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 waterborne triactino- myxon stages of Myxobolus cerebralis were evaluated by vital staining and the infectivity for juve- nile rainbow trout Oncorhynchus mykiss. A dose of 1300 mWs cm -2 was required to inactivate 100% of the triactinomyxons held under a static collimated beam of UV as determined by vital staining. Juvenile rainbow trout were protected from infections with M. cerebralis when exposed to 14 000 or 1400 triactinomyxon spores per fish that had been treated with the collimating beam apparatus (1300 mWs cm -2 ). Among all fish receiving UV-treated triactinomyxons, none had clinical signs of whirling disease, or evidence of microscopic lesions or spores of M. cerebralis after 5 mo at water temperatures of 15°C. In contrast, 100% of the fish receiving the higher dose of untreated triactino- myxons developed clinical signs of whirling disease and both microscopic signs of infection and spores were detected in all of the high and low dose trout receiving untreated triactinomyxon expo- sures. Two additional trials evaluated the Cryptosporidium Inactivation Device (CID) for its ability to treat flow-through 15°C well water to which triactinomyxons were added over a 2 wk period. CID treatments of a cumulative dose exceeding 64 000 triactinomyxons per fish protected juvenile rain- bow from infections with M. cerebralis. Rainbow trout controls receiving the same number of untreated triactinomyxons developed both microscopic lesions and cranial spore concentrations up to 10 4.6 per 1 ⁄2 head, although no signs of clinical whirling disease were observed. UV (126 mW s cm -2 , collimated beam apparatus) was also effective in killing Flavobacterium psychrophilum, the agent causing salmonid bacterial coldwater disease, as demonstrated by the inability of bacterial cells to grow on artificial media following UV treatment.