Jeffery D. Sullivan

The emergence of clade 2.3.4.4b highly pathogenic H5N1 has led to unprecedented mortality events in crane populations globally. Despite increasing concern, little is known about the prevalence of influenza A virus (IAV) and resulting antibodies in Sandhill Cranes (Antigone canadensis). To address this knowledge gap, we sampled 31 Greater and two Lesser Sandhill Cranes (Antigone canadensis tabida and Antigone canadensis canadensis, respectively) wintering in California’s Central Valley during the 2023–2024 and 2024–2025 seasons to assess active infection with IAV. None of the sampled individuals tested positive for infection with IAV via rRT-qPCR. We also sampled 29 Greater and two Lesser Sandhill Cranes for serological evidence of prior exposure to IAV; four Greater Sandhill Cranes tested positive for IAV antibodies via bELISA and antibodies specific to H5 were detected in two individuals. These findings suggest prior exposure, potentially on breeding grounds where crane mortalities related to highly pathogenic H5 have been documented. However, the low seroprevalence observed in this study suggests that the Staten Island, California, wintering population may remain largely vulnerable to future outbreaks, though small sample size does limit the broader applicability of these findings.

Highly pathogenic avian influenza viruses (HPAIV) have had disastrous, worldwide effects on wild birds and domestic poultry since the emergence of the A/goose/Guangdong/1/1996 (Gs/GD/96) lineage. The currently circulating H5N1 clade 2.3.4.4b has an expanded set of susceptible hosts, including many migratory wild birds, and is associated with higher transmission rates, increased susceptibility among wild bird hosts, and a greater number of wildlife reservoirs. Certain wild bird life‐history strategies and behaviors have been suggested to explain avian hosts’ susceptibility and exposure to HPAIV. These biological traits include gregariousness, such as colonial nesting and mixed flock foraging, predation or scavenging on wild birds, and association with aquatic habitats. Variation in host infection responses (e.g., infectability, shedding rates and duration, mortality rate, antibody development) informs the overall infection risk across avian species, yet the specific role of biological traits is often inconsistent and unclear across taxa. Moreover, the interactions and potential compounding effects among these biological traits remain largely unknown. To develop a more holistic understanding of cumulative risk across bird species, we integrate existing information on infection risk factors (i.e., susceptibility, immunological response, and behavioral traits) into a qualitative multivariate analysis. This approach enabled us to examine how infection risk factors relate to biological traits (e.g., phylogeny, physiology, behavior, species range) and to begin disentangling their complex interactions. We quantified and summarized these risk factors across host species and qualitatively ranked species by their viral responses along a proposed HPAIV response continuum, guided by expectations of traits and metrics associated with competence or vulnerability to HPAIV. In doing so, we aimed to better understand how viral responses and biological traits synergistically interact to influence cumulative risk across wild bird species. This work broadly expands on the previous avian influenza literature, which has focused on Anseriformes and Charadriiformes as primary viral reservoirs. We tie our findings to effective disease management responses with links to risk components, including descriptions of potential surveillance strategies applied to research and One Health goals, as well as a fuller understanding of how resources may be better deployed for rapid response when spillovers do inevitably occur. Additionally, we identified numerous areas where vital epidemiological information is lacking to best characterize the spread of these viruses. Ultimately, this improved understanding will help identify and inform disease management needs and decision making.

Animal movements contribute to the spread of infectious diseases and are driven in part by environmental conditions. We investigated the links among the environment, animal movement, and infectious disease dynamics in waterfowl, which are among the primary wildlife hosts of avian influenza viruses. By combining telemetry data on 4606 individuals from 26 waterfowl species with data on land cover, weather, and vegetation, we found that waterfowl moved less in areas of higher land cover heterogeneity and higher human population density. Moreover, predicted waterfowl movement distances were weakly but positively correlated with distances between detections of H5N1 highly pathogenic avian influenza in wild waterfowl, suggesting that environmental conditions might contribute to the spread of this disease via their effects on bird movements. By considering wildlife movements alongside other drivers of infectious disease dynamics, such as livestock production and human mobility, we move closer to predicting outbreaks and informing interventions.

