How to evaluate herds using swine influenza serology

For broadest surveillance against SIV strains currently known to be circulating in US swine, two H1N1 HI tests using different antigens (reassortant and H1N2-like) in combination with the H3N2 ELISA should be utilized. For routine immunization programs utilizing bivalent vaccine, the H3N2 ELISA should be adequate to determine if pigs or sows have been vaccinated according to schedule. Figure 3 shows currently available tests and recent data presented by Jackson et al.⁵ Reactivity of the H3N2 ELISA with the CO 99 sera was much lower than the responses detected for the IL 99-like outbreak. Further testing with CO 99 sera is needed to evaluate the ability of the assay to reproducibly detect antibodies against CO 99-like strains of H3N2 SIV. Other antigens expressed on the surface of the virus have been proposed as ELISA test antigens. Killed SIV vaccines do not stimulate antibodies against these antigens, which provides for recognition of natural infection in the presence of vaccinal or colostral antibodies. The primary immune response to a killed vaccine is against the dominant surface antigens of the virus, the hemagglutinin and neuraminidase. Ideally, vaccinated sows or their pigs would be seronegative to a test using an alternative antigen unless they had been naturally infected. Due to the broad antigenic spectrum of SIV strains currently circulating in US swine, it is clear that new approaches are needed to monitor herds for recent infection with SIV.