The life cycle of N. salmincola requires 3 hosts: 2 intermediate hosts and 1 definite host. The eggs of N. salmincola are passed in the stool and hatch into larvae called miracidia, which penetrate and infect snails (the first intermediate host) commonly found in the Northwest. Within these snails, larvae mature into cercariae that leave the snails and infect and encyst in the flesh of salmonid fishes or non-salmonid fishes (the second intermediate host). Those infected fishes are ingested in raw or partially cooked fish by the definitive hosts, which include dogs and humans. Once in the human host, there is an incubation period of 1-15 weeks before clinical symptoms appear.
Dogs are typically infected by feeding off the remains of fresh fish or eating dead salmon on riverbanks. This trematode worm is not known to cause clinical disease in dogs, but it carries a rickettsial bacteria known as Neorickettsia helminthoeca, which is mostly known for its association with "salmon poisoning disease" in canids, a fatal disease if left untreated. However, there are no known reported cases of human infection by this rickettsial organism.
Human nanophyetiasis usually manifests 1 week after ingestion of infected fish or 1 month after handling fresh fish. Clinical manifestations are usually nonspecific and include fatigue, abdominal discomfort, nausea, vomiting with diarrhea, and weight loss. Asymptomatic infections are common. The first human case of nanophyetiasis was reported in 1956 when a researcher experimentally infected himself; this was followed by the first report of human intestinal infection with N. salmincola from eating infected salmonid fish in Washington State in 1987. Two years later, a case series of an additional 10 patients with nanophyetiasis was reported from the same geographic region. Interestingly, a biological technician was infected with N. salmincola after he necropsied more than a thousand fresh-killed coho salmon without protective gloves in Washington State. Accidental hand-to-mouth transmission occurred during this handling, and symptoms typical of nanophyetiasis started after 1 month. Additional evidence of human nanophyetiasis reported in the literature is sparse and has been lacking since 1990. It is likely to be underreported given its nonspecific symptoms and because it does not result in long-term morbidity and mortality.
Of note, another subspecies of Nanophyetus, known as Nanophyetus schikhobalowi, has reportedly been endemic in Eastern Siberia since 1931 with similar clinical features as its counterpart N. salmincola.
Peripheral eosinophilia (>500/µL) is a significant laboratory finding seen in up to 50% of the patients. Stool examination typically shows light brown eggs with a characteristic operculum at one end and a small blunt point at the other. The eggs are usually first detected in stool approximately 1 week after ingestion of infected fish.
There are as of yet no reported cases of mortality from human nanophyetiasis. All patients treated with an anti-helminth responded well and were cured. Reportedly, one-third of patients with positive stool examination were asymptomatic, which suggests that a significant proportion of infected individuals may go undiagnosed.
B66.8 – Other specified fluke infections
417627005 – Disease due to nanophyetidae
- Diphyllobothriasis – Caused by the fish tapeworm Diphyllobothrium latum. Sources of human infection are salmon and lake trout. GI symptoms are usually absent, except the adult worms sometimes protrude from the anus. Its infestation is known to be associated with vitamin B12 deficiency and anemia. No peripheral eosinophilia is noted. The diagnosis is based on the detection of ova or proglottids in the feces. Treatment of choice is a single dose of praziquantel (10-20 mg/kg).
- Anisakiasis – Caused by infection with either the herring worm (Anisakis simplex) or the cod worm (Pseudoterranova decipiens). Human infection is acquired through ingestion of raw or undercooked marine fish. There are several different clinical presentations depending on the type. Gastric anisakiasis usually presents as acute abdominal pain, nausea, and vomiting within a few minutes to hours after ingestion of infected fish. Diagnosis is confirmed by direct visualization of the parasite in the stomach during upper GI endoscopy, and treatment – removal of the worm – is performed immediately. Intestinal anisakiasis mimics symptoms of acute intestinal obstruction by mucosal and intestinal wall thickening. The larvae only survive a few days in the human intestinal tract, and so decompression with a nasogastric tube is usually the only treatment needed. Rarely, there are reported cases of extraintestinal anisakiasis causing eosinophilic granuloma in other visceral organs, usually accompanied by systemic allergic reactions such as angioedema, urticaria, and peripheral eosinophilia. Serodiagnosis could be performed by the antigen-capture ELISA test with high (near 100%) sensitivity and specificity. Albendazole (400 or 800 mg for 1-3 weeks) has been regarded as an effective therapy.
- Intestinal capillariasis – Caused by the nematode Capillaria philippinensis. Human infection occurs after ingestion of raw freshwater or brackish-water fish. It is endemic in Southeast Asia (especially the Philippines and Thailand), and sporadic cases have been reported in various regions (eg, Japan, South Korea, Taiwan, India, Egypt, Iran). Symptoms are varied, from abdominal pain and diarrhea to progressive weight loss, anorexia, cachexia, and death if left untreated. No peripheral eosinophilia is noted. Diagnosis is confirmed by the detection of eggs on stool specimens. Standard therapy is albendazole 200 mg once daily for 10 days.
- Paragonimiasis – Caused by infection with Paragonimus lung flukes. Paragonimus westermani is the most common species of human infection in Asia. The source of infection is freshwater crabs. The larvae migrate through the peritoneal cavity and diaphragm to the pleural space and finally reach the lung parenchyma, where solid worm cysts are formed. GI symptoms are usually absent. Typical manifestations are fever, chronic cough, and hemoptysis. Diagnosis is confirmed by chest radiographic findings of nodular or cavitary lesions, along with detection of ova in the sputum, stool, or gastric aspirates. Patients are usually treated with high-dose praziquantel 75 mg/kg/day for 3 days.
- Gnathostomiasis – Gnathostoma spinigerum is the major causal nematode of the disease in Asia, Latin America, and Africa. Infection occurs due to ingestion of undercooked freshwater fish, frogs, or snakes. Gnathostomiasis typically manifests as migratory skin lesions and peripheral eosinophilia. GI symptoms are typically absent. When larvae migrate to the central nervous system (CNS), fatal eosinophilic meningoencephalitis may result. For cutaneous lesions, either albendazole (400 mg daily or twice daily for 21 days) or ivermectin (0.2 mg/kg once) is effective. A combination of albendazole and corticosteroid therapy is recommended in patients with CNS disease.
- Angiostrongyliasis – Human infection is caused by the nematode Angiostrongylus cantonensis and occurs after ingestion of raw shellfish, snails, or slugs. The majority of human cases are reported in the South Pacific islands and Southeast Asia. It is another important cause of eosinophilic meningoencephalitis in endemic regions. Its treatment is controversial, and some authors recommend a combined albendazole and corticosteroid therapy.