Background. Ectoparasitic infections of fish are considered to be stress inducing. However, only a few studies have addressed this topic. In addition, formalin bath treatment of rainbow trout is a generally applied method of controlling ectoparasites, but the stress response in rainbow trout, Oncorhynchus mykiss, as a result of such treatments with formalin, has not received much attention and therefore needs further elucidation. The present investigation addresses the stress-response induced by parasites and formalin treatment. Materials and methods. Concentrations of plasma cortisol were monitored using ELISA. Samples were taken from groups subjected to confinement stress, infection with Ichthyophthirius multifiliis and formalin bath treatment. Results. Rainbow trout clearly responded to harmful stimuli by increasing plasma cortisol concentrations. Confinement, formalin bath treatment, as well as infection with Ichthyophthirius multifiliis resulted in significant high cortisol concentrations in plasma compared to the unhandled and uninfected control groups. Conclusion. The present study showed that cortisol release in rainbow trout is associated with infection with the skin ciliate Ichthyophthirius multifiliis. Formalin, which is used to control the parasite infection, also elicited a high production of this immuno-suppressing hormone in the host.

Rainbow trout, Ichthyophthirius multifiliis, formalin, stress, handling, cortisol, fish