Pathogenicity in Chickens and Turkeys of a 2021 United States H5N1 Highly Pathogenic Avian Influenza Clade 2.3.4.4b Wild Bird Virus Compared to Two Previous H5N8 Clade 2.3.4.4 Viruses.

Highly pathogenic avian influenza viruses (HPAIVs) of subtype H5 of the Gs/GD/96 lineage remain a major threat to poultry due to endemicity in wild birds. H5N1 HPAIVs from this lineage were detected in 2021 in the United States (U.S.) and since then have infected many wild and domestic birds. We evaluated the pathobiology of an early U.S. H5N1 HPAIV (clade 2.3.4.4b, 2021) and two H5N8 HPAIVs from previous outbreaks in the U.S. (clade 2.3.4.4c, 2014) and Europe (clade 2.3.4.4b, 2016) in chickens and turkeys. Differences in clinical signs, mean death times (MDTs), and virus transmissibility were found between chickens and turkeys. The mean bird infective dose (BID 50 ) of the 2021 H5N1 virus was approximately 2.6 log 10 50% embryo infective dose (EID 50 ) in chickens and 2.2 log 10 EID 50 in turkeys, and the virus transmitted to contact-exposed turkeys but not chickens. The BID 50 for the 2016 H5N8 virus was also slightly different in chickens and turkeys (4.2 and 4.7 log 10 EID 50 , respectively); however, the BID 50 for the 2014 H5N8 virus was higher for chickens than turkeys (3.9 and ~0.9 log 10 EID 50 , respectively). With all viruses, turkeys took longer to die (MDTs of 2.6-8.2 days for turkeys and 1-4 days for chickens), which increased the virus shedding period and facilitated transmission to contacts.