Temporal changes in viral genome copy numbers in rearing water and gills during experimental infection with carp edema virus

Viral edema of carp (VEC) is a viral disease affecting carp Cyprinus carpio, caused by the carp edema virus (CEV). However, the dynamics of the virus and the levels of viral shedding in diseased and carrier fish remain to be elucidated. In this study, experimental infections with two CEV strains were performed, after which samples were collected chronologically from the rearing water and gills in order to measure viral genome copy numbers (vgcn). The vgcn in the rearing water remained below the detection limit up to 1 day post infection (dpi). However, the presence of the virus was first detected at 3 dpi, reaching a peak at 7 dpi before declining rapidly. As with the rearing water, the vgcn in the gills also began to increase from 3 dpi, peaking at 7 dpi before gradually declining thereafter. Therefore, when koi carp are experimentally infected with CEV, viral replication in the gill tissue begins around 3 dpi, and the rearing water becomes cloudy presumably due to excessive mucus secretion. Around 7 dpi, the viral level in the gill tissue and viral shedding reach their peak, with symptoms such as sunken eyes and recumbency observed. Subsequently, the viral level declines gradually, and the fish dies several days later.