- Koi herpesvirus (KHV) is a new emerging viral pathogen of koi and common carps. It has been demonstrated that KHV becomes latent in the white blood cells in the recovered koi. The latent KHV can be reactivated under stressful conditions, such as heat and injury, and the reactivated viruses can cause diseases or death in the latently infected fish. Outbreaks of KHV often occur in the spring and summer when temperature suddenly increases. The dissolved oxygen level goes down when water temperature rises and it is unknown whether the change in dissolving oxygen has any effect on KHV reactivation. In this study, the dissolving oxygen level was investigated under temperature that could lead to KHV reactivation from latency. Our studies demonstrated that by maintaining the dissolved oxygen level at the non-stressing temperature level does not prevent KHV reactivation during temperature stress, but lowers the KHV shedding in feces. We also found KHV reactivation can occur as early as day 3 post-temperature stress or at 18˚C and peaks between 20˚C and 22˚C. Reactivated KHV can be detected in both gills and droppings. By day 15 post-temperature stress, both gross lesion and histopathology were present. There were inflammation and necrosis in multiple tissues,
especially in gills, skin, eye, intestine and kidney. KHV DNA can also be detected in multiple tissues in KHV latently infected koi at day 15 post-stress. The cortisol levels between day 3 and day 15 post-temperature stress are all above normal range, which suggests that KHV reactivation is a result of physiological stress.
Key Words: KHV, reactivation, latency, low O2, post-temperature stress