Impact of Maternal Antibodies and Weaning stress on the Replication and Transmission of Human H3N2 Influenza A in Piglets

Modern swine production facilitates indoor respiratory contact between human employees and pigs in their care, creating conditions for interspecies transmission of influenza A virus (IAV). Sow vaccination is routinely practiced in the U.S.A. to transfer maternal derived antibodies (MDA) to piglets. Weaning is a highly stressful period for piglets that requires increased human interaction. Weaned piglets potentially have mixed immunity from MDA: matched, mismatched, and naïve. Since there have been multiple introductions of human seasonal H3N2 to swine, we assessed the effect of matched and mismatched MDA acquired from vaccinated sows and the stress of weaning on the susceptibility of piglets to a human-origin H3N2 IAV. The H3N2 virus was generated by reverse genetics to mimic the 2010.1 H3N2 introduction from humans to swine. Challenged seeder piglets were divided by immune and weaning status. Two days post inoculation, naïve direct contact pigs were placed with seeders. IAV quantitative reverse transcription polymerase chain reaction (qRT-PCR) and virus titration were performed on nasal swabs and bronchoalveolar lavage fluid to evaluate shedding and transmission kinetics. Matched MDA were effective in reducing shedding in challenged pigs and minimizing transmission of the human-like H3N2 to contacts. There was an increase in shedding and transmission in weaned pigs compared to littermates that remained on the sow. These results identify critical control points in production where changing practices could mitigate human-to-swine and swine-to-swine transmission to prevent establishment of novel lineages in pig populations.