Bird flu virus detected in Alaskan polar bear

In a recent study published in the journal Emerging Infectious Diseases, researchers in Alaska, United States of America (US) reported a natural infection of a highly pathogenic avian influenza (HPAI) A(H5N1) or bird flu virus in a young, free-ranging polar bear that was found dead in Alaska in 2023. They highlighted the significance of continued wildlife health surveillance by the community as well as by hunters in the Arctic.

Dispatch: Highly Pathogenic Avian Influenza Virus A(H5N1) Clade 2.3.4.4b Infection in Free-Ranging Polar Bear, Alaska, USA. Image Credit: Alexey Seafarer / Shutterstock

Background

First detected in October 2020 in Europe, the HPAI A(H5N1) clade 2.3.4.4b virus has since often infected various mammal species globally ever since. In North America, natural bird flu infections have been observed in several bear species, including Asiatic black bears, American black bears, grizzly bears, and Kodiak brown bears. Animals in captivity, including sloth bears, Asiatic black bears, and giant pandas, have been reported to contract other influenza A(H1N1) viruses. Additionally, antibodies against H3 and H6 influenza subtypes suggest previous natural exposures to bird flu viruses. Notably, seroconversion has been noted in Barent Sea polar bears and Alaskan brown bears but not in the southern Beaufort Sea polar bears.

Interestingly, genome analysis of Alaskan wildlife influenza viruses has both unreassorted and reassorted strains. The HPAI virus genotype A3 was likely introduced into Alaska via the East Asia–Australia Flyway in November 2021. Since then, it has been detected in various wild birds, including California condors in Arizona, and mammals such as red foxes, fishers, martens, raccoons, and brown bears along the Pacific Flyway.

Polar bears (Ursus maritimus) are classified as a threatened species under the US Endangered Species Act. Understanding their health threats is crucial for conservation efforts and developing effective management strategies. In the present study, researchers report the case of a male polar bear naturally infected with the HPAI H5N1 virus that succumbed to the infection.

About the study

The North Slope Borough Department of Wildlife Management (NSB DWM) in Alaska researches wildlife health and maintains programs for monitoring the harvest of marine mammals, including polar bears, by the community. The Alaska Office of the State Veterinarian oversees wildlife surveillance for the presence of notifiable infectious diseases. After identifying HPAI H5N1 in birds of prey and a red fox in April 2022, the Office of the State Veterinarian began collaborating with NSB DWM for surveillance testing of avian influenza in birds and other wildlife.

Local residents reported finding a dead polar bear near Point Barrow, Alaska, in August 2023. A postmortem examination was conducted. The bear was found to be a 120 cm-long juvenile male in a condition of moderate to advanced decomposition. After a gross examination, tissue samples were collected and fixed in 10% buffered formalin for histopathologic examination. Swab samples were also collected and stored at −50°C. They were processed for routine histopathologic examination with eosin and hematoxylin staining. Swab specimens were tested for the influenza virus matrix gene by polymerase chain reaction (PCR). Immunohistochemistry was performed using an influenza A virus polyclonal antibody. Further, molecular confirmation and virus genome characterization were carried out.

Results and discussion

Gross examination revealed liver and lung congestion, ulcerative skin lesions, sanguinal effusion, cerebral swelling and congestion, and an empty stomach. The major histopathologic findings were granulocytic and mononuclear meningoencephalitis with microgliosis, neuronophagia, vasculitis, neuronal necrosis, and parenchymal rarefaction. Additionally, the researchers identified focal lipid pneumonia, pulmonary edema, and multifocal ulcerative dermatitis. Swab specimens were found to be negative for the influenza virus matrix gene. Despite negative PCR tests, immunohistochemistry detected influenza A virus antigen in neurons and microglial cells. Molecular analysis identified HPAI virus genotype A3, a Eurasian influenza virus. The virus was first detected in Alaska in April 2022 and was the most detected genotype in the region during August–December 2023. Full genome sequences for the virus infecting the polar bear were deposited in GenBank and GISAID (short for global initiative on sharing all influenza data).

Conclusion

In conclusion, the discovery of HPAI H5N1 in a polar bear indicates the virus's ability to spill over into various mammalian species, emphasizing the need for vigilant monitoring (both community- and hunter-based) of wildlife health to detect emerging pathogens early. The successful detection of the virus through collaborative efforts between wildlife management departments and state veterinarians highlights the importance of integrated surveillance systems in monitoring and responding to wildlife health issues. Since polar bears and other wildlife are integral to the subsistence lifestyle and food security of Indigenous communities in the Arctic, detecting HPAI H5N1 highlights potential zoonotic risks. This calls for strategies to mitigate the impact on traditional food sources and safeguard public health. The findings provide valuable data on the spread and impact of HPAI H5N1 in the Arctic region, contributing to the broader scientific understanding of influenza virus ecology and cross-species transmission dynamics.