Lymphocystis is a viral disease caused by the lymphocystis disease virus (LCDV) which belongs to the family of Iridoviridae. It can affect over 140 species of marine and fresh water fish and cause wart-like growths on the skin, mouth, fins, and occasionally in the internal organs of infected fish. Lymphocystis is spread by fish-to-fish contact or contact with infected tissues. However, environmental stress factors due to intensive culture conditions in fish farms, such as crowding, shipping, poor water quality, poor diet and inappropriate temperatures can trigger disease outbreaks. Although lymphocystis is considered as self-limiting and usually not associated with fish death, opportunistic secondary infections of bacterial and parasitic pathogens can occur and lead to mass mortality. To date, there is no commercial vaccine available for LCDV infections. Therefore, an early diagnosis of LCDV asymptomatic carriers is considered as the key measure to prevent the transmission of virus in aquaculture system.
In this present study, we isolated LCDV strains through a monitoring survey in ten fish farms located on different geographical sites of Jeju Island, South Korea. The sequence comparison of the major capsid protein of the isolated strains revealed that these strains belonged to the same genotype and exhibited the highest phylogenetic relationship with the strain recently isolated from Japanese flounder. Based on the MCP coding sequences, we designed three sets of primers and Taqman probes for an absolute quantitative PCR assay. After evaluation of amplification efficacy, one set of primers and probes with the lowest detection limit of 100 copies of LCDV DNA was finally selected. To further validate the asssay’s efficacy in lymphocystis disease monitoring, we sampled tail fin tissues from 50 fish with lymphocystis symptoms and 50 fish without symptoms in the LCDV infected tanks, and subjected them to the LCDV quantification. Compared to the fish free of LCDV infection, the lymphocystis symptomatic fish showed more than 105 times higher copies of LCDV DNA in the fin tissue, while the asymptomatic fish showed approximately 10 times higher copies. Furthermore, we also conducted a monthly LCDV monitoring of seawater in five olive flounder farms using the developed assay. The results showed a high presence of LCDV virions in the olive flounder aquaculture system, especially in the winter season (November to April). Overall, our assay has great potential as a diagnostic method for detecting the presence and spread of lymphocystis in olive flounder farms.