Production-Type-Specific Spatial and Space–Time Clustering of High-Pathogenicity Avian Influenza Outbreaks in Layer and Duck Farms in the Republic of Korea, 2020–2026

High-pathogenicity avian influenza (HPAI) remains a recurrent threat to poultry production in the Republic of Korea, but production-type-specific clustering has rarely been assessed against the underlying farm population. This retrospective unmatched case–control point-pattern study examined spatial and space–time clustering across six epidemic seasons from 2020 to 2026. We analyzed 486 reported poultry-farm outbreak records together with a national registry of 66,499 poultry farms. The principal analyses compared 204 layer and 204 duck outbreak events with farms of the same production type using the Cuzick–Edwards k-nearest-neighbor statistic, an observed-to-expected clustering-intensity ratio, kernel relative-risk mapping, and spatial and space–time scan statistics. At k = 5, the observed-to-expected intensity ratio was 2.51 for layer events and 1.63 for duck events; this between-type contrast was descriptive because no direct inferential test was performed. Layer-event clustering remained elevated across all seasons, although the extreme 2023–2024 estimate was based on only 15 events. Duck-event clustering was more intermittent and increased in 2025–2026. Premises-level scan statistics identified a broad layer cluster with a relative risk of 3.12 and a smaller duck cluster with a higher local relative risk but far fewer farms. A layer space–time cluster occurred in 2024–2025. These findings support production-type-specific, density-aware surveillance. They do not establish farm-to-farm transmission or implicate egg-handling infrastructure, and they should be interpreted in light of the static registry, repeated-event design, and unmeasured flock size, movements, biosecurity, environmental exposure, and viral lineage.