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Disease in Fresh Water


Copepods are common in fish and found throughout Oregon. Usually there are few parasites present on fish and they go unnoticed, however occasionally they can become numerous and quite evident on gills, in the mouth and at the base of the fish’s fins. When present in extremely high numbers they can cause a fish health problem but this is rarely seen in the wild. The parasite poses no risk to humans or domestic animals.

Fish Health Fact Sheet - Fresh Water Copepod (pdf)

Infectious Hematopoietic Necrosis Virus (IHNV)

Infectious Hematopoietic Necrosis is a viral disease that infects salmon and trout of all age classes, but severe losses primarily occur in young fish. The virus attacks the blood forming tissues of the fish, the kidney and spleen, and currently there is no treatment or cure for the disease. IHNV occurs in Europe and Japan as well as North America and has been isolated from some marine fish species. Humans and other terrestrial animals are not affected but can spread the virus inadvertently if infected fish or fish byproducts are introduced to new areas.

Fish Health Fact Sheet - Infectious Hematopoietic Necrosis Virus (IHNV) (pdf)

Tapioca Disease

Henneguya is commonly referred to as tapioca disease because of the large tapioca-like cysts that it creates in the muscle tissue of fish. The parasite life cycle has not been described however it probably involves an annelid worm as a secondary host. Fish do not seem to be adversely affected by the presence of the parasite. Although aesthetically the flesh may be less than desirable the parasite presents no risk to humans or domestic animals.

Fish Health Fact Sheet - Tapioca Disease (pdf)

Viral Hemorrhagic Septicemia Virus (VHSV)

Viral Hemorrhagic Septicemia is a viral disease that infects salmon and trout in Europe, Japan, and North America. Fish from both freshwater and marine environments can become infected, and at least 50 species are known to be susceptible to the virus. The virus does not affect humans.

Fish Health Fact Sheet - Viral Hemorrhagic Septicemia Virus (VHSV) (pdf)

West Nile Virus

West Nile virus is carried by mosquitoes; it can infect humans, horses, and birds. Humans get the virus from the bite of an infected mosquito. There is no evidence that the disease can spread from other animals to humans or from person to person. Most infections are mild, causing fever and flu-like symptoms, but severe infections may result in encephalitis (inflammation of the brain), and, rarely, death.

More about West Nile Virus

Whirling Disease

Whirling disease (WD) affects trout and salmon and is caused by the microscopic parasite, Myxobolus cerebralis. The parasite attacks the cartilage of the head and spine. Fish with low levels of spores may not show any visible symptoms of the disease. If sufficiently affected by the parasite, young fish may become more susceptible to predation and less able to feed and survive disturbances in the environment. Where infection is severe in young fish, they may "whirl" when startled due to pressure on the nervous system from inflammation around the damaged cartilage areas or develop "black tail." Biologists have not observed these symptoms in fish in Oregon streams. The long-term effects in wild fish populations are receiving increased study among scientists.

More about Whirling Disease


Database of pathogens of Oregon fishes


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