A 15-year-old man, who was a classmate with the index case of a large outbreak oftuberculosis in a junior high school in Kochi city, showed strongly positive reaction totuberculin skin test in March 1999. After taking a chest X-ray film, which showed noabnormal finding, chemoprophylaxis with isoniazid was started in April 1999. He wascompliant with therapy, and had no symptom until July 1999, when he was checked againby chest X-ray and abnormal finding was suspected. He visited a hospital for furtherexaminations, and chest X-ray revealed cavitary lesion and sputum smears showed acid-fastbaccili. Cultures of the sputum was positive for Mycobacterium tuberculosis, and drugsusceptibility tests revealed that the organism was resistant to isoniazid (5μg/ml) andsensitive to RFP, SM, and EB. RFLP analysis of Mycobacterium tuberculosis isolatedfrom the index-case patient and the present patient demonstrated an identical pattern, although the organism obtained from the index-case patient was sensitive to isoniazid, RFP, SM, and EB. Mycobacterium tuberculosis isolated from other 7 patients in thesame outbreak showed an identical pattern in RFLP analysis and were also sensitive toisoniazid. The present patient was a close contact with the highly infectious index-casepatient. The patient was estimated to be infected around September to October, 1998, andchemoprophylaxis with isoniazid was started in April 1999, when the tubercle baccili hadgrown considerably, but not enough to show radiographic abnormality. These two factorsmight be attributable to rarely seen development of isoniazid resistance in this case.
