Studies on the Helminth Fauna of Alaska. XXVI. Some Observations on the Cold-Resistance of Eggs of Echinococcus sibiricensis Rausch and Schiller, 1954

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Abstract
Preliminary observations showed the ova of Echinococcus sibiricensis. taken from carcasses of arctic foxes, Alopex lagopus. which had been exposed to temperatures from 5[degree]C to -40[degree] C for over 6 weeks, to be infective to voles. Microtus pennsylvanicus. Later experiments revealed that red-backed voles, Clethrionomys rutilus dawsoni. harbored alveolar larvae of E. sibiricensis after being fed ova of this cestode exposed up to 54 days at -26[degree]C. Eggs in semi-fluid intestinal content from an experimentally infected fox were placed in vials at -31[degree]C and the temperature lowered 5[degree] every 24 hours to an extreme of -51[degree] C, one vial was removed every 24 hours and its contents fed to a series of 3 voles, which were autopsied after 10 days and examined for developing larvae. As exposure time and degree of cold increased, infections in the voles became lighter, but one of 3 voles became infected after receiving material which had been exposed to -51[degree]C. Infectivity of the eggs decreased in inverse proportion to the length of time exposed and the extreme of lowered temperature to which they were subjected. Eggs frozen at -51[degree]C had fractured shells. Gravid proglottids of E. sibiricensis held at -26[degree]C in situ in arctic foxes remained infective for 42 days; the foxes had been killed about 14 days before being placed in the freezer. It is suggested that the carcass of the animal protects the eggs, insuring a longer period of infectivity. All infections in test voles progressed normally.