Nanophyetus salmincola is a food-borne intestinal trematode parasite prevalent on the Pacific Northwest coast.

A subspecific parasite, Nanophyetus schikhobalowi, is endemic to Siberia, where human cases of nanophyetiasis have been reported in scientific literature since 1931.

[9] Upon further examination of the morphology, Chapin reassigned the trematode to the family Troglotrematidae and renamed the parasite Nanophyetus salmincola, since Nanophyes was already taken.

[10] Discussions regarding the correctness of classification of the parasite continued as the trematode received further scientific attention and its morphology and behavior was further scrutinized.

In 1931, Skrjabin and Podjapolskaja describe a similar parasite, Nanophyetus schikhobalowi, which was endemic to East Siberia.

[2] The study revealed 10 patients who presented with positive N. salmincola stool samples and either gastrointestinal complaints or otherwise unexplainable peripheral blood eosinophilia.

It was hypothesized that the movement, attachment, and irritation of the adult worms in the small intestine mucosa was the likely cause of gastrointestinal symptoms and peripheral eosinophilia.

A diagnosis of nanophyetiasis was made based on gastrointestinal discomfort, peripheral blood esoinophilia and a positive stool sample.

Symptoms include "diarrhea, unexplained peripheral blood eosinophilia, abdominal discomfort, nausea and vomiting, weight loss, and fatigue".

Nanophyetus salmincola is transmitted most commonly by the ingestion of raw, undercooked, or smoked salmon or steelhead trout.

Usually this is meant to be ingestion of the muscle of the fish but there have been cases reported in which the suspected agent of transmission was steelhead roe.

In a case in 1990, nanophyetiasis was diagnosed in an individual who is thought to have acquired the disease by simple handling of fresh-killed salmon.

For example, raccoons naturally spread N. salmincola because they frequently eat fish and defecate parasitic eggs in or near the water, where subsequent larval stages can continue their life cycle.

Fish that act as second intermediate hosts are different species of the families Salmonidae, Cottidae, and Cyprinidae.

In particular, salmonid fish of the genera Salmo, Oncorhynchus, and Salvelinus play a significant role in the N. salmincola life cycle.

[12] After ingestion of fish infected with N. salmincola, it takes about 1 week for symptoms to occur, namely for eggs to be detected in the stool.

[11] The oral and ventral suckers are used to grasp and crawl actively about the intestinal tissue of its host, though the worm leaves no extensive mechanical damage.

[12] Rediae and cercariae are found in all tissues of the host snail, but primarily in the gonads and the digestive gland.,[5][12] Rediae destroy the gonads, invade the hepatopancreas, damage it by 1) increased pressure from rapid growth, 2) active ingestion by the parasites, and 3) the disposition of parasitic wastes.

Once cercariae are shed from the snail, it contracts repeatedly until it contacts a fish and penetrates under its skin within 30 seconds to 2 minutes.

The cercariae penetrate further into the renal portal blood system, into the kidney and deeper tissues into the base of the tail.

Penetrations sights are easily visible, as the skin, fins, and tails of the fish appear to be heavily eroded and damaged.

The infection of snails is high in comparison to the number of cercariae it sheds, since larval development continues slowly over a long period of time.

[12] Evidence of mixed infection varied between studies, but snails with large numbers of N. salmincola were not parasitized by other trematodes.

Experiments do show that the bacteria lead to necrosis of lymph follicles, ulceration, and severe hemorrhage in its host.

Praziquantel causes immobilized contraction of the worm, such that it can no longer grasp the intestinal walls, and can be eliminated from the body.

If diarrhea recurs, general supplements may also be needed in order to maintain electrolyte balance and meet nutritional requirements.

Stream snails are found west of the Cascade Mountains in Oregon, north to the Olympic Peninsula in Washington, and in part of northern California.