In a study recently published in the journal Emerging infectious diseases, Researchers in Alaska, United States of America (USA), reported a natural infection with the highly pathogenic avian influenza virus (HPAI) A(H5N1) in a young, wild polar bear found dead in Alaska in 2023. They emphasized the importance of continuous monitoring of wildlife health by the public and hunters in the Arctic.

Report: Infection with highly pathogenic avian influenza virus A(H5N1) clade 2.3.4.4b in wild polar bears, Alaska, USA. Photo credit: Alexey Seafarer / Shutterstock

background

HPAI virus A(H5N1) of clade 2.3.4.4b was first detected in Europe in October 2020 and has since frequently infected various mammal species worldwide. In North America, natural avian influenza 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 have become infected with other A(H1N1) influenza viruses. In addition, antibodies against influenza subtypes H3 and H6 indicate previous natural exposures to avian influenza viruses. Notably, seroconversion has been detected in polar bears in the Barents Sea and in brown bears in Alaska, but not in polar bears in the southern Beaufort Sea.

Interestingly, genome analysis of influenza viruses in Alaska has revealed both non-reassorted and reassorted strains. 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, fisher martens, martens, raccoons, and brown bears along the Pacific Flyway.

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

About the study

The North Slope Borough Department of Wildlife Management (NSB DWM) in Alaska conducts research into wildlife health and maintains programs to monitor community hunting of marine mammals, including polar bears. The Alaska Office of the State Veterinarian oversees wildlife surveillance for reportable infectious diseases. After HPAI H5N1 was detected in birds of prey and a red fox in April 2022, the Office of the State Veterinarian began collaborating with the NSB DWM on surveillance testing for avian influenza in birds and other wildlife.

Local residents reported finding a dead polar bear near Point Barrow, Alaska, in August 2023. An autopsy was performed. The bear was found to be a 120 cm long male cub in moderate to advanced decomposition. After macroscopic examination, tissue samples were collected and fixed in 10% buffered formalin for histopathological examination. Swab samples were also collected and stored at −50 °C. They were processed for routine histopathological examination with eosin and hematoxylin staining. The swab samples were tested for the influenza virus matrix gene using polymerase chain reaction (PCR). Immunohistochemistry was performed using a polyclonal antibody against influenza A virus. In addition, molecular confirmation and characterization of the virus genome were performed.

Results and discussion

Macroscopic examination revealed liver and lung congestion, ulcerative skin lesions, hematoma, brain swelling and congestion, and an empty stomach. The main histopathological findings were granulocytic and mononuclear meningoencephalitis with microgliosis, neuronophagia, vasculitis, neuronal necrosis, and parenchymal thinning. In addition, the researchers noted focal lipid pneumonia, pulmonary edema, and multifocal ulcerative dermatitis. Swab samples were 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 discovered in Alaska in April 2022 and was the most commonly detected genotype in the region between August and December 2023. Complete genome sequences of the virus that infects polar bears have been deposited in GenBank and GISAID (short for Global Initiative on Sharing All Influenza Data).

Diploma

In summary, the discovery of HPAI H5N1 in a polar bear indicates that the virus can jump to different mammal species. This highlights the need for vigilant surveillance of wildlife health (by both community and hunters) to detect emerging pathogens early. The successful discovery of the virus through collaboration between wildlife management departments and government veterinarians highlights the importance of integrated surveillance systems in monitoring and responding to wildlife health issues. Because polar bears and other wildlife are essential to the livelihoods and food security of indigenous communities in the Arctic, the discovery of HPAI H5N1 indicates potential zoonotic risks. This requires strategies to mitigate impacts on traditional food sources and protect public health. The findings provide valuable data on the distribution and impact of HPAI H5N1 in the Arctic region and contribute to a more comprehensive scientific understanding of influenza virus ecology and interspecies transmission dynamics.