Influence of temperature on the infectivity of eggs of Echinococcus granulosus in laboratory rodents.

Abstract

A comparison was made of the susceptibilities of Mongolian jirds (Meriones unguiculatus) and CF1 mice to infection with embryos of Echinococcus granulosus administered either intraperitoneally or orally. Optimal infectivity was obtained in both hosts using a dose of 1,000 eggs by the intraperitoneal route. This procedure was subsequently used in a series of experiments on the effects of exposure to subfreezing temperatures or moist heat on the survival and infectivity of eggs of E. granulosus. Eggs were found to survive for 24 hr at -30 C with no detectable effect on infectivity. One animal became infected after receiving eggs exposed to -50 C for 24 hr. No infections developed in animals inoculated with eggs subjected to -70 C. After 5 min incubation at 55 C the infectivity was significantly reduced, and incubation for an identical period of time at higher temperatures was sufficient to inhibit cystic development completely. These findings are discussed in relation to available information on the survival of taeniid eggs exposed to extremes of temperature. The influence of exposure to low temperatures on the survival of taeniid eggs has been investigated by a number of workers both in the field and in the laboratory. Jepsen and Roth (1949) showed that some eggs of Taenia saginata retained their infectivity for cattle after 23 weeks of exposure to winter temperatures on pastures in Denmark, and Sweatman and Williams (1963) demonstrated that eggs of Echinococcus granulosus were still infective for sheep after 4 winter months on grass plots in New Zealand. Gemmell (1969) found that a small proportion of embryos of Taenia hydatigena and Taenia ovis which had been exposed to -15 C for 24 hr produced viable cysts in lambs. Resistance to the effects of freezing was also shown by Schiller (1955) who was able to infect redbacked voles with eggs of E. multilocularis maintained at -26 C for 54 days, or at -51 C for 24 hr. Few data are available on the capacity of taeniid eggs to survive elevated temperatures. Nosik (1952) found that eggs of E. granulosus in water were no longer infective for sheep after 1 hr at 50 C, or 20 sec at 100 C, and Williams (1963) showed that eggs of T. pisiformis were no longer able to infect rabbits after immersion for 5 sec in water at 100 C. Meymarian and Schwabe (1962) were unable to activate eggs of E. granulosus in vitro after exposure to moist heat at 60 C for 10 min, 70 C for 5 min, or 100 C for 1 min. Received for publication 7 January 1971. There is a need for more precise definition of the factors which influence the survival of eggs of E. granulosus (WHO Expert Committee on Hydatidosis, 1968). This information would be of immediate use in the assessment of environmental contamination under a variety of climatic conditions and in the establishment of suitable handling and sterilization procedures for laboratory work. The extrapolation of data obtained for other taeniid species to E. granulosus is hazardous, but the assessment of survival and infectivity of this parasite is problematical. Many authors have used direct observation of the movement of embryos in artificial digestive media (Meymarian, 1961; Pamell, 1965; Laws 1967, 1968). However, activation has not been shown to be an indicator of infectivity, and embryos which do not activate may still be infective for laboratory rodents (Williams and Colli, 1970). The use of oral infections in the natural hosts (sheep or swine) has been largely precluded as an indicator of infectivity due to the high cost and extended duration of such experiments, the requirement for large numbers of cestode-free animals, and the extreme variability of infections produced by a standard dose of eggs. In the present study, the infectivity of eggs of E. granulosus for laboratory rodents was established, using both the intraperitoneal and oral routes of administration. Optimal conditions for manifestation of infectivity were chosen and the influence of exposure to sub-