacteria Mycobacterium tuberculosis is the causative agent of TB. Figure 2 summarises the important microbiology of this organism. Infection is acquired by inhalation of aerosolised droplet nuclei from the airways of individuals with active TB. Coughing facilitates spread in this way.
Pulmonary TB is the most common disease resulting from TB infection and Figure 3 summarises the clinical features.
Diagnosis of pulmonary TB is based on the clinical features of the patient, radiology (PA chest X-ray) and identification of AFB (acid-fast bacilli). The current gold standard in diagnosing TB is culture of the organism but this is a slow process, taking 6-8 weeks. Figure 4 summarises relevant investigations. Treatment should be started prior to obtaining culture results if the clinical picture is consistent with TB.
Drug resistance is an increasing problem in managing TB. Incomplete courses of anti-tuberculous drugs are the main driving factor in the development of resistance.
Extra-pulmonary TB is the consequence of haematogenous spread of the bacteria and can result in two scenarios:
1. Miliary TB: bacteria seed the lungs and many other sites with multiple small foci of infection (1-5mm caseating lesions). Liver, bone marrow, spleen, adrenal glands, meninges and kidneys can all be seeded
2. Extra-pulmonary TB: non-miliary disease outwith the lungs. More common in HIV positive individuals.
Mycobacterium tuberculosis complex:
Non-tuberculous Mycobacteria (NTM):
Mycobacterium avium-intracellularecomplex (MAC) infection is uncommon except in AIDS, where disseminated MAC infection (commonly involving lungs & GI system) is often a terminal event.
Oxford Handbook of Infectious Diseases and Microbiology (1st ed., 2009, Torok E, Moran E, Cooke F). p.407-414
Mim’s Medical Microbiology (4th ed., 2008, Goering RV, Dockrell HM, Wakelin D, Zuckerman M, Chiodini PL, Roitt IM, Mims C). p.245-248
Davidson’s Principles and Practice of Medicine (20th ed., 2006, Boon NA, Colledge NR, Walker BR, Hunter JAA). p.695-703.
Robbins and Cotran Pathologic Basis of Disease (7th ed., 2005, Kumar, Abbas, Fausto). p381-388.
World Health Organisation: Tuberculosis
National Institute for Health & Clinical Excellence: Tuberculosis Guidelines
For more in depth discussion:
Barry CE 3rd, Boshoff HI, Dartois V, Dick T, Ehrt S, Flynn J, Schnappinger D, Wilkinson RJ & Young D. The spectrum of latent tuberculosis: rethinking the biology and intervention strategies. Nature Reviews Microbiology 2009; 8: 45-55.
Giri PK. How could we have better vaccines against tuberculosis? Expert opinion in biological therapy 2008; 8: 1759-1772.
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