Nanophyetus is a common trematode parasite of fish in Oregon, infecting small and large native fish and some amphibians (salamanders); rare in introduced warmwater fishes. In internal fish tissues, the parasite forms cysts (metacercaria), which appear as small black spots. Sometimes there can be so many spots – millions of trematodes per gram – that entire organs appear black. In extremely high numbers the parasite can cause the fish to become blind, but usually it is not a serious problem. The life cycle of Nanophyetus requires both fish and snail intermediate hosts and a mammal final host. Salmon can become infected in freshwater, and the parasite can survive in the fish when they migrate to the ocean and back.

In the Pacific Northwest, Nanophyetus is commonly referred to as the “salmon poisoning fluke” because it can carry the bacterium Neorickettsia helminthoeca. This bacterium is capable of causing illness in dogs that have eaten uncooked, infected fish. Untreated, the sickness is fatal in about 40% of dogs that contract it, however it is easily treated with antibiotics. Dogs become immune to the bacterium if they survive the first infection.

Distribution: Widespread in freshwater; endemic across much of the U.S.

Detection Methods: Microscopic exam – wet mount

Hosts: Salmonid fishes, but other fish hosts possible

Target Tissue: Kidney, internal organs (site of metacercaria cysts)