Gill diseases have a significant impact on fish health and a very negative impact on aquaculture, mainly due to the multifunctional properties of the gills in fish physiology. Carp edema virus (CEV) is a large DNA poxvirus that primarily infects the gills of common carp (Cyprinus carpio L.), causing a highly contagious and fatal disease known as Koi Sleepy Disease (KSD). Our previous studies have shown that, when experimentally infected with genogroup IIa CEV, different strains of carp exhibit high (Amur wild carp - AS) or low (koi carp) resistance to this virus. The increased susceptibility of koi leads to severe impairment of gill function.
In the present study, blood parameters, viral load and expression of selected immune-related genes were determined in the gills of both carp strains. The experiments were carried out at two temperatures: 12 and 18 °C. In the case of the koi carp, we also introduced a salt rescue model based on the addition of 0.5% NaCl to the tank water, which prevents mortality of the fish.
Nanoscale qPCR analysis of 40 genes showed that CEV induced a significant antiviral response in the gills of all infected fish groups compared to uninfected controls. We also found that the viral load was higher at 18°C than at 12°C in all fish groups studied, and at both temperatures the highest viral load was present in koi carp compared to the koi salt rescue group and AS carp. Interestingly, CEV-infected koi carp had higher glucose and cortisol levels and lower plasma sodium levels than the koi salt-rescue group and AS carp at both temperatures. This clearly indicates that CEV infection in a susceptible strain correlates with high stress parameters that can trigger immune modulation. Further analysis showed that disease severity and higher stress response in koi were indeed associated with immunomodulation, as reflected by a down-regulation of T-cell responses.
In conclusion, our data indicate a clear, temperature-dependent relationship between CEV-induced gill disease, stress and immunomodulation in susceptible koi carp.
Acknowledgements: This research was supported by the German Research Foundation (DFG project number 426513195) and the National Science Centre of Poland (NCN project number UMO-2018/31/F/NZ6/02311).