A standardized method to propagate Cryptocaryon irritans on a susceptible host pompano Trachinotus ovatus

A standardized procedure for Cryptocaryon irritans propagation was established by means of infecting the animal model pompano Trachinotus ovatus (167.8 ± 15.1 g) with a sub-lethal dose of theronts. By using this method, up to 40 consecutive cycles were successively established. Briefly, theronts collected within 2 h of excystment were used to infect pompanos (8000–15,000 theronts per fish) in dark for 3 h starting between 22:00 and 24:00, at 27 ± 0.5 °C and 29–32‰ salinity. The trophonts on the skin and gill appeared evidently 60 h post-infection. Their growth phase culminated when the mature trophonts spontaneously exited the host after another 24 h and fell to the bottom as protomonts. All of the protomonts were collected by using a special collection unit. After a 60-h incubation, the protomonts evolved into tomonts and began to release theronts. The excystment of theronts reached the peak after 24 h, and at this time point the theronts were collected for the next cycle of propagation. One cycle of propagation took one week and produced approximately 1 million theronts per fish. The maximum parasite yield of propagation was 122-fold and the minimum was 61-fold. The longest survival time of theronts was 36 h. The infection rate of theronts upon excystment was 64%, and then declined evidently by 4 h. At 8 h, the infection rate declined to 9.6%. The present study demonstrated that C. irritans could be efficiently propagated on T. ovatus, a suitable experimental host. The standardized method for C. irritans propagation provides a foundation for future studies of C. irritans such as its pathogenicity and control of the disease it causes.