Parasite survival in a freeze-tolerant host
We examined whether Ribeiroia ondatrae, an ecologically important parasitic trematode, can survive below 0°C within the freeze-tolerant wood frog (Lithobates sylvaticus) and whether metacercariae survival is dependent on the pattern of freezing. We exposed 107 wood frog tadpoles from interior Alaska to 30 R. ondatrae cercariae each. Metamophosed frogs were divided into three groups: control, single freezing event, or repeated freeze-thaw, which is a pattern that results in higher accumulation of cryoprotectants (glucose) in naturally freezing wood frogs. For all groups, parasites were considered alive if motility was observed within cysts or after excystment. We necropsied 10 frogs from each group prior to freezing and found no significant difference in parasite survival. Control wood frogs (n=22) were held for two weeks at 2°C. Linearlyfrozen wood frogs(n=26) were cooled from 2°C to -5°C over 12 hours, nucleated at -1.5°C, and then held for two weeks at -5°C. Wood frogs experiencing repeated freeze-thaw(n=29) werecooled over 12 hours from 2°C to -5°C, nucleated at -1.5°C, and then warmed over 12 hours to 2°C; this cycle was repeatedtwice before wood frogs were then held at -5°C for two weeks
Freezing significantly impacted parasite survival (P<0.05)and survival was dependent on the pattern of freezing of the host. Wood frogs averaged 19.1 motile metacercariae prior to freezing, and all metacercariae survived in the unfrozen control group after 2 weeks. No parasites survived in the single freezing event group; however, parasite survival was 23% in the naturally freezing group. Our results demonstrate that ecologically relevant conditions must be used to accurately evaluate parasite survival. Further, freezing diminishes parasitism in a freeze-tolerant host. We hypothesize that parasite and host overwinter survival is enhanced in wood frogs that experience successive freeze and thaw cycles, possibly due to the higher concentrations of glucose that accumulate with this pattern of freezing that characterizes natural conditions.