Highly Pathogenic Avian Influenza H5N1 in a Commercial Turkey Breeder Farm with Structured Entry Biosecurity: A Descriptive Field Investigation

Highly pathogenic avian influenza (HPAI) remains a major threat to commercial poultry. We investigated an HPAI H5N1 outbreak that became clinically apparent in House 6 of a 15-house, fully enclosed turkey breeder holding in eastern Hungary with structured site- and house-entry biosecurity. The holding contained 17,876 48-week-old birds, including 454 males in House 6. Acute mortality began on 6 February 2025 and reached 371/454 (81.7%) by 8 February; depression, lethargy, and somnolence predominated. Gross and histopathological lesions were consistent with systemic HPAI, while RT-qPCR and HA cleavage-site sequencing confirmed HPAI H5N1. A separate infected laying-hen holding was located 4.70 km away. Regional meteorological reconstruction showed directional compatibility with the inter-holding axis during 54.7% of hours from 25 January–1 February and 83.3% during 25–28 January, including a 47-h continuous compatible episode; no compatible hourly direction occurred on 1 February, when culling was completed. These findings describe a temporally and directionally compatible regional exposure context but do not demonstrate atmospheric transport, source attribution, or culling-associated transmission. Wild-bird-associated contamination and other indirect pathways remain plausible; comparative whole-genome data, environmental sampling, exact culling times, and complete trace-back information were unavailable.