[Generation-to-generation TB transmission in Aichi, Central Japan -- an epidemiological study of 701 TB patients in 290 clusters].

OBJECTIVES To elucidate characteristics in generation-to-generation TB transmission from smear positive pulmonary TB patients. SUBJECTS AND METHODS The subjects of this retrospective study were 10,088 TB patients registered in Aichi prefecture between 1989 and 2003. Smear-positive pulmonary TB was found in 3,332. All registration files were reviewed to identify epidemiological links of patients. When linked patients with an interval of the dates of registration of less than 10 years were found, the first case was considered as the index case, and the other patients were regarded as secondary patients. All patients were subdivided by every 10 years of age as a generation. All patients were also grouped as young generations aged 0 to 29, middle-aged generations aged 30 to 59, and elder generations aged 60 to 99. A young generations rate (YGR) for a generation of index cases was defined as following; YGR=NY/NA, where NA: Number of whole TB secondary patients transmitted from all index cases belonging to generation A, and NY: Number of secondary patients aged 0-29 in NA. A same generation rate (SGR) for a generation of index cases was defined as following; SGR=NS/NA, where NA: Number of whole TB secondary patients transmitted from all index cases belonging to generation A, and NS: Number of secondary patients belonging to the same generation A in NA. RESULTS A total of 290 clusters were found with 290 index cases and 411 secondary cases. Of the 290 index cases, the biggest number of patients was 58 for the forties, followed by 43 patients for the twenties and the fifties. Of the 411 secondary patients, the biggest number of patients was 91 for the twenties, followed by 58 patients for the thirties, and 158 patients or 38.4% belonged to the young generations. High YGRs were found in the young and middle-aged generations ranging from 37.5% to 75.0%, while YGRs were low in the elder generations ranging from 15.9% to 26.7%. The difference was significant between the fifties and the sixties (p<0.01). The YGRs were calculated 57.1% for the young generations, 43.5% for the middle-aged generations, and 18.9% for the elder generation. The differences were significant between the young and the middle-aged (p<0.05), as well as between the middle-aged and the elder (p<0.001). The biggest SGR was calculated 42.2% for the twenties. Low SGRs were 18.2% for the forties and 17.5% for the fifties. The differences were significant between the twenties and the forties (p<0.01), as well as between the twenties and the fifties (p<0.01). The secondary patients aged 0 to 29 were counted 158: 123 were transmitted through intra-familial infection and 35 were through extra-familial infection. The major index case generation was the middle-aged generations with 82 or 66.7% secondary patients in the case of intra-familial infection, while it was the young generations with 20 or 57.1% patients in the case of extra-familial infection. CONCLUSION These findings suggest that the major sources of TB transmission to young people are smear-positive pulmonary TB patients aged 0 to 59.