Lack of evidence of increased West Nile virus disease severity in the United States in 2012.

Abstract. In the United States, West Nile virus (WNV) causes annual seasonal outbreaks that fluctuate in size and scope. There was a large multistate outbreak of WNV in 2012, with more human disease cases reported nationally than any year since 2003. We evaluated national surveillance data to determine if the higher number of WNV cases reported in 2012 was associated with changes in the epidemiology or severity of disease compared with 2004-2011. Despite an increased incidence of neuroinvasive disease in 2012, national surveillance data showed no evidence of changes in epidemiology or increased disease severity compared with the previous 8 years.