Parasite kills rainbow trout at three Oregon hatcheries

Published 8:55 am Thursday, March 21, 2024

CORVALLIS — Oregon wildlife officials have found a novel parasite in sick rainbow trout raised at three fish hatcheries, and the effects of the discovery will slightly reduce the number of trout stocked in the state this spring and summer.

The affected hatcheries are at Klamath Falls, Elk River (near Port Orford) and Rock Creek (about 23 miles east of Roseburg).

Employees from the Oregon Department of Fish and Wildlife’s fish health lab detected the parasite, the first time it was has been confirmed in Oregon, according to ODFW.

Symptoms of the parasite were seen in several raceways raising about 160,000 legal-sized rainbow trout (8 to 10 inches). ODFW hatchery staff have disposed of these trout to stop the spread of the pathogen. Because of the risk of spreading the parasite, the fish exposed cannot be safely released in Oregon.

Hatchery managers have identified approximately 104,000 surplus trout being raised at other hatcheries to cover most of what was lost. Altogether, ODFW’s hatcheries raise and release about 3.8 million rainbow trout annually.

Trout stocking across the state will be slightly decreased to make up the shortfall, minimizing the effect on any one trout stocking. ODFW will update the trout stocking schedule when more details on final stocking numbers are available.

“This is a disappointing loss but allowing the parasite to spread could be much worse and put even more fish at risk,” said Dr. Aimee Reed, aquatic veterinarian and ODFW’s senior fish pathologist. “Situations like this are why Oregon’s state hatcheries have vigorous biosecurity measures and monitoring programs.”

Late last year, employees at the fish health lab and the hatcheries noticed higher than usual numbers of rainbow trout dying at the three hatcheries. Initial tests for bacteria and viruses didn’t detect any known pathogens. Hatchery workers treated sick trout with two rounds of antibiotics, but the fish didn’t improve.

To determine what was causing the fish to get sick, ODFW began working with Oregon State University’s College of Veterinary Medicine and Department of Microbiology. Workers there identified an unknown parasite within the blood vessels, gills, and kidney tissue of the affected trout. Preliminary genetic testing places this parasite as a Myxidium species in the class Myxozoa which includes more than 2,400 parasitic organisms.

This parasite has a two-host life cycle. It first infects an invertebrate host such as an aquatic worm, then moves into the water where it infects fish, then leaves the fish to infect a worm again. Sometimes these parasites are associated with disease in the fish.

The parasite is similar to others previously identified in fish in the Pacific Northwest.

“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,” Reed said.

It’s not known where the parasite came from. Due to damage during the 2020 wildfires, the Klamath Falls and Rock Creek hatcheries aren’t able to to raise fish at their earliest life stages (the egg and alevin stage).

The rainbow trout that died were hatched at Trask Hatchery in February 2023. They were transferred to Klamath Falls in June 2023 during their fry stage (when fish are about 2 inches long) and from there to Elk River and Rock Creek in the fall of 2023.

The parasite can infect the fish’s kidney and urinary systems. Where spores can be excreted into water, they’re available to continue their life cycle in more worms and more fish.

ODFW hatcheries take many biosecurity measures to reduce the risk of bacteria, parasites or other disease-causing agents from spreading. Fish are raised in separate raceways and not comingled. Staff sanitize any equipment that comes into contact with water or fish including boots and nets. Ponds and raceways are also sanitized once a cohort of trout leave the facility.

ODFW’s fish health services conduct monthly examinations at all 33 hatchery facilities and test fish for bacteria, viruses and parasites and other illnesses.

ODFW and OSU have secured a safe research laboratory for 500 of the infected trout so they can learn more about the parasite. They will work to further identify the exact myxozoan species and determine how and when it infected the fish and caused disease.

As the changing climate results in warmer rivers and conditions that favor pathogens, ODFW’s fish health monitoring is becoming increasingly important as a first line of defense, according to a press release from the agency.

“We know that high temperatures and low flows are more inviting for parasites like myxozoans to infect fishes,” Reed said. “Climate change is causing exactly these conditions so it could be playing a role in why we are seeing this parasite now.”

ODFW is also looking at how hatchery infrastructure can be modified to adapt to the changing environment and improve fish health.