Swine Influenza A Virus Seropositivity in Greek Pig Farms: Sampling-Occasion and Age-Group Patterns, 2020–2022

Swine influenza A virus (swIAV) is a respiratory pathogen of pigs with zoonotic potential. This investigation evaluated overall and subtype-specific seropositivity in 2889 sera with available haemagglutination inhibition (HI) results from 94 sampling occasions on 64 Greek pig farms during 2020–2022. Antibodies against the H1avN1, H1huN2, H3N2 and H1pdmN1 subtypes were investigated, while age group, clinical signs potentially compatible with, but not specific to, swIAV infection, and sow vaccination status were recorded. Seropositivity was detected in 70.21% of sampling occasions, and at least one seropositive sampling was recorded on 70.31% of farms. Reactions against one or multiple subtypes were identified, with H1pdmN1 being the most common single subtype. Mixed-effects logistic regression showed no significant association between sampling-occasion-level seropositivity and herd or sampling factors. Subtype-specific seropositivity was associated with sow vaccination and the numbers of animals sampled. Age-group-level seropositivity differed among age groups and was associated with sow vaccination and the number of animals examined in each age-group sample. Seropositivity was not significantly associated with such clinical signs, whereas sow vaccination and herd size were associated with their presence. The findings demonstrate frequent antibody detection and support age-stratified serological surveillance. Vaccination should be considered when interpreting HI results.