Chronological decrease of tuberculosis incidence rates by organ classification based on a birth cohort study in Japan, 1975-2005.

BACKGROUND AND OBJECTIVE The incidence and annual risk of infection of tuberculosis (TB) have decreased rapidly in Japan because of the development of anti-TB medicines and nutritional and hygienic improvements after World War II. The incidence of tuberculosis is currently high among elderly people, reflecting the fact that the prevalence rate of TB infection had been extremely high during their youth. This would suggest that most current cases of TB in the elderly are reactivation of infections acquired in their youth. TB reactivation in various organs have both common and unique aspects. We evaluated the frequency of endogenous reactivation of TB in various organs by examining the TB incidence rate over a 30-year period (1975-2005) in Japan. METHODS The incidence rate of TB in each organ was obtained for each 10-year birth cohort, using reports of newly registered TB patients in Japan in 1975, 1985, 1995, and 2005. Specifically, the incidence rates of pulmonary TB, lymph node TB, bone-joint TB, kidney TB, and meninges TB were analyzed. RESULTS Chronological changes in TB incidence rates in each organ were characterized by a rapidly declining phase followed by a stationary phase in every organ TB except pulmonary TB. Incidence rates among the already infected population in the stationary phase were 3.0 (lymph node TB), 1.2 (bone-joint TB), 0.5 (kidney TB), and 0.3 (meninges TB) per 100,000 cases, respectively. CONCLUSIONS Once infected with TB, the incidence rate of TB in these organs does not decrease below the above-mentioned values.