Salmonpoisoning fluke

Nanophyetus salmincola

ORDER Plagiorchiida




Nanophyetus salmincola Chapin, 1927, dogs, Canis lestes, Procyon psora pacifica, Lynx fasciatu.


None known.


The ovate adult form of N. salmincola is about 0.03-0.08 in (0.8-2.5 mm) long and about 0.011-0.3 in (0.3-0.8 mm) wide. Two similar-sized suckers are present, one located at the anterior edge, and the other about a third of the way back on the body. The tegument is spiny. The pair of testes are large oval structures that extend from the central to the posterior body. In contrast, a small round ovary is located near the ventral sucker. A genital pore opens just behind the ventral sucker.


North Pacific basin, stretching from the Columbia River basin through British Columbia to Alaska.


N. salmincola is a fluke of freshwater streams when it is not infesting other animals. The eggs hatch in fast-moving streams, the miracidia infect snails (commonly Oxytrema silicula), and the cercariae penetrate the skin of frogs and fish, especially salmon and trout. The definitive hosts include birds and mammals, particularly skunks and raccoons, and occasionally humans. Immature and adult flukes inhabit the wall of the small intestine. Cercariae live free (not embedded) in the snail's tissues and enter the muscular tissue of fishes.


The eggs of this parasitic flatworm incubate in freshwater streams for a period of about three to seven months. The newly hatched miracidia randomly find and then burrow into snails, where they develop into cercariae. The cercariae exit the snails and move in random patterns through the water. They can survive in this free-living form for as long as two days. When they encounter a frog or fish, they quickly penetrate its tissue—a process that takes 30-120 seconds—and migrate to the muscles, kidneys and fins. The host sometimes dies from heavy infestations. Migratory fishes like salmon may carry N. salmincola many miles. Predatory birds and mammals ingest infected fish and frogs, and the flatworms encyst in the small intestine, where they mature within seven days and lay eggs. The eggs pass from the definitive host in the feces, and the life cycle begins again.

Salmon poisoning in dogs and other canids is often blamed on N. salmincola. The actual poisoning agent is a rickettsia bacteria known as Neorickettsia helminthoeca, which infects all life stages of N. salmincola. If untreated, infection by N. helminthoeca causes distemper-like symptoms and may be fatal in dogs. It does not, however, affect humans or cats. Dogs should never be fed raw salmonids.


N. salmincola is dependent on hosts to meet its nutritional needs. Its hosts include the snail Oxytrema silicula as the first intermediate host, salmonid fish as the second intermediate host, and a variety of fish-eating mammals, including dogs, cats, and humans, as definitive hosts. Metacercarial encyst in fishes. Immature flukes in dogs attach to the intestinal wall.


The unembryonated eggs of N. salmincola are yellowish brown in color, and about 0.003-0.0039 in (80-100 pm) long and 0.0015-0.0019 in (40-50 pm) wide. Incubation may take from 87-200 days at room temperature. The rediae are elongated with birth pores located toward the front. The cercariae are also elongated, but have a ventral sucker and short tail. Thousands of cercariae may be deposited at a time in long strings of mucus.



Humans who eat raw or underprocessed infected fish are susceptible to the disease called nanophyetiasis or "fish flu," but such cases are rare. The symptoms of fish flu, which arise 5-8 days after eating infected fish, may include diarrhea or abdominal discomfort. Humans are unaffected by N. ricksettia, which is carried by N. salmincola, but such an infection can be fatal to dogs if left untreated. Heavy infestations in salmon and trout may cause some losses in populations of these sport fishes. ♦

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