Persistence of antibodies after natural infection with swine influenza virus and epidemiology of the infection in a herd previously considered influenza-negative

A herd previously considered influenza-negative became infected with swine influenza virus (SIV). After virus circulation had apparently stopped in the herd, antibodies to SIV could still be demonstrated in sows 28 months postinfection. The source of infection was not determined, and various possibilities are discussed.