Infectivity of Trichinella from the Temperate and Arctic Zones of North America

Abstract

Infectivity of Trichinella spiralis from the polar bear and from hogs has been tested in a number of host species. Rats, hamsters, and mice are relatively refractory to the polar bear form but are readily infected with the form in hogs. Cats, dogs, and deer mice seem to be readily infected with trichinella from either source. It has sometimes been tacitly assumed that trichinella occurring in arctic mammals (Rausch et al., 1956; Rausch, 1962) is specifically identical with Trichinella spiralis occurring in various mammals in temperate regions. Ten years ago we found that trichinella from polar bears (Thalarctos maritimus) and from hogs reared in the southern U. S. showed very different capacities to infect laboratory hosts. These observations were briefly mentioned by Chandler and Read (1961). Nelson and Mukundi (1963) found that a strain of trichinella from Kenya showed low infectivity for rats and domestic pigs. Kozar and Kozar (1965) reported differences in infectivity and in pathogenicity of trichinella from Kenya and from Poland. Nelson et al. (1965, 1966) found that an Alaskan strain of trichinella showed low infectivity for laboratory rats. Although we have not been able to carry out further studies on trichinella from arctic hosts, it seems desirable to publish our observations. MATERIALS AND METHODS Diaphragm muscle from polar bears (Thalarctos maritimus) collected near Wainwright, Alaska, was the source of the Arctic trichinella, while skeletal muscle from a hog killed on a farm near Houston, Texas, was the source of the U. S. strain. To infect hosts, muscle containing trichinella cysts was finely minced and digested with artificial gastric juice at 37 C with stirring. After 2 hr, the partially digested slurry was briefly centrifuged and the supernatant fluid discarded. The sediment was washed once with Ringer's solution, suspended in 50 ml of Ringer's and an estimate made of the number of trichinella in the suspension. Additional Ringer's solution was added to give the desired concentration and hosts were given a measured dose of the worms by stomach tube. In the case of the polar bear worm, hosts were fed 100 to 500 cysts and, in the case of the hog strain, 200 to 300. Host muscle was examined 50 to 70 days after infection, using a muscle press and a comReceived for publication 1 October 1968. 72 pound microscope. If trichinella was not detected, the skeletal muscle of each animal was removed and digested overnight. The sediment from the digestion was examined with the microscope. RESULTS AND DISCUSSION The results of the experiments are summarized in Table I. The rat, hamster, and mouse appear to be relatively refractory to the arctic trichinella but are highly susceptible to T. spiralis from hogs. Cats, dogs, and deer mice (Peromyscus leucopus) seem to be quite susceptible to both the polar bear and hog strains. Unfortunately, facilities did not permit testing of the polar bear form in hogs nor of the hog form in polar bears. These results give no information on the possible mechanism of refractoriness of certain hosts to establishment of the polar bear trichinella. It might involve failure of development of adult worms, a cellular reaction of the type occurring with T. spiralis in the Chinese hamster (Ritterson, 1959), or some other mechanism. There are sufficient data in the literature to conclude that humans are susceptible to trichinella from hogs in the Temperate Zone and from polar bears in the Arctic (Rausch, 1962; Abs and Schmidt, 1954; Kagan, 1959; and others), but it cannot be assumed that humans are equally susceptible to both. The apparent differences in infectivity of polar bear trichinella and hog trichinella for laboratory animals seem to raise the question: Do the Arctic and Temperate Zone forms represent a single species? Comparative study of the morphology of the adults would be highly desirable. LITERATURE CITED ABs, O., AND H. W. SCHMIDT. 1954. Die Arktische Trichinose und ihr Verbreitungweg. Norsk Polarinstitutt Skrifter No. 105, Oslo. CHANDLER, A. C., AND C. P. READ. 196