Molecular epidemiology of tuberculosis in Montreal.

Molecular markers provide an important method for detecting tuberculosis (TB) transmission that has been followed by rapid progression to active disease. Restriction fragment length polymorphism (RFLP) typing of Mycobacterium tuberculosis has been used to confirm outbreaks, through the demonstration of shared isolates. In population-based studies, it has therefore been inferred that patients with TB whose infecting organisms are characterized by shared RFLP “patterns” (“fingerprints”) have likely been involved in recent transmission. We used public health reporting data and RFLP-based DNA fingerprinting to determine the extent to which ongoing transmission contributes to TB incidence in Montreal and to evaluate factors associated with such TB transmission. RFLP analysis using the insertion sequence (IS) 6110 was used to classify the 303 culture-positive cases of TB reported in Montreal during 1997 and 1998. We restricted the analysis to those isolates with 5 or more bands, because isolates with few or no copies of the IS6110 element cannot be reliably classified by this method. Reactivation of remotely acquired tuberculous infection was considered likely if the RFLP pattern was unique within the database. Recent transmission was considered likely if an isolate matched at least one other by identical, or nearidentical, criteria. “Identical” isolates were characterized by identical numbers of bands on gel electrophoresis, following digestion by the restriction endonuclease; all such bands had to have matching molecular weights. “Near-identical” isolates were characterized by a single band difference (a single band addition, band loss or band shift). This study was approved by the research ethics committee of the Montreal General Hospital, which is responsible for the infectious diseases unit of the Montreal public health department. Molecular epidemiology of tuberculosis in Montreal