[The epidemiology of nontuberculous mycobacterial diseases in Hiroshima Prefecture]

The patients of diseases caused by nontuberculous mycobacteria (NTM) newly diagnosed in 1993 in Hiroshima Prefecture were investigated retrospectively. During 1993, 59 newly diagnosed patients of NTM disease, who were the resident of Hiroshima Prefecture were reported from 6 hospitals. Mycobacterium avium complex (MAC) disease was most frequent (53 patients, 90%) and M. kansaii (MK) disease, which accounts for more than 20 percent of all the NTM diseases in the national survey of Japan, was found only in 3%. Other pathogenesis were M. chelonae and M. fortuitum. Incidence rate for all NTM disease per 10(5) population was 2.1. Age-specific incidence rate calculated from the estimated population of Hiroshima Prefecture was; less than 0.4/10(5) under the age 40, 4.0/10(5) in the age group 50 to 59, 4.5/10(5) in the age group 60 to 69 and 9.4/10(5) in the age group over 70. Thirty-one patients (57%) had underlying pulmonary disease such as previous lung tuberculosis (TB) and previous history of TB was found in 41 percent of patients over 60 yr. of age. The data suggest that epidemiological picture of NTM diseases may change in the future. Relative increase of elder population will lead to the increase of NTM diseases as a whole; the incidence rate calculated from the age specific incidence rate and predicted population of Japan in 2025 is 3.1/10(5), which is 1.5 fold higher than the present rate. However, secondary NTM diseases with healed TB will decrease because of the sharp decline of TB morbidity in Japan. Assuming that secondary NTM disease with healed TB decreases to one-fourth, estimated incidence rate will be 2.3. Further, the upward tend of MK disease in Japan should be taken into consideration. Thus, the future trends of NTM diseases will increase fairly due to the increase of elder population as well as the MK disease. It was pointed out that many NTM disease cases had been counted as tuberculosis in the surveillance system of Japan. In our present study, 52 (82%) were registered as tuberculosis in the beginning and only 19 cases were reported later to health centers as NTM disease. As tuberculosis classification and surveillance system in Japan were changed from 1996 to separate NTM disease from TB, NTM disease cases will be counted separately in the tuberculosis statistics. This revision in the management of NTM disease should be taken into account when discussing the trend of tuberculosis statistics in Japan.