[A case of isoniazid-resistant tuberculosis diagnosed during chemoprophylaxis with isoniazid]

A 15-year-old man, who was a classmate with the index case of a large outbreak of tuberculosis in a junior high school in Kochi city, showed strongly positive reaction to tuberculin skin test in March 1999. After taking a chest X-ray film, which showed no abnormal finding, chemoprophylaxis with isoniazid was started in April 1999. He was compliant with therapy, and had no symptom until July 1999, when he was checked again by chest X-ray and abnormal finding was suspected. He visited a hospital for further examinations, and chest X-ray revealed cavitary lesion and sputum smears showed acid-fast baccili. Cultures of the sputum was positive for Mycobacterium tuberculosis, and drug susceptibility tests revealed that the organism was resistant to isoniazid (5 micrograms/ml) and sensitive to RFP, SM, and EB. RFLP analysis of Mycobacterium tuberculosis isolated from 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 the same outbreak showed an identical pattern in RFLP analysis and were also sensitive to isoniazid. The present patient was a close contact with the highly infectious index-case patient. The patient was estimated to be infected around September to October, 1998, and chemoprophylaxis with isoniazid was started in April 1999, when the tubercle baccili had grown considerably, but not enough to show radiographic abnormality. These two factors might be attributable to rarely seen development of isoniazid resistance in this case.