A technique for the preservation of Cryptocaryon irritans at low temperatures

Applied and basic research to evaluate the ciliate Cryptocaryon irritans requires serial passage in fish hosts to maintain the laboratory stock. However, continual repeated passage is labour-intensive and time-consuming and results in the senescence of parasite clones over time. To address these issues, this study was conducted to investigate the preservation of C. irritans in vitro at low temperatures. Trophonts and tomonts of C. irritans were collected and preserved at 5–25 °C. The excystation rate of preserved parasites and the infectivity of the theronts produced by the preserved parasites were then determined every other month for six months. The results revealed that trophonts and tomonts of C. irritans could survive at 12.0 °C for 5 and 4 months, respectively. Additionally, the excystation rate of the preserved parasites and the infectivity of theronts produced by the preserved parasites decreased as the preservation period increased. Specifically, trophonts preserved for 4 months and tomonts preserved for 3 months could produce theronts with infectivity, and these theronts were capable of generating a large number of descendants after two serial passages in the fish. Moreover, the virulence of the descendants did not decrease. Overall, the results of this study confirmed the practicality of preserving C. irritans in vitro at low temperatures. Furthermore, the findings presented here suggest that C. irritans can survive during winter in a dormant status and can resume infectivity during spring.