The Effect of Freezing on Encysted Anisakis Larvae

Abstract

Immature heterocheilid nematodes, usually Contracaecum or Anisakis, are commonly found encysted in commercial fish brought to the Seattle docks. Sites of predilection are the serosal surfaces of the stomach, pyloric caeca, and adjacent intestine, the ventral mesentery, and less commonly, somatic musculature in contact with these areas or along the vertebrae. The heaviest parasite load occurs in bottom fish, such as rock fish, cod, and flat fish, which may carry several hundred parasites per host. Herring and salmonids are usually more lightly infected. Criteria for specific identification of the immature nematodes in this group are not established. Anisakis adults have been reported as occurring in various marine mammals (pinnipeds and cetaceans), and occasionally from marine birds. Recently, immature stages were reported from man (Hitchcock, 1950). There is nothing to suggest that more than one species of Anisakis is present in the material from the intermediate hosts encountered in Seattle. Adult Contracaecum occur locally in marine mammals, birds, and fish (Sebastodes, cod, etc.). Since a temperature of 37? C. is lethal in vitro for most excysted Contracaecum from fish in this area, it seems likely that the common final host is a piscivorous fish. No published material dealing specifically with death by freezing has been found for this group. The only example of encysted nematode larva investigated with respect to freezing is that of Trichinella spiralis, which has been the subject of repeated testing to establish a basis for control of trichinosis. This literature has been most recently reviewed by Gould and Kaasa (1949). Discrepancies among the opinions expressed have been attributed to the type of freezer employed, the medium surrounding the sample, the size of the sample, the rate of cooling, the age of the larvae, individual differences in the larvae, and the host in which they are encysted. It seems probable from this work that trichina larvae may survive for two weeks or more at - 10? C., less than 3 days at - 20? C., and only a few hours at - 30? C. Studies to determine the effect of freezing on heterocheilids were initiated following inquiries from the Woodland Park Zoo, Seattle, where frozen herring are fed to seal, cormorants, and other potential hosts. Interest has also been expressed in this problem by animal breeders using fish scrap, and by individuals with public