Washington bird flu outbreak spread to seals, study finds

An outbreak of bird flu may have had lasting effects on the Caspian tern population on the Washington coast.

A study conducted by the Washington Department of Fish and Wildlife and researchers from Washington State University found that 56% of a large breeding colony died from a 2023 outbreak on Rat Island, and no birds have successfully bred there since.

“This Caspian tern event was the first big marine environment avian flu outbreak for Washington,” said Katherine Haman, a wildlife veterinarian for WDFW and lead author of the study in the journal Frontiers in Veterinary Science. “It caused significant, punctuated mortality for the Caspian terns, which were already a species in decline throughout this flyway.”

Researchers also found that the virus was transmitted to harbor seals for the first time in the northeastern Pacific. They found 15 dead harbor seals in the area which usually only sees one to two annually. Through tissue samples, WSU researchers at the Washington Animal Disease Diagnostic Laboratory (WADDL) were able to first identify H5N1 in the birds, but the tests in seals were harder to confirm.

“We tend to think of avian influenza as a respiratory disease, but the seal respiratory samples were initially negative for H5N1,” said co-author Kevin Snekvik, a WSU veterinary pathologist and WADDL executive director. “That seemed odd because there was a relatively high mortality in the seals, and there were also birds that were sick at the same time.”

Further testing in other organs revealed that the disease had a different pathology in the harbor seals causing an inflammatory response in their brains. The team performed a whole genome sequencing of the virus confirming that the seals likely contracted the virus from the terns.

There is no vaccine or treatment yet available in the U.S. for animals impacted by the avian flu. It is difficult to control in wildlife given the rapid spread and the difficulty associated with the capture and handling of wild animals. At this stage, researchers are trying to gain insights into the spread of the disease in wild populations by tracking the spread and understanding the impact.

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