Comparative study of experimental infection against koi carp using Carp edema virus

Viral edema of carp is caused by Carp edema virus and is a major carp disease listed on the WOAH list of emerging infectious diseases. However, an effective experimental infection method that can control virus copy number has not been established. In this study, we compared three experimental infection methods：immersion infection using gill suspensions from diseased fish (immersion method), infection by rearing water from diseased fish (rearing water method) and co-infection method. Cumulative mortality in the immersion groups ranged from 40 to 60%, while that in the rearing water infected and co-infected groups ranged from 80 to 100%. The rearing water method can be used to control the viral DNA copy number and to perform experimental infections when frozen storage is possible. Thus, we performed the same experimental infection using frozen rearing water and found no disease or mortality, indicating that stable experimental infection using the rearing water method is difficult. The immersion method has a lower mortality rate than the other two methods, and it requires a large amount of gill tissue because of the low number of virus DNA copies that can be obtained from the gill tissue. However, it is possible to control the number of virus DNA copies and to preserve the gill tissue suspension by freezing. Therefore, the immersion method was considered the most suitable for stable experimental infection under the same conditions,