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Impact of Avian Influenza Outbreak on Caspian Terns in Washington State

A recent epidemiological study revealed a harrowing statistic: 56% of a substantial breeding colony of Caspian terns succumbed to a 2023 outbreak of highly pathogenic avian influenza on Rat Island, located in Washington state. This alarming mortality rate has raised critical concerns regarding the future of the Pacific-coast population of these birds, which was already on the decline prior to the outbreak.

A collaborative research effort involving the Washington Department of Fish and Wildlife (WDFW) and Washington State University (WSU) not only examined the impact of the outbreak on the terns but also documented a notable transmission of the avian flu virus H5N1 to harbor seals, marking a first for the northeastern Pacific region.

While there has not been another major wildlife outbreak of H5N1 in coastal areas since this incident, researchers estimate that the overall Caspian tern population in the Pacific flyway has decreased by approximately 10-14% due to H5N1-related infections.

“The outbreak of avian influenza among Caspian terns was unprecedented in the marine environment of Washington, leading to considerable mortality among a species already experiencing decline across this region,” stated Katherine Haman, a wildlife veterinarian at WDFW, who served as the lead author of the study published in the journal Frontiers in Veterinary Science.

Caspian terns are a widespread species in North America. Until recently, their largest breeding colony was situated on an island in the lower Columbia River, downstream from Portland. Due to the terns preying on young endangered salmon, conservation efforts had previously discouraged nesting in that area. This likely led some of the large population to relocate to Rat Island, near Fort Flagler Park in Jefferson County, where the avian flu outbreak transpired in 2023.

Haman acknowledged the critical role played by citizen volunteer docents from Friends of Fort Flagler and local kayaking guides, who were among the first to notice unusual bird deaths on Rat Island. Their early reports prompted a swift response from officials, enabling researchers to gather deceased birds and attend to sick ones effectively. The outbreak ultimately led to the death of 1,101 adult terns and 520 chicks.

Additionally, the researchers recorded 15 deceased harbor seals in the vicinity—a significant rise from the typical annual reports of one or two seal deaths. Through tissue analysis, scientists at the Washington Animal Disease Diagnostic Laboratory (WADDL) successfully identified H5N1 in the terns, although confirming the presence of the virus in the seals proved more complex.

“Avian influenza is commonly considered a respiratory ailment; however, initial respiratory samples from the seals came back negative for H5N1, which raised questions due to the high mortality observed among them alongside sick birds,” noted Kevin Snekvik, a veterinary pathologist at WSU and executive director at WADDL.

Subsequent examinations of other organ tissues demonstrated that the avian flu affected harbor seals differently, triggering an inflammatory response in their brains. Whole genome sequencing of the virus indicated that the seals likely contracted H5N1 through interaction with the terns.

Globally, avian influenza has led to significant deaths among seals and sea lions in various regions, particularly in South America. However, similar widespread impacts have not yet been reported in the Pacific Northwest. The consequences for the region’s terns have proven much more severe. Currently, there are no available vaccines or treatments for wildlife afflicted by avian flu in the U.S., complicating efforts to manage the disease due to its rapid transmission and the challenges of handling wild populations. Researchers are now focused on tracking the disease’s spread and determining its effects on wild animal populations.

According to the Centers for Disease Control and Prevention, human infections with avian flu in Washington and other states have predominantly occurred among agricultural workers with close proximity to infected domesticated animals.

Researchers urge the public not to approach sick or deceased wildlife but to report their observations through the WDFW website.

This research was supported by various entities, including the Washington state legislature, the Centers for Disease Control and Prevention, the National Oceanic and Atmospheric Association, the Washington Department of Health, WDFW, and WADDL. The collaborative effort involved multiple organizations, enhancing the investigation’s effectiveness.

“The success of our research highlights the importance of inter-organizational cooperation in wildlife studies,” remarked Tom Waltzek, a WADDL researcher and member of WSU’s College of Veterinary Medicine. “Coordination across varied entities is absolutely essential for wildlife investigations.”

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