Norway rat population in Baltimore, Maryland, 2004.

Norway rats are reservoirs for several zoonotic agents, including hantaviruses, and are implicated in the transmission of pathogens to humans in urban environments. The rat population of Baltimore, Maryland was estimated from surveys in 1949 and again in 1952, but has not been evaluated for more than 50 years. Previously identified sociodemographic risk factors for rat infestation, including median income, human density, and percentage of rental properties, were used to categorize census block groups in Baltimore. Rat infestation risk factors, including median income and human density, have improved over the last 50 years in Baltimore. Rat infestation was determined both by observation and trapping of rats in alleys that were representative of the different strata of risk factors. Despite improvements in risk factors, the outdoor, residential rat population of Baltimore in 2004 was estimated to be approximately 48,420 +/- 14,883 rats, which is comparable to the 1949 and 1952 estimates. Approximately half of the rats trapped in Baltimore City had detectable antibody against Seoul virus. The failure to substantially impact rat population levels in the past 50 years indicate that alternative control strategies for rat infestation are needed to reduce the risk of rat-borne pathogen spillover to the human population.