[Clustered secondary case rate in 10,088 patients with tuberculosis]

To elucidate characteristics in clustered secondary TB patients transmitted from culture positive pulmonary TB patients.

The subjects of this retrospective study were 10,088 TB patients registered in Aichi Prefecture between 1989 and 2003. Pulmonary TB was found in 8,629 patients, and 1,459 had extra-pulmonary TB. Bacteriological examination revealed sputum smear-positive (SPBP) in 3,332, sputum smear-negative bacillus-culture-positive (SNBP) in 2,139, and smear-negative bacillus-culture-negative (SNBN) in 3,158. 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 cases. A clustered secondary case rate (CSR) for a category of patients was defined as follows; CSR=NCS/NA, where NA: number of TB patients in a category A, and NCS: number of secondary cases in category A. A cluster rate for a category of bacillary pulmonary patients was defined as follows; Cluster rate= (NIC+NCS)/ NA, where NA: number of TB patients in a category A, NIC: number of index cases in category A, and NCS: number of secondary cases in category A.

A total of 417 patients were considered as clustered secondary cases, and the CSR was 4.1% in total. The CSRs were 3.5% for the SPBP patients, 3.8% for the SNBP patients, 5.4% for the SNBN patients, and 3.4% for the extra-pulmonary patients. The CSR in SNBN patients was significantly higher than the SPBP patients (p<0.001). The significant differences in the CSRs were found between the SNBN patients and the SNBP patients (p<0.01), as well as between the SNBN patients and the extra-pulmonary patients (p< 0.01). The CSRs were 42.5% in patients aged 0-9, 30.3% in those aged 10-19, 11.2% in those aged 20-29, 7.4% in those aged 30-39, 4.6% in those aged 40-49, 3.2% in those aged 50-59, 2.4% in those aged 60-69, 1.8% in those aged 70-79, 1.3% in those aged 80-89, and 0.6% in those aged 90-99. There were significant differences in the CSRs between those aged 10-19 and those aged 20-29 (p< 0.001), between those aged 20-29 and those aged 30-39 (p<0.05), and between those aged 30-39 and those aged 40-49 (p< 0.05). The male patients showed significantly lower CSR than female patients (2.9% vs 6.3%, p<0.001). The cluster rate for the 5,471 bacillary patients was 8.8%. The cluster rates were significantly different between those patients aged 10-19 and those aged 20-29 (37.1% vs 21.1%, p<0.001), as well as between those aged 40-49 and those aged 50-59 (16.4% vs 8.5%, p<0.001).

These findings suggest that the CSR is closely related with patient's age, gender, and bacillary findings, and that the CSR is significantly high in young, female, and SNBN patients.