Evaluation of transmission metrics in a slow-spreading highly pathogenic avian influenza (HPAI) outbreak in a commercial upland game bird system.

In 2022, highly pathogenic avian influenza virus (HPAIV) H5N1 clade 2.3.4.4b was detected in United States (US) poultry, quickly escalating into an outbreak that surpassed the 2014/15 HPAI US event in scale and impact. Unlike in 2014/15, the 2022/3/4/5 outbreak has included numerous HPAI detections in previously infrequently affected commodities such as broilers and commercially-raised upland game birds. Here, we describe H5 HPAI detections that occurred between December 2023 and January 2024 in a multipremises, commerical upland game bird system located in the Midwestern US. We used an approximate Bayesian computation algorithm and stochastic within-flock HPAI transmission model to estimate the following: (1) time of HPAI introduction onto each affected premises, (2) number of days between estimated times of introduction and times of detection, and (3) the adequate contact rates and basic reproduction numbers within individual barns. Clinical signs and mortality observed in the infected pheasant flocks were largely consistent with other pheasant H5 clade 2.3.4.4b outbreaks. Across the system, the estimated transmission metrics were noticeably lower than those calculated from outbreaks in other poultry species. However, times to detection were similar to HPAI outbreaks that have occurred in other commodities.