Nanophyetus salmincola Chapin, 1928

(Figure 2-31)

ETYMOLOGY:Nano = posterior and phyetus = genitalia along with salmincola = for the salmonid second hosts.

SYNONYMS: Chapin, 1926 first described this fluke as Nanophyes salmincola. He later (1928) changed the name to be Nanophyetus salmincola. A synonym or sister species from the Siberian coast is Nanophyetus schikhobalowi Skrjabin and Podiapolskaia, 1931. Another synonym is Distomulumoregonensis Ward and Mueller, 1926. The genus Nanophyetus is also considered by some to be synonymous with the genus Troglotrema.

HISTORY: The fluke was noted be associated with salmon-poisoning disease in dogs because salmon that did not contain metacercariae did not transmit the disease to dogs (Donham et al., 1926). The infectious agent was later identified to be Neorickettsia helminthoeca. This rickettsia is passed from dog to dog within the body of the fluke.

GEOGRAPHIC DISTRIBUTION: This trematode is found distributed around the northern Pacific Rim mainly in various species of canids. The cat does not appear to be a major host of this parasite (Schlegel et al., 1968).

LOCATION IN HOST: Small intestine.

PARASITE IDENTIFICATION: Members of the genus Nanophyetus are small flukes, about 1 mm long. There is a muscular pharynx. The genital opening is just posteriad to the ventral sucker, with the ovary being at about the same level as the ventral sucker. The testes are large and are opposite each other in the middle of the hind body.

The adult flukes may be anywhere from 0.8 mm to 2.5 mm in length. The eggs, 87 µm to 97 µm in length by 38 to 55 µm wide, are light brown in color, operculate, and are not embryonated when passed in the feces.

LIFE CYCLE: The miracidium within the egg, hatches and swims by means of its cilia. The miracidium develops into a redia within the body of the freshwater snail Oxytrema silicula. Ultimately, cercariae are produced. The cercariae are microcercous, i.e., have a very small tail, and are grouped together by strands of mucus which probably help them come into contact with the surface of a fish swimming by. The cercariae penetrate the skin of fish at the site of contact. This fish hosts utilized are members of the salmon family that become parasitized during the freshwater portion of their life cycle. After infection, the metacercariae are found to develop predominantly within the kidneys and muscles of the fish. The metacercariae are capable of persisting in the tissues of the fish for almost two years after they return to the sea, and after this period are capable of transmitting the causative agent of salmon poisoning disease to dogs. Mammals and birds become infected when they ingest raw salmon. The flukes mature into adults with eggs in 6 to 10 days after the fish has been eaten.

CLINICAL PRESENTATION AND PATHOGENESIS: Lesions in cats infected with Nanophyetussalmincola have not been described. Hoeppli (1926) examined the damage to the intestinal mucosa of the dog induced by this parasite and believed it to be highly pathogenic; however, Hoeppli’s studies were performed before the discovery of the rickettsial cause of this disease.

TREATMENT: Probably praziquantel, but not reported.

EPIZOOTIOLOGY: Cats become infected by eating raw fish. In the wilds of Oregon, many other hosts are infected (Schlegel et al., 1968). The most important hosts in the wild are probably the raccoon, coyote, lynx, spotted skunk, and even birds (e.g., the hooded merganser).

HAZARD TO OTHER ANIMALS: The rickettsia can be lethal to dogs that have ingested fish containing the rickettsial bearing trematode. If infected with the trematode, cats need to be treated to prevent the parasite from being able to complete its life cycle.

HAZARD TO HUMANS: Human beings have been found infected with this parasite in Siberia. It is also possible that the rickettsial agent may cause disease in human beings. People, like cats, obtain their infections by the ingestion of raw fish.

CONTROL/PREVENTION: The prevention of the ingestion of raw fish.


Chapin EA. 1926. A new genus and species of trematode, the probable cause of salmon-poisoning in dogs. N Am Vet 7:36-37.

Chapin EA. 1928. Note. J Parasitol 14:60.

Donham CR, Simms BT, Miller FW. 1926. So-called salmon poisoning in dogs. Progress report. JAVMA 71:215-217.

Hoeppli R. 1926. Anatomische veranderungen des Hundarms, hervorgerufen durch Nanophyes salmincola Chapin. Arch Schiffs Tropen-Hyg 30:396-399.

Schlegel MW, Knapp SE, Millemann RE. 1968. "Salmon poisoning” disease. V. Definitive hosts of the trematode vector, Nanophyetus salmincola. J Parasitol 54:770-774.

Figure 2-31. Nanophyetussalmincola from the small intestine of an experimentally infected cat collected in Washington D.C. in the 1920's. Note the two very large suckers and the large ovaries that are present on both sides of the body.