[The index and secondary patients in tuberculosis transmission--a retrospective epidemiological study of 3783 patients newly registered in recent 15 years in Aichi, Japan].

OBJECTIVES: To elucidate the characteristics of the index and secondary patients in clusters containing multiple TB patients who were examined epidemiologically. SUBJECTS AND METHODS: The subjects of this retrospective study were 3783 TB patients registered in Aichi between 1989 and 2003. All recorded files were reviewed to identify multiple TB patients in the same transmission group. When multiple patients with a registration interval of less than 10 years were found in the same transmission group, the first patient was considered as the index case, and the other patients were regarded as secondary cases. RESULTS: A total of 106 index patients and 132 secondary patients were found. The index patients occupied 2.8% of the total 3783 registered patients, or 0.9 per 100,000 population. Secondary patients occupied 3.5% and 1.2 per 100,000. Of the 106 clusters, 77.4% consisted of two patients, 15.1% three patients, and 7.5% four or more patients. Smear-positive patients were found in 66.0% of the index patients, 27.3% of the secondary patients, and 30.3% of the 3783 overall patients, and the differences were highly significant between the index and secondary patients, as well as between the index and overall registered patients (p < 0.001). The proportion of the index patients to all registered patients was 6.1% for smear-positive patients, 1.8% for culture-positive patients, and 1.1% for bacilli-negative patients. The differences were highly significant between smear-positive and culture-positive patients, as well as between smear-positive and bacilli-negative patients (p < 0.001). CONCLUSION: The findings of this retrospective study suggest that 2.8% of the total registered patients, 6.1% of smear-positive patients, and 0.9 per 100,000 population was the index patients.