Clade 2.3.4.4b highly pathogenic avian influenza A virus (HP H5 IAV) has caused mortality in a diversity of wild bird species since its introduction to North America late in 2021. Despite continued global importance of HP H5 IAV to wild bird health, comparisons incorporating pathobiology across species are scarce. We conducted postmortem investigations on 39 individuals representing 19 species across the eastern and central United States from 2022-2024. We assessed clinical history, gross and histopathology, and tested tissues, oropharyngeal and cloacal swabs by 2.3.4.4b H5 real-time reverse transcription polymerase chain reaction (rRT-PCR) to compare species group patterns. Prominent gross lesions in waterfowl included pancreatic mottling (8/21; 38%), histologically correlating to pancreatic necrosis, which was more commonly evident (17/21; 81%). Gross lesions of mottling were evident in raptor livers (2/7; 29%), and hepatic necrosis was the most common histopathology finding in this group (6/7; 86%). Noteworthy gross lesions in passerines and other aquatic birds included splenomegaly (1/2; 50% and 2/3; 75%, respectively) and pancreatic mottling in shorebirds (2/7; 29%). Nonsuppurative encephalitis and neuronal necrosis were the most common histologic lesions in shorebirds (3/3; 100%), passerines (2/2; 100%) and other aquatic birds (3/3; 100%) but were not grossly evident. Testing by rRT-PCR showed varied tissue tropism across species, although the lowest cycle threshold values (i.e., levels of viral RNA) were most consistently detected in brain among all species groups. Our results help lay the foundation for further elucidating differences in HP H5 IAV epidemiology and pathobiology across North American wild bird species.

Tori Andreasen1, Rebecca L Poulson1, Lyndon Sullivan-Brugger1, Alisia Weyna2,3, Chloe Goodwin1,2, Aidan O’Reilly1, Robert Stilz1, Nicole M Nemeth1,2 Affiliations: 1Southeastern Cooperative Wildlife Disease Study, Department of Population Health and 2Department of Pathology, College of Veterinary Medicine, University of Georgia, Athens, GA, USA, 3Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI, USA.

Transmission pathways, pathobiology, immune responses