Susceptibility of the topmouth gudgeon (Pseudorasbora parva) to CyHV-3 under no-stress and stress conditions

Cyprinid herpesvirus 3 (CyHV-3), also known as koi herpesvirus, is the causative agent of the highly contagious koi herpesvirus disease, which is restricted to koi and common carp and causes significant losses in both fish stock. Some experimental investigations have shown that other cyprinid or non-cyprinid species may be asymptomatically susceptible to this virus and might play roles as potential carriers of CyHV-3 or might contribute to persistence of this virus in environment. Therefore, it seems important to verify not only the susceptibility of other cyprinid or non-cyprinid species, but also their ability to transmit CyHV-3 infection to susceptible species. Our previous investigation of the susceptibility of the topmouth gudgeon (Pseudorasbora parva) did not reveal the presence of CyHV-3 DNA in the tissues of this species after cohabitation with infected koi. Consequently, we changed the experimental conditions and applied two stress factors (removal of skin mucus and scaring) which would presumably mimic the stress most commonly encountered in the wild. Both experiments (without and with stress factors) consisted of primary and secondary challenges. In both the no-stress and stress experiments, the first challenge was focused only on testing the susceptibility of the topmouth gudgeon to the virus. With the secondary challenge, we investigated potential viral transmission from the topmouth gudgeon to healthy naive koi after exposure to stress factors. All fish (dead, surviving and sacrificed) were tested for the presence of CyHV-3 DNA using nested PCR (no-stress experiment) and real-time PCR (stress experiment). After the primary challenge of the no-stress experiment, PCR did not reveal the presence of CyHV-3 DNA in any specimen of cohabitated topmouth gudgeon, but all specimens of dead koi were CyHV-3 DNA-positive. PCR of fish tissues subjected to the secondary challenge did not show the transfer of virus to naive fish. After exposure to stress (removal of skin mucus), qPCR revealed four out of five samples (80%) of topmouth gudgeon to be positive for CyHV-3 DNA. Two out of five samples (40%) of topmouth gudgeon treated by scaring were found to be positive for the presence of viral DNA. Real-time PCR after the secondary challenge did not reveal any viral DNA positivity in specimens of topmouth gudgeon from groups previously exposed to stress. The stress experiments show that removal of skin mucus might potentially lead to susceptibility of topmouth gudgeon to CyHV-3 infection, but the transmission of the virus to koi carp was not observed.