[Examination of onset cases in contact investigation].

Objectives: To analyze and evaluate onset cases of tuberculosis detected in contact investigations and to apply the results to future countermeasures.

Methods: Index and secondary cases in contact investigations in which the secondary cases occurred in Osaka City between 2005 and 2008 were enrolled. The tuberculin skin test or QFT (QuantiFERON-TB Gold) was conducted to diagnose whether the contacts were infected with tuberculosis. X-ray examination of the chest was conducted to determine whether tuberculosis had developed, immediately or 6 months, 1 year or 2 years after the contact investigation.

Results: (1) Index cases: Index cases followed by secondary cases numbered 131 patients. Regarding the chest X-ray findings, a cavity was observed in 67.7% of the index cases, and a sputum smear of 3 + was observed in 51.5% of the index cases. A 3-month or longer delay in index case-finding was noted in 50.4% of the index cases. (2) Secondary cases: Secondary cases numbered 177 patients, consisting of 107 patients who showed an onset of less than 6 months after their last contact with index cases. Of 70 secondary cases in whom tuberculosis was detected in the investigation conducted 6 months to 2 years after the contact investigation, 50% of them were not tested for infection due to their older age, consisting of 11 patients in their 40's, 8 in their 50's, and 16 in their 60's or over, and 17.1% of them refused or discontinued the treatment for latent tuberculosis infection, leading to onset.

Discussion: Many secondary cases were detected immediately after the contact investigation, suggesting the importance of reducing the delay in index case-finding. Regarding the onset of secondary cases who showed an onset 6 months after the last contact with index cases, many cases showed an onset without being tested for infection due to their older age. More attention should be paid to the cases who refused or discontinued the treatment for latent tuberculosis infection, leading to onset.