[Estimation of the future epidemiological situation of tuberculosis in Japan]

Abstract

Objective: To estimate the future trends of all forms of tuberculosis (TB) and sputum smear positive pulmonary TB in order to consider the emerging issues of TB control and eliminating TB in Japan. MATERIALS AND METHODS] Annual reports of TB registrations were used for observing past trends of TB, and predictions were then made assuming that past trends would continue. At first, to obtain the number of TB patients by sex and age-group, sex-age-specific incidence rates were estimated for the years 2010, 2015, 2020, 2025 and 2030, and then applied to a sex-age-specific population which was projected by the National Institution of Population and Social Security Research. According to the different methods used to calculate the reduction rates of incidence, we adopted model A and model B. In model A, the reduction rate was calculated by using two groups of the same age group but different members by calendar year. In model B, the reduction rate was calculated by using the same birth cohort but different age by calendar year. We also adopted two sub-models by the observation period of past trends. The incidence rates for the period from 1987 to 2005 were used in model 1 and the incidence rates for the period from 1998 to 2005 were used in model 2. The incidence rate in 1999 was excluded from both model 1 and 2, because the TB incidence rate increased abnormally due to the declaration of a state of emergency concerning tuberculosis in 1999. The speed of decline among particular several sex-age-groups was weighted taking into account the influence of foreign, homeless and elderly cases. The future number of sex-age-specific sputum smear positive pulmonary patients was estimated by applying various parameters, i.e. pulmonary TB rate, sputum smear positive rate and its trend, to the estimated future number of TB incidence.

Results: The TB incidence rate, which was 22.2 per 100,000 population as of 2005, would reach 9.8 in model A-1, 5.4 in model A-2, 7.5 in model B-1 and 3.2 in model B-2 by 2030. On the other hand, the sputum smear positive pulmonary incidence rate, which was 8.9 per 100,000 population as of 2005, would decline to 5.5 in model A-1, 3.0 in model A-2, 4.2 in model B-1 and 1.7 in model B-2 by 2030. The future number of TB patients and incidence rates by sex and age were discussed based on a mixed model which used the middle series of estimates, and was obtained by combining model A-2 and model B-1. The number of TB patients by the mixed model will become about 12 thousand with 10.1 per 100,000 population in 2020, and about 7.4 thousand with 6.5 per 100,000 population in 2030. From 2005 to 2020, the age composition of TB patients will change from 0.4% to 0.2% at 0-14 years old, 4% to 4% at 15-24 years old, from 10% to 11% at 25-34 years old, 8% to 11% at 35-44 years old, from 9% to 12% at 45-54 years old, from 15% to 10% at 55-64 years old, 18% to 15% at 64-74 years old, from 24% to 17% at 75-84 years old, and from 11% to 20% at 85 years of age or older. Although the proportion of the elderly aged 65 years or higher will not be so different, the proportion of TB patients aged 85 years or older will almost double.

Conclusion: The year when the TB incidence rate will reach the level of low-incidence countries, which is defined as a country with a TB incidence rate of less than 10 per 100,000 population, might be around 2020 in Japan. At that time, the age composition of TB patients will tend to be very old patients, and the young adult and middle-aged patients. Problems such as delay in diagnosis and difficulty of treatment are expected among very old patients.