Rat Coronavirus Epizootiology in Wild <i>Rattus norvegicus</i>

From 2006 through 2008, 543 synanthropic <i>Rattus norvegicus</i> were sampled from 20 alleys clustered in five high-density human neighborhoods throughout Baltimore, Maryland, USA. Results indicated that rat coronaviruses (RCV) circulated endemically and infected most rats in the region. Collected animals were tested by serology and RT-PCR for evidence of exposure to RCV and the results were used to characterize the epizootiology of natural infection. Serology showed that 71.8% of rats had IgG antibodies to RCV, indicating previous exposure, while only 3.0% showed evidence of recent infection based on positive PCR results. Incidence was estimated at 16% per month. Antibody and PCR results were uniformly distributed throughout the city. Consistent with historical laboratory studies, RCV appeared to be horizontally transmitted and generated an acute infection with a short period of infectiousness. Evidence of past infection increased with rat age but did not differ significantly between males and females. The low incidence (6/1000) of infectious rats (PCR positive/IgG negative) limits the risk of spillover to nearby species, including humans. These results encourage surveys for other wildlife agents to collect sufficient samples to estimate the power to detect the agents and use serological surveys as a screening tool if active infection is of short duration. Otherwise, substantial efforts may be expended with a low likelihood of detection.