Tuberculosis transmission with multiple secondary patients

Abstract

To elucidate TB transmission with multiple secondary patients (MSP) in comparison with a single secondary patient (SSP). 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-positive (SNBP) in 2,139, and smear-negative bacillus-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. An index case rate (ICR) for a category of patients was defined as following; ICR=NI/NA, where NA: Number of TB patients in a category A, and NI: Number of index cases in category A. An epidemic source rate (ESR) was used instead of ICR when the index case and the TB patients in a category were smear-positive. A total of 337 patients were considered as index cases. Sixty-nine patients had MSP, while 268 had a SSP. The ICRs for MSP were 1.74% for the SPBP patients, 0.33% for the SNBP patients, 0.06% for the SNBN patients, and 0.14 % for the exrtra-pulmonary TB patients. These ICRs for SSP were 5.43%, 1.73%, 1.14%, and 0.96%, respectively. The ESR differences with MSP were highly significant (p<0.001) between patients aged 0-59 and those aged 60-99 (3.8% vs 0.5%), between patients with cavitary lesion and those with non-cavitary lesion (2.6% vs 0.4%), and between patients with large amount of bacilli and those with small amount of bacilli (2.9% vs 0.9%). These differences were also found in those with SSP. These findings suggest that TB transmission with multiple secondary patients is closely correlated with aging, cavitary lesion, and bacillary amount, and that no characteristic changes were found between index cases with multiple secondary patients and those with a single secondary patient.