To clarify the epidemiology of influenza A viruses in coordinated swine production systems to which no animals from outside the system are introduced, we conducted virologic surveillance during September 2012-September 2013. Animal age, geographic location, and farm type were found to affect the prevalence of these viruses.
Keywords: H1N1; H3N2; North America; United States; influenza; surveillance; swine; viruses; zoonoses.