Red Sea Bream Iridovirus Kinetics, Tissue Tropism, and Interspecies Horizontal Transmission in Flathead Grey Mullets (Mugil cephalus)

SIMPLE SUMMARY: Red sea bream iridovirus (RSIV) can infect more than 30 species of farmed fish and has been reported in several Asian countries. Previous studies have reported the detection of RSIV in flathead grey mullets (Mugil cephalus), but research on pathogenicity, histopathological lesions, viral shedding, and the effects of infected fish on other species is very limited. In this study, we analyzed pathogenicity and viral dynamics in flathead grey mullets through immersion infection, simulating natural conditions of RSIV, and confirmed the risk of horizontal transmission to other fish species through cohabitation. Our data verify that RSIV is pathogenic to flathead grey mullets and that there is a potential for spreading the disease from fish farms to other species. Additionally, our findings revealed a high correlation between viral load and histopathological lesions in fish. ABSTRACT: Red sea bream iridovirus (RSIV) causes significant economic losses in the aquaculture industry. We analyzed the pathogenicity of RSIV in flathead grey mullets (Mugil cephalus), the correlation of histopathological lesions, and interspecies horizontal transmission, through immersion infection and cohabitation challenges. Flathead grey mullets, which were challenged by immersion infection, exhibited mortality at 14 and 24 days after RSIV exposure. Viral shedding in seawater peaked 2–3 days before or after the observed mortality. Specific lesions of RSIV were observed in the spleen and kidney, and the correlation between histopathological grade and viral load was the highest in the spleen. In a cohabitation challenge, flathead grey mullets were the donors, and healthy rock bream, red sea bream, and flathead grey mullets were the recipients. Viral shedding in seawater was the highest in flathead grey mullet and rock bream at 25 °C, with 10(6.0) RSIV copies L/g at 14 dpi. No mortality was observed in any group challenged at 15 °C, and no RSIV was detected in seawater after 30 dpi. The virus shed from RSIV-infected flathead grey mullets caused horizontal transmission through seawater. These findings suggest that rapid decision-making is warranted when managing disease in fish farms.