|dc.description.abstract||Given that parasites demonstrate different sensitivity to contaminants and environmental stressors, there is an increasing interest in the use of parasites as biological or ecological indicators of their fish host life conditions as well as bioindicators of heavy metal pollution of aquatic ecosystems. In order to obtain insight into host-parasite relationships in selenium (Se) polluted environments, a study on nematode parasitism of rainbow trout exposed to subchronic dietary selenomethionine (Se-Met) was initiated. Se-Met is the major dietary form of selenium, and exposure to elevated dietary Se has been associated with detrimental effects on fish health and survival.
In the current study, rainbow trout (Oncorhynchus mykiss) were experimentally infected with larval stages (L3) of the intestinal nematode Raphidascaris acus and subsequently exposed to an environmentally relevant concentraton of dietary Se-Met (4.54 mg/kg wet weight (WW)) for 70 days. The specific research objectives were to determine to what extent R. acus and its host accumulate Se from Se-Met fortified diet. In addition, non-specific biomarkers such as condition factor (K), hepatosomatic index (HSI), and gross energy (GE) were calculated to demonstrate the general effects of Se-Met exposure and parasitic infection on trout health. To further investigate joint effects of parasitism and Se-Met exposure on oxidative stress in rainbow trout, levels of superoxide dismutase (SOD) and gluthathione s-transferase (GST) were assessed in the liver and in the head kidney, as these enzymes are involved in the antioxidant response. Fish health variables, muscle Se concentration, and enzyme activity levels were compared between infected trout and uninfected fish exposed to same dietary Se-Met concentration for 56 days, as well as control fish.
Subchronic exposure of uninfected rainbow trout to dietary Se-Met lead to an initial increase in muscle Se concentration reaching a plateau concentration at 5.27 mg Se/kg dry weight (DW), while a continuous increase in muscle Se was observed in infected trout exposed to Se-Met, reaching 7.52 mg Se/kg DW on day 70. Results suggest that continuous Se accumulation by parasitized fish could be due to increased susceptibility of rainbow trout to metal exposure as a result of infection with parasites. Furthermore, depletion in GE stores in infected rainbow trout exposed to Se-Met was observed (p<0.05), while no changes in K and HSI were detected between treatments, suggesting that challenging parasitic infection can significantly deplete energy reserves of the fish. In addition, in fish exposed to Se-Met an increase (p<0.05) in GST activity was recorded for the initial 7 days of Se exposure, whereas SOD activity was elevated (p<0.05) for the duration of the study. In contrast, fish exposed to both parasites and Se-Met showed a decrease (p<0.05) in antioxidant enzyme levels when compared to trout exposed to Se-Met alone, suggesting an important role of parasitic infection in suppressing oxidative stress response of the host.
In summary, parasites such as R. acus can upregulate absorption of Se in the final host, leading to increased deposition of Se into the muscle tissue. In contrast to its host, R. acus did not accumulate large concentrations of Se (1.91 mg Se/ kg DW), and therefore may not be very useful as a biomarker of Se pollution. In addition, parasites negatively influenced expression of general oxidative stress biomarkers (SOD, GST) and therefore should be considered in ecotoxicological studies. To our knowledge, this is the first study to demonstrate effects of combined Se-Met exposure and parasitic infection on rainbow trout health and the oxidative stress response.||en_US