© 2003 American Thoracic Society
Limited transmission of multidrug-resistant tuberculosisTo the Editor:We were impressed with Nitta and colleagues' report on the limited transmission of multidrug-resistant tuberculosis (MDR-TB) in Los Angeles County (LAC), and we applaud the aggressive surveillance and case management activities of their MDR-TB unit (1). We also read with interest the editorial by Daley, who offers valuable insight into alternative explanations for the findings of the study (2). The question of whether drug-resistant Mycobacterium tuberculosis bacilli are less likely than drug-susceptible strains to produce infection and disease has persisted for decades. Reports have demonstrated that transmission of drug-resistant M. tuberculosis occurs even among immunocompetent persons (3, 4). Another report found no lower risk of infection among contacts of isoniazid- and streptomycin-resistant tuberculosis (TB) cases than among contacts of drug-susceptible cases (5). The recent report implies that persons with MDRTB may less readily transmit M. tuberculosis, as evidenced in part by the low proportion of tuberculin skin test (TST) conversions among contacts. The ability to detect conversions in the population tested, however, must be considered. Using TST conversion to detect recent transmission of M. tuberculosis in persons from areas in which TB (and Bacille Calmette-Guérin) is endemic is problematic. The significance of a positive TST in an MDR-TB contact who did not meet the authors' criteria for conversion is unclear. The reaction may either represent remote infection with drug-susceptible M. tuberculosis or more recent infection with MDR bacilli. Given that most of the MDR-TB cases had been previously (and ineffectively) treated, many of these cases could have infected contacts over an extended period of time. Again, such contacts would not meet the criteria for conversion. In this context, the rate of TST conversions among contacts of MDR-TB cases would underestimate the actual degree of transmission. Although the number of MDR-TB cases in the United States continues to decline, the proportion of foreign-born persons with MDR-TB continues to increase (6). Accordingly, it is imperative that TB control programs continue aggressive efforts to eliminate MDR-TB, such as those successfully used in Los Angeles County.
Centers for Disease Control and Prevention Atlanta, Georgia REFERENCES
From the Authors:We thank Drs. Spradling and Ridzon for their thoughtful comments, and concur that multiple studies have documented the transmissibility of drug-resistant Mycobacterium tuberculosis. Shortly after we submitted our study (1) for publication, researchers in Milan, Italy published results of a 2-year, population-based study in which all strains were typed by restriction fragment length polymorphism analysis (2); multidrug resistance was one of several factors with statistical significance associated with clustering. This study adds to the body of evidence that indicates the phenomenon of drug resistance does not prevent the spread of TB in susceptible populations.Although our findings suggested limited transmission of multidrug-resistant TB in Los Angeles County, California, we acknowledge the limitations of currently available investigative tools such as the TST. We carefully selected cut points for TST interpretation in an effort to discriminate between true conversions and booster effects due to bacillus Calmette-Guérin or atypical mycobacteria. Nevertheless, the TST remains an imperfect tool that cannot differentiate between current and distant infection, nor can it detect differences between the drug resistance patterns of infecting strains of M. tuberculosis. Better diagnostic and analytical tools for TB are sorely needed, and would greatly add to our current understanding of the epidemiology of TB.
Los Angeles County Department of Health Services Los Angeles, California REFERENCES
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