Infection dynamics and shedding kinetics of RSIV in red sea bream (Pagrus major) using immersion and cohabitation challenge models

Abstract

Red sea bream iridovirus (RSIV) poses a significant threat to global aquaculture and wild fish populations, leading to substantial economic and ecological losses. Despite its recognition as a major viral disease, our understanding of the dynamics of RSIV infection and transmission in naturalistic conditions remains limited. Therefore, the present study aimed to investigate the kinetics, shedding, and interspecies transmission of RSIV in red sea bream (Pagrus major) through immersion and cohabitation challenge models. The primary objectives were to (1) elucidate the pathogenicity of RSIV in red sea bream through naturalistic immersion challenges, (2) analyze the viral load and shedding dynamics in RSIV-infected fish, and (3) assess the risk of horizontal transmission to other species. Our results indicated that red sea bream exposed to high doses of RSIV (101–107 RSIV copies/mL) experienced rapid cumulative mortality, particularly at 25 °C. Notably, red sea bream infected by immersion at 25 and 15 °C showed a significant increase in RSIV shedding into seawater when the viral load exceeded approximately 102.48 RSIV copies/mg, highlighting a critical threshold for understanding disease spread in aquaculture settings. Histopathological analysis revealed that the spleen and kidney were primarily affected, with significant lesion scores correlating with viral loads. We observed a strong correlation between viral load and histopathological infection grades, particularly in these primary tissues. Furthermore, cohabitation experiments highlighted the risk of RSIV transmission to other fish species, including rock bream (Oplegnathus fasciatus) and flathead grey mullet (Mugil cephalus), indicating the possibility of horizontal spread in multi-species farming systems. These findings emphasize the importance of continuous surveillance and robust biosecurity measures in aquaculture systems to manage RSIV infection effectively. Overall, this study contributes to understanding RSIV dynamics in red sea bream, offering insights into the impact of this virus in aquaculture environments, particularly concerning viral shedding and transmission.