Variation among tadpole species in their ability to clear encysted trematode parasites

Background/Question/Methods

The persistence time of parasitic organisms in their hosts has important implications for parasite transmission, immune evasion, and age-intensity relationships. Persistence is especially important for parasites with dormant stages that need to wait extended time periods before passing to the next host. Most trematode parasites have dormant stages called metacercariae that encyst within a second intermediate host, which must be consumed by a definitive host to complete the trematode life cycle. These cysts have generally been assumed to be persistent, walled off from the host immune response and able to wait long periods of time for the intermediate host to be eaten. However, recent studies of trematodes in tadpoles have found decreasing numbers of encysted metacercariae at longer time points post-infection, suggesting that at least some tadpole species might be capable of clearing trematode metacercariae entirely. However, these patterns might also result from differential mortality of heavily infected tadpoles, or reduced detectability of dead metacercariae (i.e., empty cyst walls). To distinguish these hypotheses, we used a novel fluorescent-tag technique to track the presence of Ribeiroia ondatrae through time in live tadpoles, comparing the abilities of eight different tadpole species to clear metacercariae over the course of a three-week time period.

Results/Conclusions

We found that tadpoles are definitely capable of clearing metacercariae, which disappeared over time in every host species. Tadpoles that had cleared cysts were occasionally observed to have fluorescent livers or gall bladders, suggesting that fluorescently tagged lipids in the parasites had been metabolized by the tadpoles and converted into bile. Several metacercariae were also observed in the process of being cleared. Counts of fluorescent metacercariae were found to be highly accurate based on validation with preserved tadpoles using traditional dissection or clearing methods. There was great variation in clearance rates among tadpole species, with the three treefrog species clearing metacercariae much more rapidly than the three ranid or one bufonid species. However, the shapes of the survival curves for metacercariae in different tadpole species were remarkably similar, all fitting the special case of a Weibull distribution in which the hazard (clearance rate) increases linearly through time (Scale parameter = 2). This generality suggests that metacercaria clearance occurs via similar mechanisms in various species of tadpoles, despite differences in overall clearance rates. These results have important implications for the ecology of R. ondatrae infection, which is associated with amphibian limb deformities and mortality events throughout North America.