Reply to Lee and Howden

To the Editor—We thank Dr Lee and Prof Howden for their letter and for giving us an opportunity to better articulate our interpretation of findings, especially with respect to transmission of Mycobacterium tuberculosis among patients within 2 southern Indian districts. Based on analysis of single nucleotide polymorphism (SNP) differences between 223 M. tuberculosis strains from 196 patients within the Thiruvallur and Madurai districts of Tamil Nadu, we report recent intradistrict, but no recent interdistrict, transmission of strains among patients. In drawing these conclusions, we limited our interpretation to the data available to us, which showed that the closest SNP distance between M. tuberculosis isolated from patients in different districts was 85 SNPs, which is substantially higher than the very small numbers of SNPs (as few as 0) observed when comparing isolates from patients treated in the same district and treatment center.