Tuberculosis genotyping in six low-incidence States, 2000-2003.

Background: As tuberculosis incidence declines in the United States, a new tool for TB control efforts is Mycobacterium tuberculosis genotyping. Colorado, Iowa, Montana, New Hampshire, West Virginia, and Wisconsin began routine genotyping of all culture-confirmed TB cases in October 2000.

Methods: M. tuberculosis isolates from cases reported October 2000 through December 2003 were genotyped by spoligotyping, mycobacterial interspersed repetitive units, and IS6110-based restriction fragment length polymorphism methods. Genotyping results were linked to demographic variables from national surveillance records. Patients who were in genotype clusters were interviewed and their records reviewed to determine possible transmission links among clustered patients. Final analysis was completed during April 2004 through June 2005.

Results: Of 971 reported TB cases, 774 (80%) were culture-confirmed, of which 728 (94%) were genotyped. Most genotyped isolates (634 [87%]) were unique. Within 36 clusters linking 94 individuals, four clusters involved both U.S.- and foreign-born individuals. For eight clusters, genotyping results led to the discovery of previously unsuspected transmission. Transmission links between individuals were established in 21 (58%) of the 36 clusters.

Conclusions: In these six low-incidence states, most isolates had unique genotypes, suggesting that most cases arose from activation of latent infection. Few TB clusters involved the foreign-born. For 58% of genotype clusters, epidemiologic investigation ascertained that clustering represented recent M. tuberculosis transmission.