Swine influenza virus is the cause of an acute respiratory disease affecting swine worldwide. Inactivated influenza vaccines used in breeding females provides passive immunity to neonatal piglets through colostrum. However, this antibody does not prevent infection of susceptible piglets. Endemic circulation of influenza in breeding herds may be due to re-infection of neonatal swine with inadequate levels of colostrum or non-protective maternal antibodies. Infected, but non-clinical piglets may be responsible for the transmission of influenza to littermates or nursery pigs as colostral immunity declines during the pre- and post-weaning phase of production.

 The objectives of this study were to: 1) evaluate the level of influenza infection in neonatal swine on a farm using inactivated influenza vaccines in breeding females; 2) determine the influenza subtype(s) present in neonatal and nursery pigs in the same production system; 3) determine the presence of colostral antibody in neonatal piglets; 4) compare the influenza sequences isolated from neonatal and nursery pig populations and their antigenic relationship with the vaccine strains used in the sow herd vaccine.

 Four breeding farms from the same production system located in the Midwest United States using killed influenza vaccines in the breeding herd were selected for the study. Samples included 4,320 nasal swabs and serum from 12-17 day old piglets within the same litter collected every-other-week between March-May and July-August 2013 for a total of eight collections. Serum was also collected from 330 breeding females corresponding with the litters. Three to eight oral fluid samples were collected from the same group of piglets after transport to the nursery at approximately four weeks of age. Serum samples were evaluated for influenza antibodies by an ELISA test representing colostral antibody transferred from dams to piglets. Nasal swabs and oral fluids were evaluated for the presence of influenza by a diagnostic test that indicated if samples were positive and the subtype. Genetic sequences were compared between influenza isolates and vaccine strains. Serum from breeding females was used to detect the presence of influenza ELISA antibody and for hemagglutination inhibition (HI) tests against vaccine strains and influenza viruses recovered from neonatal and nursery piglets.
 Influenza virus was detected in 1.8% (76/4,320) of individual nasal swabs from piglets nursing dams. The majority of positive nasal swabs were detected early during the sample period (March-April). Overall, 56 H1N2, 3 H3N2 and 12 H1/H3 mixed infections were identified. Five samples were positive for only the N2 subtype. The largest number of H1N2, H3N2 and mixed infections were detected at week 1 and 2 in farm 2 corresponding with clinical influenza illness. Virus was not detected in NS from farm 2 during the final 6 collections. In addition, H1N2 was the only subtype detected in farms 1, 3 or 4 except for a suspect H3 detected at farm 1 during week 7 ( July 22-26). Farm 2 was the only location that demonstrated mixed H1N2/H3N2 influenza infection. Fourteen H1N2 influenza isolates derived from Farm 2 were sequenced. All isolates belonged to the δ-1 cluster with a 2002 lineage N2 and pandemic Matrix gene.
In the downstream nurseries, 31.0% (49/158) of oral fluids were positive for influenza virus. Farm 2 demonstrated the highest number of positive oral fluid samples, 51.0% (25/49). Farms 1, 3 and 4 demonstrated approximately 15.9%, 20.0% and 30.0% positive oral fluid samples, respectively. Overall, 28.6% of the positive oral fluid samples were H1N2 (14/49), 8.0% H3N2 (3/49), and 16.3% (9/49) subtyped N2 only. Approximately 18.4% (9/49) of the positive oral fluid samples were considered suspect H1N2 and H3N2 in the subtyping test and 28.6% (14/49) were untypeable.
Despite the use of inactivated vaccines in dams to prevent influenza-associated respiratory disease and infection, a low level of influenza circulates endemically in nursing pigs on each of the four farms and may remain subclinical. The low prevalence of influenza virus in neonates was in contrast to the much higher prevalence of influenza in the down-stream nurseries. Overall, this study helps guide the management and biosecurity measures that may affect the level of influenza circulation on breeding farms and nurseries and provides guidance for using influenza vaccines in swine.