Serological Evidence of Influenza A Virus Exposure and Reactivity to an H1N1pdm09 Reference Strain in Backyard Pigs in Chile

This study assessed serological exposure to influenza A virus (IAV) in swine backyard production systems (BPS) across 14 regions of Chile. Sera originated from official surveillance for exotic swine diseases in 2020. Database reconciliation identified 928 sample records linked to 123 distinct registered premises; two empty tubes were excluded from sample-level analyses. Nineteen of 926 evaluable samples were ELISA-positive (2.05%; descriptive 95% confidence interval [CI]: 1.32–3.18%), from six premises (4.88%; 95% CI: 2.25–10.23%). Seven initially doubtful samples were retested, yielding six positive and one negative result. Thirteen of the 19 ELISA-positive samples had sufficient residual serum for hemagglutination inhibition (HI) testing; all 13 reacted to the H1N1pdm09 reference strain (titers, 1/40–1/640). Positive premises were located in O’Higgins, Biobío, and Ñuble. The results support IAV exposure and HI reactivity to an H1N1pdm09 reference strain, but do not identify the infecting genotype, active circulation, or direction of transmission. Surveillance-based selection and within-premises clustering limit population-level inference.