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Investigating the effects of per-/polyfluoroalkyl substances (PFAS) on host-parasite interactions in a larval amphibian-echinostome system
thesisposted on 27.04.2021, 19:50 by Melissa Eden Lech
Per- and polyfluoroalkyl substances (PFAS) are a large class of ubiquitous, persistent environmental contaminants of emerging concern due to their detrimental effects on humans and wildlife. Chronic exposure to PFAS at environmentally relevant concentrations has been shown to cause immunosuppression in a variety of taxa. This is concerning because changes in immune function can alter host-parasite dynamics, which are common in nature. Additionally, because contaminants can affect both hosts and parasites, there is a need to examine the effects of PFAS on both members of these interactions. There is currently little information about how PFAS affect host-parasite interactions; therefore, using a larval amphibian-echinostome system, we conducted three experiments to address this knowledge gap. In our first experiment, we used a controlled laboratory experiment to investigate the effects of PFAS mixtures commonly found at aqueous film-forming foam (AFFF) contaminated sites on host susceptibility to parasites. We exposed larval American bullfrogs (Rana catesbeiana) to 4 ppb perfluorooctanesulfonic acid (PFOS), 10 ppb PFOS, and three PFAS mixtures for 62 days. The composition of the mixtures allowed us to assess potential additive, synergistic, or antagonistic effects of exposure to multiple PFAS. After chemical exposure, tadpoles were challenged with echinostome cercariae. We found that exposure to PFAS increased parasite loads 42-100%. Additionally, we found potential antagonistic effects of PFAS mixtures on host susceptibility to parasites. In our second experiment, we investigated the effects of PFAS on host susceptibility, parasite infectivity, and cercariae longevity under controlled laboratory conditions. We exposed larval northern leopard frogs (Rana pipiens) and echinostome-infected ramshorn snails (Helisoma trivolvis) to 10 ppb PFOS, perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA), perfluorohexanesulfonic acid (PFHxS), perfluoropentanoic acid (PFPeA), and a 10 ppb PFAS mixture for 21 days. We then exposed PFAS-free tadpoles to PFAS-free and PFAS-treated cercariae and PFAS-treated tadpoles to PFAS-free and PFAS-treated cercariae. We also conducted a time-to-death assay with the cercariae from the snails that were exposed to PFAS. We found that host-only and parasite-only exposure to PFAS resulted in a 15% and 10% reduction in infection rates, respectively. However, when both tadpoles and parasites were exposed to PFAS, there was no difference in parasite load. Additionally, we found significant differences in cercarial longevity outside of the host following PFAS exposure. In our third experiment, we investigated the interactive effects of atrazine and PFAS on host susceptibility to parasites under semi-natural conditions. Atrazine and PFAS are likely to co-occur in aquatic systems and exposure to both chemicals has been shown to increase parasite loads. Because of this, there could be additive or synergistic effects of atrazine and PFAS on host susceptibility. Larval American bullfrogs and infected ramshorn snails were exposed to atrazine and PFAS in mesocosms for 30 days. Here, we found that there was no effect of chemical exposure on parasites loads, but we did find interactive effects of parasites and chemicals on tadpole growth and development. Collectively, our work indicates that PFAS can alter host-parasite interactions and that effects may depend on length of exposure and the chemical mixture exposure compositions. Moreover, our results highlight the need for additional studies examining the effects of PFAS on the complex ecological interactions found in natural systems.