Thermotactic Response of Spirometra Plerocercoids

It appears that low temperatures are well tolerated by spargana whereas higher temperatures are unfavorable, and the direction of migration of the worms in the body of the host suggests that their performance in a thermal gradient might differ from what has previously been observed with other species.

Abstract

Migration of spirometrid spargana in warm-blooded hosts (mouse, man) suggests they may react negatively to a thermal gradient, in contrast to the observed behavior of other parasitic worms. A test of this hypothesis showed that anterior fragments containing the scolex, or whole worms, placed in such a gradient, react positively and migrate into the zone of thermal damage and death. Fragments of worms lacking the scolex remain active but fail to show linear progression. Earlier studies (McCue and Thorson, 1964) gave evidence for the presence of a positive thermotaxis in a wide range of nematode, cestode, trematode, and acanthocephalan parasitic helminths. The response was faulty in that the organisms did not seek an optimum temperature but rather migrated in the gradient until thermal damage occurred. Human cases of sparganosis are usually first recognized by the appearance of an itching nodule or tumor under the skin which on incision or removal proves to contain the worm. When plerocercoid larvae (spargana) of Spirometra mansonoides are fed to mice, they tend to wander outward in the host as though seeking a cooler environment, frequently coming to rest under the skin. Whereas, when scoleces are injected under the skin of the mouse to begin with, they show little tendency to migrate and usually proceed to develop in situ (Mueller, 1961). When injected into the tissues or body cavity of rabbits or birds (pigeon, chicken), spargana survive for a time but in a few weeks or months disappear and are absorbed (unpublished observations by Mueller), unlike their behavior in mice or man in which they survive and grow indefinitely. Both birds and rabbits have a somewhat higher temperature than man, whereas that of the mouse is approximately the same (Spector, 1956). Spargana show increasing activity with rise in temperature up to 116 F (1 min exposures in physiological saline). At 118 F their movements slow down, and activity ceases from 120 to 126 F, but recovery occurs on removal to room temperature. A temperature of 129 F causes irreversible damage and death. Spirometrid spargana infect, and thrive in, a number of poikilothermic hosts: tadpoles, certain frogs, water snakes, the alligator, etc. (Mueller, 1951) and withstand hibernation in these animals or storage in suitable media at 4 C for periods of several weeks. Thus it appears that low temperatures are well tolerated by spargana whereas higher temperatures are unfavorable, and the direction of migration of the worms in the body of the host suggests that their performance in a thermal gradient might differ from what has previously been observed with other species. MATERIALS AND METHODS Worms were removed from infected mice and placed in the gradient as soon as possible after removal. The spargana were approximately 20 cm long, and since the track was only 25 cm long, 1-cm segments containing the anterior end (larval scolex) were used for most tests. Several whole worms were also used but in no instance did their migration differ from that of the 1-cm segments. Although the initial rate of movement was slower, by the time the 1-cm segments had reached the end point the whole spargana had done likewise. The temperature gradient was set from 22 to 45 C as described by McCue and Thorson (loc. cit.). Locke's solution was used as the medium in the migration chamber. Received for publication 15 April 1964. * This investigation has been supported by PHS Grants Al-01876 and AI-03049.