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Epidemiology of Tuberculosis

Tuberculosis (TB) is one of the most ancient human plagues, having been present in humans in prehistoric times. It became epidemic in Europe during the Middle Ages, and by the early 19th century it caused up to 1/4 of all deaths in European cities. You can find more details in a brief history of spread of tuberculosis disease.

Although its incidence and prevalence have declined in most technologically advanced countries during this century, TB remains an enormously important global problem responsible for an estimated 3 million annual deaths. During recent years, there has been a reversal of historically declining trends in the United States, and by 1991, the number of new cases of TB reported annually had increased 18% compared to 1983. Also, 7 of 15 Western European countries (Denmark, Ireland, Italy, Netherlands, Norway, Spain and Switzerland) have showed an increase in TB cases. While New Zealand and Australia have presented a slight increase, Canada has showed no difference in the number of notifications and rates over the last years.

Progression from latent to active TB is not extremely common ; it is believed that about 5 to 10% of TB infected individuals develop clinical TB. The likelihood of progression towards active TB depends on age of infection as well as on several other factors that correlate with socio-economic status. Most exposed individuals mount an effective immune response to the initial infection. This response limits proliferation of the bacilli leading to what appears to be long-lasting partial immunity against re-infection or re-activation of latent bacilli. Apparently, the longer that an individual carries this bacilly, the less likely that he develops active TB unless his immune system becomes seriously compromised by infection with other diseases, by malnutrition, or via re-exposure to TB (exogenous re-infection). Consequently, age of infection as well as chronological age are important factors in disease progression. The rate of progression towards active TB may be accelerated after an individual is re-exposed to TB bacilli via repeated contacts with active TB individuals. The impact of endogenous re-activation of latent bacilli, i.e., re-activation of a pre-existing dormant infection from either re-infection (exogenous infection) or from changes in the immune system, must be studied at the population level.

Socially, TB is increasingly a disease of the social and economically disadvantaged. The general view is that TB has increased due to the growth experienced in high risk groups that include migrant workers, intravenous drug users, HIV-infected individuals, the homeless, institutionalized and incarcerated persons, and nursing home residents. TB morbidity and mortality rates are also strongly affected by urban living conditions.

In 1921, Calmette and Guérin introduced a live attenuated vaccine obtained by serial subculturing for 13 years of a bovine strain of tuberculosis on a glycerinated bile potato medium. During this process, it gradually lost its virulence. The strain, bacilli Calmette-Guérin (BCG), not only had lost its virulence but also conferred protection against virulent challenge. BCG vaccination started in France in 1924 but mass vaccination campaigns were started by The Scandinavian Red Cross Societies after World War II. Currently, it is the most widely used vaccine in the world. UNICEF estimated that coverage with BCG is greater than 80%. The protective effect appears to wane by 10 to 15 years after vaccination. However, there is controversy about the effectiveness of BCG.

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