Abstract / Summary
Swine influenza A virus (swIAV) is an endemic respiratory pathogen of pigs with zoonotic relevance. We analysed retrospective serological data from routine diagnostic submissions (January 2023–May 2026) to a single laboratory, comprising 2003 sera from 66 reports on 44 commercial farrow-to-finish farms in seven regions of mainland Greece, tested with a nucleoprotein ELISA that cannot distinguish vaccine-, maternally, or infection-derived antibodies. Overall seroprevalence was 46.4% (929/2003; farm-clustered 95% CI 39.1–53.8%); at least one seropositive animal was found in 51 of 61 farm-years and in 36 of 44 farms. Seroprevalence differed by production stage (p < 0.001): it was highest in multiparous sows (75.9%) and suckling piglets (70.7%), both likely influenced by sow vaccination and maternally derived antibodies, intermediate in gilts (46.0%) and nursery pigs (38.6%), and lowest in growers (24.8%) and finishers (24.0%), in which antibodies indicate prior exposure rather than current circulation. No consistent temporal trend was detected, although some adjusted models suggested higher seroprevalence in 2025–2026. Regional seroprevalence ranged from 10.1% (Thessaly) to 61.0% (West Greece); the association was significant with the standard but not the bias-reduced cluster-robust estimator (p = 0.014 and p = 0.39) and persisted after adjustment for year and stage, but it rests on few farms per region and is considered exploratory. As a convenience sample of diagnostic submissions, these data provide descriptive rather than nationally representative estimates and support the addition of molecular, subtype-level surveillance.