Ultraviolet irradiation inactivates the waterborne infective stages of Myxobolus cerebralis: a treatment for hatchery water supplies

The effects of ultraviolet (UV) irradiation on the viability of the waterbo rne triactinomyxon stages of Myxobolus cerebralis were evaluated by vital s taining and the infectivity for juvenile rainbow trout Oncorhynchus mykiss. A dose of 1300 mWs cm(-2) was required to inactivate 100% of the triactino myxons held under a static collimated beam of UV as determined by vital sta ining. Juvenile rainbow trout were protected from infections with M. cerebr alis when exposed to 14000 or 1400 triactinomyxon spores per fish that had been treated with the collimating beam apparatus (1300 mWs cm(-2)). Among a ll fish receiving UV-treated triactinomyxons, none had clinical signs of wh irling disease, or evidence of microscopic lesions or spores of M. cerebral is after 5 mo at water temperatures of 15 degrees C. In contrast, 100% of t he fish receiving the higher dose of untreated triactinomyxons developed cl inical signs of whirling disease and both microscopic signs of infection an d spores were detected in all of the high and low dose trout receiving untr eated triactinomyxon exposures. Two additional trials evaluated the Cryptos poridium Inactivation Device (CID) for its ability to treat flow-through 15 degrees C well water to which triactinomyxons were added over a 2 wk perio d. CID treatments of a cumulative dose exceeding 64000 triactinomyxons per fish protected juvenile rainbow from infections with M. cerebralis. Rainbow trout controls receiving the same number of untreated triactinomyxons deve loped both microscopic lesions and cranial spore concentrations up to 10(4. 6) per 1/2 head, although no signs of clinical whirling disease were observ ed. UV (126 mWs cm(-2) collimated beam apparatus) was also effective in kil ling Flavobacterium psychrophilum, the agent causing salmonid bacterial col dwater disease, as demonstrated by the inability of bacterial cells to grow on artificial media following UV treatment.