- Fish Flu -



Contents on this page:

What is it?


History of Discovery

Clinical Presentation





Life Cycle




Public Health and Prevention Strategies

Useful Web Links





What is It?

Nanophyetiasis is the name of the disease caused by ingestion of the parasitic worm species Nanophyetus in humans.  It has been labeled as the Fish Flu by several media outlets because of its usual method of transmission through infected fish.  Nanophyetus is also of interest because it both the cause of disease and also a vector for another disease – Neorickettsia helminthocea, or salmon poisoning diesease – caused by a rickettsia bacteria carried by the worm.  This disease affects dogs with 80% mortality if left untreated.  Neorickettsia has not been proven to be transmissible to humans.


Pictures of Nanophyetus


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The agent of transmission of the disease is the North American or Russian troglotremtoid trematode (or fluke) of the Phylum Platyhelminthes, Order Digenea – Nanophyetus salmincola in North America and Nanophyetus schikhobalowi in Eastern Siberia respectively. 


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History of Discovery:

The disease was first noticed in the mid-19th century when it was noticed that dogs which were fed a diet of raw salmon soon died.  It was later discovered that the fish were infected with the worm.  It was first identified as a parasitic worm in humans in 1931 and studied in detail in the U.S.S.R. beginning in 1959. 


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Clinical Presentation:

Patients have presented with gastrointestinal complaints and unexplainable eosinophilia.  Typical symptoms include:

*    Increased frequency of bowel movement or diarrhea

*    Abdominal discomfort

*    Nausea and vomiting

*    Weight loss

*    Fatigue

Most patients recalled eating fish prior to onset of symptoms.  Patients however can also be asymptomatic.


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Transmission is through ingestion of the metacercaria larval stage of the worm encysted in the flesh of raw or undercooked fish.


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The primary hosts are raccoons, spotted skunks, and minks.  Secondary hosts are species which feed on the infected fish, including dogs, fish-eating birds and humans.  Nanophyetus also has two intermediary hosts: the Oxytrema stream snail and salmonid fish. 


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The vector of transmission to humans is freshwater and anadromous fish.  So far salmon and steelhead trout have been implicated in transmission. 


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Small elongated flukes 2 X .5 mm in size, with a cirrus sac dorsal to the acetabulum but no cirrus.  Vitelline follicles are prominent and the uterus typically contains about a half-dozen eggs.  The eggs are a distinctive yellowish-brown with indistinct operculum.


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Life Cycle:

Nanophyetus has a fairly standard life cycle.  Unembryonated eggs are shed by the hosts.  After a long incubation period (87-200 days), the eggs hatch and the redia stage is taken up by snails.  Cercariae larvae then burrow into the fish and enter the mercercariae stage, which then form cysts.  Cysts are distributed primarily in the kidneys, muscles, and fins.  The cysts themselves are able to survive for long periods of time out of freshwater as the fish migrate from streams to the ocean and back.  Once in the human host, there is an incubation period from 1 to 15 weeks before symptoms appear.  


Diagram of Nanophyetus spp. life cycle


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Detection of eggs in stool through direct wet mounts, formalin-ehtyl acetate concentration, and trichrome (Gomori’s or Wheatly-modified) staining and examination under microscope.  The worm itself can be identified from other flatworms by morphological characteristics including observation of the uterus, which has only a few loops and fewer than a half dozen eggs at time. 


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Antihelminth drugs have proven to be effective in curing the disease.  The usual prescription is 2 doses of bothionol or 3 doses of niclosamide (Mebendazole is ineffective in controlling Nanophyetus).  Fritsche et al demonstrated that praziquantel was effective in curing the disease (1989).  Dosage is 3 x 20 mg/kg.


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Essentially the distribution of the Nanophyetus is the same as the range of the intermediate hosts.  It is endemic to North America, particularly Washington, Oregon, Northern California, the Pacific Northwest around the Columbia River Basin and tributaries, British Columbia, and Alaska.  It is also endemic to Eastern Siberia.  There have been few reported cases in the US.  Whether this is because the disease is not very prevalent in the US or because it has been severely underreported is not certain.  However in endemic areas of Russia, reported infection rates in some Siberian villages are 95-98% and spreading.  


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Public Health and Prevention Strategies:

The only realistic prevention strategy is to insure freshwater fish are properly cooked before ingestion.  Detection of the flukes in fish is difficult.  Careful examination of fish under a dissection microscope using candling will only detect heavily infected fish.


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Useful Weblinks:

U.S. Food and Drug Administration Center for Food Safety & Applied Nutrition, Foodborne Pathogenic Microorganisms and Natural Toxin’s Handbook.


Agricola Articles




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Demaree, Richard, Grieve, Robert, Marquardt, William.  Parasitology Vector Biology 2nd Ed.  Harcourt Academic Press, New York, NY, 2000.


Eastburn, Richard L., Fritsche, Thomas R., Wiggins, Lloyd H., Terhune, Charles A., Jr.  Praziquantel for Treatment of Human Nanophyetus salmincola (Troglotrema salmincola) Infection.  Journal of Infectious Disease 1989; Nov 160(5); 896-9.


Eastburn, Richard L., Fritsche, Thomas R., Terhune, Charles A., Jr.  Human Intestinal Infection with Nanophyetus salmincola from Salmonid Fishes.  American Journal of Tropical Medical Hygiene 1987; May 36(3); 586-91.


Mehlhorn, Heinz (Ed.).  Encyclopedic Reference of Parasitology: Biology Structure Function 2nd Ed.  Springer-Verlag, New York, NY, 2001.


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