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160,000 fish were killed in Oregon hatcheries, officials say. What made them sick?

Sacramento Bee
About 160,000 rainbow trout were killed in three Oregon hatcheries after workers noticed they were getting sick, officials said.

Wildlife officials learned the fish were infected with a novel parasite, the Oregon Department of Fish and Wildlife said in a March 19 news release.

Workers decided to have the fish tested when they noticed an increase in deaths at Klamath Falls, Rock Creek and Elk River hatcheries, officials said. A fish hatchery is where fish are bred and raised in a controlled and safe setting before being released into natural waters, the U.S. Fish and Wildlife Service said.

Officials try to uncover mysterious illness in fish

At first, the trout were tested for bacteria and viruses, but nothing turned up, officials said. They were also given antibiotics, but the fish “did not respond” to the treatment.

The wildlife agency partnered with the Oregon State University’s College of Veterinary Medicine and Department of Microbiology to test the fish again.

They discovered an “unknown parasite” living in the blood vessels, gills and kidney tissue of the infected rainbow trout, officials said.

“Preliminary genetic testing places this parasite as a Myxidium species in the class Myxozoa which includes over 2,400 parasitic organisms,” officials said.

Parasite causing illness in trout for first time

The parasite typically infects a worm, which then infects a fish, and the fish eventually infects a worm again. Officials call this a two-host life cycle.

“While this parasite is thought to already exist in the Pacific Northwest region, it is the first time we have seen it associated with illness and mortality in fish in Oregon,” the agency’s senior fish pathologist Dr. Aimee Reed said in the release.

When a fish is infected with the parasite, it attacks the kidney and urinary systems, officials said. Spores are then passed into the water from the fish, which can infect the worms.

It’s not clear where the parasite came from, officials said.

Over 160,000 rainbow trout were killed to stop the parasite from spreading in Oregon waters, officials said.

“This is a disappointing loss but allowing the parasite to spread could be much worse and put even more fish at risk,” Reed said.

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