Contextualizing tuberculosis risk in time and space: Comparing time-restricted genotypic case clusters and geospatial clusters to evaluate the relative contribution of recent transmission to incidence of TB using nine years of case data from Michigan, USA

Novel approaches must address the underlying factors sustaining the TB endemic in the U.S., specifically what maintains new Mycobacterium tuberculosis (Mtb) transmission.
Culture-confirmed TB cases reported to the Michigan Department of Health and Human Services (2004-2012) were analyzed for time-restricted genotypic and/or geospatial clustering. Cases with both types of clustering were used as a proxy for recent, local transmission. Modified, multivariate Poisson regression models were fit to estimate this prevalence in relation to various individual- and neighborhood-level demographic and socio-economic variables.
Those individuals that were spatially clustered were 1.7 times as likely to also be time-restricted genotypically clustered. The prevalence of recent, local transmission was higher among U.S.-born cases, males, and Non-Hispanic Blacks. Moreover, people living in neighborhoods in the highest poverty quartiles had 13.8 times the prevalence of recent, local transmission compared to those in the lowest poverty neighborhoods.
Our results suggest geographic areas with high concentration of TB cases are likely driven by ongoing transmission, rather than enclaves of individuals who have reactivated a case of latent TB. Furthermore, efforts to continue reducing Mtb transmission in the U.S., and other low-incidence settings, must better identify community-level sources of risk, manifested through the complex social interactions among people and their environments.