Evaluation of parasitic contamination and effects on estimates of mitochondrial enzymatic activities in infected fish livers

Parasites can impair host performance through various physiological processes, including alterations to host metabolism. Since mitochondria are responsible for cellular energy production, it is likely that disruptions in host cellular metabolism contribute to changes in metabolism at the organismal level. However, some studies investigating parasite-induced alteration in cellular metabolism have not taken the presence of parasites in the target tissues into account, potentially biasing results. It is therefore critical to confirm that the measured enzyme activities reflect those of the hosts rather than the parasites themselves. Here, we tested a parasite extraction protocol to evaluate the extent to which parasite contamination impacts estimates of cellular enzyme activities in hepatic tissues of wild pumpkinseed sunfish (Lepomis gibbosus) infected with bass tapeworm cestodes (Proteocephalus ambloplitis). We tested four treatments: uninfected livers, cleaned infected livers, infected livers (repopulated) and parasites alone. We then compared the activity of key metabolic enzymes among groups. PCR tests were used to assess parasitic contamination of samples after applying the parasite extraction protocol on hepatic tissue. Enzyme activities of cleaned livers and contaminated livers were similar despite PCR tests revealing contamination. The intensity of cestode infection also did not influence enzyme activity, which suggests that parasite presence in liver tissues does not impact the accuracy of the enzyme activity estimates. These results suggest that studying the organs of heavily parasitized individuals is possible. Nevertheless, we recommend that our cleaning protocol is applied to infected organs to avoid any perception of biases in highly infected individuals.