Myxobolus cerebralis is a microscopic metazoan parasite (Phylum Myxozoa: Myxosporea) associated with salmonid whirling disease. There are currently no vaccines to minimise the serious negative economical and ecological impacts of whirling disease among populations of salmonid fish worldwide. UV irradiation has been shown to effectively inactivate the waterborne infective stages or triactinomyxons of M. cerbralis in experimental and hatchery settings but the mechanisms by which the parasite is compromised are unknown. Treatments of triactinomyxons with UV irradiation at doses from 10 to 80mJ/cm 2 either prevented (20-80mJ/cm 2) or significantly inhibited (10mJ/cm 2) completion of the parasite life cycle in experimentally exposed juvenile rainbow trout (Oncorhynchus mykiss). However, even the highest doses of UV irradiation examined (80mJ/cm 2) did not prevent key steps in the initiation of parasite infection, including attachment and penetration of the epidermis of juvenile rainbow trout as demonstrated by scanning electron and light microscopy. Furthermore, replication of UV-treated parasites within the first 24h following invasion of the caudal fin was suggested by the detection of concentrations of parasite DNA by quantitative PCR comparable to that among fish exposed to an equal concentration of untreated triactinomyxons. Subsequent development of parasites treated with an 80mJ/cm 2 dose of UV irradiation however, was impaired as demonstrated by the decline and then lack of detection of parasite DNA; a trend beginning at 10days and continuing thereafter until the end of the study at 46days post parasite exposure. Treatments of triactinomyxons with a lower dose of UV irradiation (20mJ/cm 2) resulted in a more prolonged survival with parasite DNA detected, although at very low concentrations, in fish up to 49days post parasite exposure. The successful invasion but only short-term survival of parasites treated with UV in rainbow trout resulted in a protective response to challenges with fully infective triactinomyxons. Prior treatments of juvenile rainbow trout with UV-treated triactinomyxons (10 and 20mJ/cm 2) resulted in a reduced prevalence of infection and significantly lower concentrations of cranial myxospores (two direct measures of the severity of whirling disease) compared with trout receiving no prior treatments when assessed 5months post parasite exposure to fully infective triactinomyxons.
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