Analysis of stress factors associated with KHV reactivation and pathological effects from KHV reactivation

Koi herpesvirus (KHV) is a highly pathogenic virus of common carp and koi. KHV becomes latent in recovered koi or exposed koi without symptoms, and the latent infection can reactivate under stress conditions. KHV reactivation from latency often occurs when water temperature rapidly rises above 17°C. Dissolved O₂ is lower at ≥17°C than at non-stress temperatures ≤15°C. To determine whether reduced dissolved O₂ level has a role in KHV reactivation during temperature stress, KHV reactivation was investigated in KHV latently infected koi (KHV⁺ koi) under stress temperatures by maintaining dissolved O₂ consistent with the O₂ level at 15°C. There was no significant difference in the amount of reactivated virus between KHV⁺ koi maintained with and without O₂ supplementation during temperature stress. Both handling and sampling were found to be stressful to koi and can contribute to KHV reactivation from latency. There was an increase in KHV genome within white blood cells (WBC) during KHV reactivation, which is about 3-4 fold higher than the amount of KHV genome detectable in WBC during the latency stage. At day 15 post-temperature stress (PTS), inflammation and necrosis were observed in multiple tissues, especially in the gills, eye, intestine, skin and kidney. KHV DNA was also detectable in multiple tissues on days 6, 9 and 15 PTS. Following day 3 PTS, the plasma cortisol levels were higher than that observed in koi before temperature stress, suggesting that KHV reactivation is associated with physiological stress in KHV⁺ koi.