Advancements in technology have facilitated a rapid expansion in the range of species for which tracking studies can be implemented. Such efforts often require unique attachment methods or materials for which careful evaluation is required to fully understand potential tagging effects and subsequent implications. During the 2021 breeding season, our research team affixed two types of light-weight transmitters to adult and pre-fledgling juvenile Common Terns (Sterna hirundo) with leg-loop harnesses made of elastic cord. Our initial observations of these marked birds detected minimal differences in behavior and nesting success of marked adults, and failed to detect any notable impacts on the growth rate or behavior of marked juveniles, but were limited to a single year of data. To provide further insight into the potential effects of these tagging methods, we evaluated return and nesting rates for the 2022 through 2025 breeding seasons. We found that all individuals observed on the breeding grounds in the years following transmitter attachment had not retained the transmitter. The observed tag loss suggest that tag retention could be considered when planning future efforts, and limits our analyses to understanding lagged effects of the unknown duration of actual attachment. With this limitation in mind, we found that birds marked as adults with center-weighted and rear-weighted tags were 1.00 and 0.76 times as likely, respectively, to return to the colony site, and 1.09 and 0.77 times as likely, respectively, to be observed nesting when compared with unmarked controls. Similarly, birds marked as juveniles with rear-weighted transmitters were 0.54 as likely to return, and 0.72 times as likely to nest in subsequent years. These results suggest no negative impacts of center-weighted attachment on return or nesting rates, but moderate support declines in both metrics for birds marked with rear-weighted transmitters.

While wild waterfowl are known reservoirs of avian influenza viruses and facilitate the movement of these viruses, there are notable differences in the response to infection across species. This study explored differential responses to infection with highly pathogenic avian influenza in Snow Geese (Anser caerulescens) located in the California Central Valley. Though H5 antibody prevalence was high across years among birds sampled in the winter (75% in both years via hemagglutination inhibition), these values were even higher among birds sampled in summer that failed to migrate (i.e., August 2023 = 100% and August 2024 = 93% via hemagglutination inhibition). Birds that failed to migrate were also generally lighter than birds sampled in the winter and presented notable damage to cerebrum and cerebellum. In December 2022, a single individual positive for infection with H5N1 at the time of sampling indicated reduced movement during the 14 days following sampling but completed spring migration comparably with uninfected conspecifics. However, while no birds were actively infected during sampling and marking in 2023, two marked geese departed for migration late and one did not migrate at all. Additional banded birds marked in August have been reencountered in scenarios ranging from hunter harvest at a different site over a year later to found dead shortly after banding. Our data indicate that Snow Geese infected with HPAI have the potential to express variable outcomes following infection with highly pathogenic H5N1, ranging from rapid recovery within a migratory season to death. These data also suggest that the abnormal failure of some Snow Geese to migrate from the Central Valley is likely driven by HPAI infection.

Infección por influenza aviar y anticuerpos no identificados en pavos silvestres del este (Meleagris gallopavo silvestris) muestreados en la península de Delmarva, en los Estados Unidos. Aunque la influenza aviar altamente patógena representa una amenaza notable para los pavos domésticos (Meleagris gallopavo domesticus), se han identificado infecciones mínimas en pavos silvestres (Meleagris gallopavo) en todo Estados Unidos. De hecho, las infecciones recientes en pavos silvestres se han relacionado con o se sospecha que son el resultado de la propagación de pavos domésticos infectados cercanos. Este estudio presenta datos de muestreo de influenza aviar para pavos silvestres del este (Meleagris gallopavo silvestris) en la porción de Maryland de la península de Delmarva, en el invierno de 2023-2024, con el objetivo de identificar cualquier virus de influenza A (IAV) circulante, así como determinar el grado de exposición previa a el virus de influenza A en esta población. Se recolectaron muestras de hisopos de 36 pavos silvestres hembras. No se detectó ARN viral del virus de influenza A en ninguna muestra, según la transcripción inversa y PCR en tiempo real. Se recolectaron sueros de 31 de los 36 individuos muestreados, todos los cuales tenían valores de bELISA S/N >0.7 (rango: 0.79–1.29) que indican una falta de anticuerpos contra el virus de influenza aviar. Aunque nuestro tamaño de muestra es limitado, estos resultados no indican evidencia de infección actual o previa reciente por el virus de influenza A en las aves muestreadas.

