Abstract

In an attempt to determine whether hymenolepidid oncospheres may overwinter and contribute to the pool of cysticercoids in the intermediate host population the following spring, we studied the survival of the oncospheres of the cestode Microsomacanthus hopkinsi following storage at 7 and 20 °C, following short-term freezing under laboratory conditions, and after overwintering (October–May) under natural conditions at depths of 0.5, 1, and 1.5 m in a local lake. Viability was tested by feeding the oncospheres to laboratory-reared amphipods (Hyalella azteca). The percentage of surviving amphipods that became infected following exposure to oncospheres stored at 7 °C declined steadily from 96% following 4 weeks storage to 43% after 24 weeks. Only 3% (2 of 60) were infected following exposure to oncospheres stored for 28 weeks. Oncospheres held overwinter under natural conditions failed to produce infections. These were slightly older (29 weeks) than the oldest oncospheres maintained at 7 °C in the laboratory, at the time of feeding to the amphipods. It appears that 28 weeks approaches the maximum survival time for the oncospheres of this species. Oncospheres subjected to short-term freezing also failed to produce infections when fed to amphipods. Oncospheres survived in sufficient numbers for up to 24 weeks to be of potential significance in the formation of infective pools in amphipods in the spring. However, there is a northern limit beyond which the inactive period of the amphipod host is too long for the oncospheres to bridge. Under these circumstances, they are of no consequence in the formation of the infective pool. Only 5% (3 of 60) of the amphipods exposed to oncospheres stored at 20 °C for 4 weeks became infected. Foci established in warmer conditions, as expected, are infective for much shorter periods.