Thursday March 21, 2024

Oregon Department of Fish and Wildlife

ODFW’s fish health lab has detected a novel parasite in sick rainbow trout that were raised at Klamath Falls, Rock Creek and Elk River hatcheries. The detection marks the first time this parasite has been associated with disease symptoms in Oregon’s fish. 

Late last year, ODFW fish health and hatchery staff noticed increased mortality in several raceways of rainbow trout at the three hatcheries. Staff from ODFW’s fish health lab tested the fish for bacteria and viruses but did not detect any known pathogens. Hatchery staff treated the trout with two rounds of antibiotics and other supportive care but the fish did not respond.   

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. They 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 over 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.    

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