[An outbreak of tuberculosis in which environmental factors influenced tuberculosis infection]

We encountered a contact group investigation in which differences in environmental factors, including the ventilation frequency and the airflow, led to differences in the infection risk.

The index case was diagnosed with tuberculosis after cough and sputum persisted for 9 months. The patient was an instructor working at vocational schools A and B. Sixty-six instructors/staff and 446 students had contact with this patient at the schools. The patient taught 24 regular courses and 21 short-term courses at the 2 schools after symptom onset through to the final day of work.

In a contact investigation of instructors/staff, one person with latent tuberculosis infection (LTBI) was identified. Subsequently, 30 and 240 students with the closest contacts and those with 8-hour or longer contact with the index case, respectively, were examined. In School A, of the 162 students examined, one student developed tuberculosis, 7 were QFT-positive, one was QFT-doubtful, 147 were QFT-negative or judged as not infected (either QFT-negative, or a tuberculin skin test of erythema less than 20 mm, including past history of LTBI treatment or TB treatment), and 6 were not examined. In School B, of the 108 students examined, no one developed tuberculosis nor was QFT-positive, 4 were QFT-doubtful, 98 were either QFT-negative or judged as not infected, one was QFT-indeterminate, and 5 were not examined. Since the onset of tuberculosis and QFT-positivity occurred in only School A, the difference in the incidence of infection between the 2 schools, despite the levels of contact being similar, was assumed to be due to environmental factors. Thus, the ventilation frequency, which had been not reported initially, was reinvestigated. The frequency of air change was as low as 0.45-1/hour in School A, whereas it was better (3.57-7/hour) in School B. Moreover, the air flew from the instructor side toward students in School A, while it was reversed in School B.

It was clarified that the ventilation frequency and airflow markedly influenced infection. It is important to investigate environmental factors on epidemiological investigations and to educate people regarding the importance of ventilation.