Wild waterfowl are considered to be the reservoir of avian influenza, but their distinct annual life cycle stages and their contribution to disease dynamics are not well understood. Studies of the highly pathogenic avian influenza (HPAI) virus have primarily focused on wintering grounds, where human and poultry densities are high year-round, compared with breeding grounds, where migratory waterfowl are more isolated. Few if any studies of avian influenza have focused on the molting stage where wild waterfowl congregate in a few selected wetlands and undergo the simultaneous molt of wing and tail feathers during a vulnerable flightless period. The molting stage may be one of the most important periods for the perpetuation of the disease in waterfowl, since during this stage, immunologically naïve young birds and adults freely intermix prior to the fall migration. Our study incorporated empirical data from virological field samplings and markings of Swan Geese (Anser cygnoides) on their breeding grounds in Mongolia in an integrated agent-based model (ABM) that included susceptible–exposed–infectious–recovered (SEIR) states. Our ABM results provided unique insights and indicated that individual movements between different molting wetlands and the transmission rate were the key predictors of HPAI perpetuation. While wetland extent was not a significant predictor of HPAI perpetuation, it had a large effect on the number of infections and associated death toll. Our results indicate that conserving undisturbed habitats for wild waterfowl during the molting stage of the breeding season could reduce the risk of HPAI transmission.

Clade 2.3.4.4b Eurasian-origin H5N1 entered North America in late 2021 and spread across the continent. While studies have characterized the antibody response mounted by dabbling ducks following exposure, little data are available for diving ducks. This study sought to identify influenza A virus (IAV) infection and antibodies in Lesser and Greater Scaup captured in Maryland, Illinois, and Rhode Island. In Maryland, IAV seroprevalence increased from the 2021/2022 to 2022/2023 sampling season, with IAV antibody prevalence increasing for juvenile (38%–80%) and adult (82%–90%) Lesser Scaup. While adult Lesser Scaup sampled in Illinois in 2021/2022 had IAV antibody prevalence comparable to those sampled in Maryland (76% and 82%, respectively), they had higher antibody prevalence to both H5 (48% and 18%) and N1 (68% and 35%), potentially due to being sampled in March versus December and January. Our data suggest that Lesser Scaup had limited antibodies to highly pathogenic H5 IAV prior to the introduction of clade 2.3.4.4b H5N1 to North America, but relevant antibodies were widely observed in the months and year following. Our more limited data suggest similar trends may have occurred in Greater Scaup as well.

The lack of consolidated information regarding the response of wild bird species to infection with avian influenza virus (AIV) is a challenge to both conservation managers and researchers alike, with related sectors also impacted, such as public health and commercial poultry. Using two independent searches, we reviewed published literature for studies describing wild bird species experimentally infected with avian influenza to assess host species’ relative susceptibility to AIVs. Additionally, we summarize broad-scale parameters for elements such as shedding duration and minimum infectious dose that can be used in transmission modelling efforts. Our synthesis shows that waterfowl (i.e. Anatidae) compose the vast majority of published AIV pathobiology studies, whereas gulls and passerines are less represented in research despite evidence that they also are susceptible and contribute to highly pathogenic avian influenza disease dynamics. This study represents the first comprehensive effort to compile available literature regarding the pathobiology of AIVs in all wild birds in over a decade. This database can now serve as a tool to all researchers, providing generalized estimates of pathobiology parameters for a variety of wild avian families and an opportunity to critically examine and assess what is known and identify where further insight is needed.