Meeting Abstract
I examined whether larval cysts of the trematode Ribeiroia ondatrae can survive within freeze-tolerant wood frogs (Lithobates sylvaticus) and whether survival was dependent on host physiological adaptations to freezing. I exposed 107 wood frog tadpoles from interior Alaska to 30 R. ondatrae cercariae each. Metamorphosed frogs were divided into three groups: unfrozen control, single freezing event, or repeated freeze-thaw. For all groups, parasites were considered alive at experiment end if motility was observed either within cysts or after excystment. Control wood frogs (n=22) were held for two weeks at 2°C. Wood frogs experiencing a single freezing event (n=26) were cooled from 2°C to -6°C over 12 hours, nucleated with ice to initiate freezing at -1.5°C, and then held for two weeks at -6°C. Wood frogs experiencing repeated freeze-thaw (n=29) were cooled over 12 hours from 2°C to -6°C, nucleated at -1.5°C, and then warmed over 12 hours to 2°C; this cycle was repeated twice before wood frogs were then held at -6°C for two weeks. Wood frogs (n = 10 per group) averaged 18.7±2.7 motile metacercariae and there were no changes in motile metacercariae abundance among unfrozen wood frogs 2 weeks after exposure. Freezing significantly impacted parasite survival. No parasites survived in the single freezing event group; however, parasite survival was 23% in the freeze-thaw group. Parasite survival had a positive linear relationship with the concentration of cryoprotectant (glucose) produced by the frog host. My results demonstrate that ecologically relevant conditions must be used to accurately evaluate parasite survival in vitro. This research also demonstrates how overwintering physiology of a host can detrimentally affect parasite survival. Additionally, these results indicate overwintering parasites may use host cryoprotectants to survive freezing.
