Studies on the molluscan and fish hosts of the "salmon poisoning" fluke, Nanophyetus salmincola (Chapin) Public Deposited

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  • This study¹ was undertaken: (1) to obtain information on the distribution of the snail, Oxytrema silicula, in three coastal rivers in Oregon, and the seasonal incidence of infection in these snails and in snails from an inland stream, with the cercariae of the trematode, Nanophyetus salinincola; (2) to follow cercarial development in the snail under natural and experimental conditi is; (3) to determine the species of animals naturally infected with the metacercariae and those susceptible to experimental infection; and (4) to follow development of the metacercariae in the fish hosts. The snail was found widely distributed in the Alsea, Siletz, and Yaquina Rivers both in fresh and brackish water, Salinities in the brackish water areas were as. high as 11.2 parts per thousand. Large snails were observed in all rivers during each season of the year. The incidence of infection with N. salmincola was highest in snails with aperture diameters from 10 to 1.3 mm, and varied from 27 percent in snails from the Alsea River to 29 percent in snails from the Siletz, and Yaquina Rivers. Thirty-five percent of snails from Oak Creek, a stream near Corvallis, were found infected with cercariae of N. salmincola. There was no apparent change in the intensity of infection in the snails during the study period. Immature, but not mature, cercariae of N. salmincola were found in snails during the months of December through March. Identity of these cercariae as N. salmincola was shown by holding infected snails at room temperature for 15 days. At the end of this time 90 percent of the snails infected with N. salmincola contained mature cercariae. Mature cercariae were noted in snails during the months of April through November. The kidneys from 116 of 152 ocean-caught coho salmon, Oncorhynchus kisutch, and from 11 of 1.5 ocean-caught chinook salmon, O. tshawytscha, were found infected with 1 to 2400 metacercariae of N. salmincola. A dog fed two of these kidneys developed "salmon poisoning" disease. These results demonstrated for the first time that salinity was without any "cleansing" effect on either the trematode or the etiologic agent, though this effect was previously believed to occur. The fishes Cottus perplexus, Lampetra richardsoni, L. tridentata, and Richardsonius balteatus, and the Pacific giant salamander, Dicamptodon ensatus, all from western Oregon streams, were found naturally infected with the metacerariae of N. saimincola. This is the first report of natural infections in an animal other than a fish and in nonsalmonid fishes. Fourteen species of fishes were experimentally infected: Salmo gairdneri, S. salar, S. trutta, Salvelinus fontinalis, S. namaycush, L. richardsoni, C. perplexus, Carassius auratus, R. balteatus, Catostomus macrocheilus, Lepomis macrochirus, Gasterosteus a. aculeatus, G. a. microcephalus, and Gambusia affinis. This extends the number of salmonid and nonsalmonid fishes susceptible to experimental infection. Cysts from all five of the naturally infected animals and from 12 of the 14 experimentally infected fishes were given to hamsters by stomach tube. The identification of the parasites as N. salmincola was confirmed by recovery of adult flukes from the hamsters in all instances. except one (probably because of low dosage). Metacercariae from 1 to 106 days old from the experimentally infected fishes, and from naturally infected animals were studied. The stylet was absent in rnetacerariae approximately 50 days and older. The excretory bladder was always filled with round granules in metacercaniae 15 days and older. The diameter of the cyst and thickness of the cyst wall increased with age of the parasite.
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