Nanophyetus salmincola is a food-borne intestinal trematode parasite prevalent on the Pacific Northwest coast. The species may be the most common trematode endemic to the United States. The life cycle of the N. salmincola requires three hosts. The first intermediate host is an Juga plicifera stream snail. The second intermediate host is a salmonid fish, though some non-salmonid fishes also play a role. Lastly, the definitive host is most commonly a canid, though many other mammals are also definitive hosts, including humans. Transmission of N. salmincola to the definitive host occurs upon ingestion of parasite-infected fish. The parasite is most known for its association with "salmon poisoning disease", which, left untreated, is fatal to dogs and other canids. However, canids are affected by the Neorickettsia helminthoeca bacteria, for which N. salmincola acts as a vector, and not by the parasite itself. Very few known cases of naturally acquired human infection with N. salmincola are found in the literature, though it is likely that many cases are unreported, since most people are asymptomatic, or symptomatic with non-specific symptoms like gastrointestinal discomfort. Disease caused by N. salmincola, or nanophyetiasis, is easily preventable by thoroughly cooking fish before consumption. There are no known cases of human infection by the Rickettsia bacteria carried by N. salmincola. A subspecific parasite, Nanophyetus schikhobalowi, is endemic to Siberia, where human cases of nanophyetiasis have been reported in scientific literature since 1931.
Discovery The first record of salmon poisoning disease (SPD) was reported in northwestern Oregon in 1814 when a writer for Henry's Astoria Journal noted the death of dogs after consumption of raw salmon. At first, investigators believed that SPD was caused by poisonous blood in the ingested fish. In 1911, small white cysts were observed in the kidneys of disease-causing salmon and trout, but the cysts were mistakenly identified as amebae. Small trematodes in the intestines of dogs that died after eating infected salmon were finally found in 1925 and the cysts present in the salmon were correctly identified as intermediate stages of the trematode. In an experimental follow-up study, researchers showed that the small intestinal parasite did in fact cause SPD in dogs, and that the cysts did develop into the adult worm found in the intestine. The trematode was first named by Chapin as Nanophyes salmincola in 1926, as a member of the family Heterophyidae. 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. 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. Ultimately, Nanophyetus salmincola was agreed upon, though Troglotrema salmincola remains a synonym. In 1931, Skrjabin and Podjapolskaja describe a similar parasite, Nanophyetus schikhobalowi, which was endemic to East Siberia. Argument regarding whether or not N. schikhobalowi and N. salmincola were the same or different species recurred until 1966 when the two were granted subspecific status in order to reflect their biological and geographic differences, but little significant morphological differences. Since its discovery, N. schikhobalowi has been known to naturally infect humans and research reveals surveys indicating rates of infection in endemic Siberian villages of up to 98%. In contrast, N. salmincola was not recognized to be a source of an infection until a researcher purposefully infected himself in a scientific experiment in 1958. Besides Philip experimentally infecting himself with the North American N. salmincola, the first naturally acquired human intestinal infection cases were observed between September 1974 and October 1985. The study revealed 10 patients who presented with positive N. salmincola stool samples and either gastrointestinal complaints or otherwise unexplainable peripheral blood eosinophilia. 7 patients recalled ingestion of undercooked or raw fish. Of those who were not given effective treatment, symptoms and/or eggs in stools persisted for 2 or more months before spontaneously resolving. 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. Two years after the first 10 cases of human infection with N. salmincola were reported in 1987, Fritsche et al. reported ten additional cases of human nanophyetiasis. Five presented with gastrointestinal complaints and the other five had unexplained peripheral eosinophilia. Nine out of ten recalled eating inadequately cooked fish. This time, praziquantel was the effective treatment of choice. In 1990, the first case of human infection with N. salmincola without ingestion of raw or undercooked contaminated fish was reported. A man was infected through hand contamination while handling highly infected, fresh-killed, coho salmon. A diagnosis of nanophyetiasis was made based on gastrointestinal discomfort, peripheral blood eosinophilia and a positive stool sample. Treatment with praziquantel was effective. None of the human cases of infection with either the North American or Siberian subspecies reveal infection by the Neorickettsia helminthoeca carried within the trematode, which was discovered in 1950. Infection by rickettsia helps to explain the more fatal outcome afflicting canids.
Clinical presentation in humans Upon infection with N. salmincola, humans are normally asymptomatic. If symptoms are present, they are usually non-specific and mistaken for indication of other gastrointestinal problems. Symptoms include "diarrhea, unexplained peripheral blood eosinophilia, abdominal discomfort, nausea and vomiting, weight loss, and fatigue". Eggs of N. salmincola appear in stools approximately one week after ingestion of infected fish.
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