Ten years ago, the World Health Organization declared tuberculosis (TB) a global health emergency. Today, TB ranks second to human immunodeficiency virus as the leading cause of death worldwide from an infectious disease.
Mycobacteria are ubiquitous. Mycobacterium tuberculosis, M bovis, M africanum and M microti can all cause TB in humans, but most cases are caused by M tuberculosis. Over 60 other species of mycobacteria have been identified, some of which are low-grade pathogens in man.
About two billion people (one third of the world’s population) are infected with TB. The number of new cases each year (estimated at eight million) is rising, mainly as a result of the increasing burden of HIV infection. 2 Rates (incidence and prevalence) of TB are highest in sub-Saharan Africa but the total number of cases is highest in Asia. India, China, Indonesia, Bangladesh and Pakistan account for more than half of all cases.
All but 2 per cent of the estimated two million deaths annually caused by TB occur in developing countries, predominantly among young adults. Almost all of these deaths are preventable.
TB is transmitted almost exclusively by people with active pulmonary or laryngeal forms of the disease, who expectorate bacilli in droplet nuclei as they cough, sneeze or talk. In poorly ventilated, enclosed environments bacilli can remain airborne for several hours. The tuberculosis bacillus is an intracellular pathogen. Transmission requires inhaled bacilli to reach the alveoli in the lung periphery and to be ingested by alveolar macrophages. Each macrophage then rapidly transports bacilli via the lymphatic system to the hilar lymph nodes and, if replication is not checked, infection can reach almost any other organ. Actively replicating bacilli destroy their host cell and are liberated into the blood and lymph to invade other macrophages.
Depending on the host’s immune response and the virulence of the invading bacilli, infection leads to eradication, containment resulting in latent infection (where the host is asymptomatic but continues to harbour small numbers of quiescent bacilli) or primary disease.