Parasitic infections in pond-reared rainbow trout Oncorhynchus mykiss in Denmark

From October 1993 to December 1995, 5 Danish freshwater rainbow trout Oncorhynchus myklss farms were surveyed for parasites, the most comprehens~ve survey to date in Denmark.A total of 805 f ~s h , aged 0+ and l+, were examined, and the parasites were identified by light microscopy, scanning electron microscopy and histochemistry.Prevalence (protozoans and metazoans) and abundance (metazoans) were recorded.The 12 protozoans iound were: an intestinal diplomonadid flagellate, the ectoparasitic kinetoplastid Ichthyohodo necator, and the clhates Ichthyophthlnus multifiliis, Chilodonella piscicola, Capriniana (Tr~chophrya) sp., Tr~chodina nigra, 7. mutabills, 7 fultoni, Trichodinella epizootlca, Apiosoma sp., Amblphrya sp and Epistylis sp The 10 metazoan species found were: the inyxosporean organism PKX, the monogeneans Gyrodactylus derlavinl and G. salaris, the digeneans Diplostomum spathaceum and Tylodelphys clavata, the cestodes PI-oteocephalus exlguus.P. long~coll~s, Eubothnum crassum, and Triaenophorus nodulosus, and the crustacean Argulus fohaceus.The diplomonad occurred predominantly in anorectic undersized fish, and was associated with mortality.I necator produced discoloration a n d epidermal hyperplasia and I. niultlflllls infections were associated with mortalities.G. salai-IS and G. derjavini produced alterations (lesions and hyperplasia) of the host epidermis visible in the scanning electron microscope.Soine D spathaceum infections caused cataract.Prevalence of I. mult~filiis increased with temperatul-e (maximum at 16 to 20°C), whereas the diplomonad had h ~g h e s t prevalence at l to 5°C Likewise, the gyrodactylids occurred more abundantly at lower temperatures KEY WORDS R a ~n b o w trout Oncorhynchus mykiss Parasites Protozoa hletazoa Fish farming Aquaculture Ep~demiologyTable 1 Previous investigations on parasitic infections In farmed salmonids Author Geographical Host No. of fish Period Parasites recorded region sxamincd p p -Zitnan & Cankovic Bosnia-Herzegovina Oncorhynchiis ~n y k i s s No 1966-1968 Protozoa and (1970) information metazoa Wootten (1972) England 0, mykiss