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
Simple Summary High-pathogenicity avian influenza repeatedly affects Korean poultry farms, but outbreak maps can be misleading because poultry farms themselves are geographically concentrated. We compared outbreak events in layer and duck production with the corresponding national farm populations using complementary spatial methods. Layer-farm outbreak events had higher observed-to-expected local clustering ratios across several neighborhood sizes, whereas duck-farm clustering was more variable among epidemic seasons. The analyses also identified broad and localized high-risk areas in west-central poultry-production regions. These results describe where outbreak events were unusually concentrated relative to farms at risk; they do not identify a specific transmission route or imply greater biological transmissibility in chickens. Because the study used a single 2020 farm registry and retained repeated outbreak events at recurrent premises in event-level analyses, the magnitude of clustering should be interpreted cautiously. The findings support production-type-specific surveillance and closer investigation of farm density, flock size, service movements, biosecurity, wild-bird exposure, and viral lineage in future studies.