Increasing drug resistance of Mycobacterium tuberculosis isolates in Ontario, Canada, 1987-1998

We examined trends in resistance to first-line antituberculous agents for M ycobacterium tuberculosis strains isolated in Ontario, Canada from 1987 thr ough 1998 (n = 8069), The proportions resistant were as follows: isoniazid, 9.6%; rifampin, 1.9%; streptomycin, 4.9%; ethambutol, 1.3%; and pyrazinami de, 1.7%. Resistance to isoniazid has increased markedly since 1990, wherea s resistance to streptomycin, ethambutol, and pyrazinamide increased from 1 997 through 1998, Resistance to both isoniazid and rifampin did not increas e. The incidence of persistence and reactivation (early or late treatment f ailure) was 1-2 per 100 person-years (PY) in the first 7-12 months and 0.3- 0.9 per 100 PY from 13 months to 5 years thereafter. For initially suscepti ble strains, the incidence of resistance to isoniazid was 0.11 per 100 PY a nd for and rifampin was 0.06 per 100 PY in the first year and negligible th ereafter, with an overall risk of 0.14% for isoniazid and 0.10% for rifampi n, Resistance of M. tuberculosis to antituberculous agents, and in particul ar to isoniazid, is a growing problem in Ontario and is higher than elsewhe re in Canada.