[Transmission of tuberculosis from smear-negative bacillus-positive pulmonary TB patients]

To elucidate TB transmission from smear-negative bacillus-positive patients.

The subjects of this retrospective study were 8,339 TB patients registered in Aichi Prefecture between 1989 and 2003. Of these patients, 7,122 had pulmonary TB and 1,217 had extra-pulmonary TB. Bacteriological examination for the pulmonary patients revealed sputum smear-positive in 2,640, sputum smear-negative bacillus-positive (SNBP) in 1,794, and smear-negative bacillus-negative (SNBN) in 2,688. 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 initial case was considered as the index case, and the other patients were regarded as secondary cases. An index case rate (ICR) for a category of patients was defined as follows; ICR=NI/NA, where NA: Number of TB patients in a category A, and NI: Number of index cases in category A.

A total of 287 patients were considered as index cases, and the ICR was 3.4%. The ICRs were 2.3% for the SNBP patients, 7.5% for the smear-positive patients, 1.2% for the SNBN patients, and 1.1% for the exrtra-pulmonary tuberculosis patients. The SNBP patients had a significantly higher ICR than the SNBN patients (p<0.01), and a significantly lower ICR than the smear-positive patients (p<0.001). In the SNBP patients, the ICRs were 5.0% for those aged 40 to 49 years and 1.6% for those aged 50 to 59, and the difference was statistically significant (p<0.05). All patients were grouped into two groups; younger patients with an age of less than 50 years and elder patients with an age of 50 years or higher. The ICRs showed significant differences between the younger patients and the elder patients as follows: 4.3% vs 1.7% for the SNBP patients (p<0.01), 15.5% vs 5.0% for the smear-positive patients (p<0.001), and 1.9% vs 0.9% for the SNBN patients (p<0.05). The chest X-ray showed cavitary lesion in 453 SNBP patients. The ICRs were 8.7% for 126 cavitary younger SNBP patients, 2.6% for 311 non-cavitary younger SNBP patients, 2.4% for 327 cavitary elder SNBP patients, and 1.5% for 1,030 non-cavitary elder SNBP patients. The ICR for the cavitary younger SNBP patients was significantly higher than those for the other three subgroups (p<0.01 to p<0.001).

These findings suggest that TB transmission from SNBP patients depends on the patient's age and the cavity formation, and that patients aged less than 50 years with cavitary pulmonary TB is the most important target for the epidemiological research.