Mandarin fish ranavirus (MRV) is a distinctive member among the genus Ranavirus of the family Iridoviridae. The persistently covert infection of MRV was previously observed in a natural outbreak of MRV, but the underlying mechanism remains unclear. Here, we show that mandarin fish peripheral B lymphocytes are implemented as viral reservoirs to maintain the persistent infection. When mandarin fish were infected with a sublethal dosage of MRV under a nonpermissive temperature (19 °C) and a permissive temperature (26 °C), all of the fish in the 19 °C group survived and entered the persistent phase of infection, characterized by a very low viral load in white blood cells, whereas some of the fish died of MRV infection in the 26 °C group, and the survival fish then initiated a persistent infection status. Raising the temperature, vaccination and dexamethasone treatment can reactivate the quiescent MRV to replicate and result in partial mortality. The viral reservoir investigation showed that IgM⁺-labeled B lymphocytes, but not CD3Δ⁺-labeled T lymphocytes and MRC-1⁺-labeled macrophages, are target cells for the persistent infection of MRV. Moreover, the reactivation of the quiescent MRV was confirmed through a non-TLR5 signal pathway manner. Collectively, we demonstrate the presence of the B cell-dependent persistent infection of ranavirus, and provide a new clue for better understanding the complex infection mechanism of vertebrate iridovirus.

Scale drop disease virus (SDDV) poses an escalating threat to global aquaculture, prompting an urgent need for research. Our study found that SDDV infection upregulates genes related to iron, oxidative stress, and lipid metabolism, causing iron overload, reactive oxygen species (ROS) accumulation, and ultimately ferroptosis. Among the tested antioxidants, vitamin C (VC) demonstrated the most potent inhibitory effect in mandarin fish, reducing SDDV-induced mortality by 37.5%. qPCR and IFA results showed that VC effectively suppressed SDDV infection; decreased ROS, lipid peroxidation (LPO), and iron levels; and enhanced glutathione peroxidase 4 (GPX4) expression in infected cells. Mechanistically, VC’s inhibitory effect was reversed by the nuclear factor erythroid 2-related factor 2 (Nrf2) inhibitor ML-385, indicating an Nrf2-dependent pathway. VC promoted Nrf2 nuclear translocation and activated downstream antioxidant genes. Moreover, VC modulated inflammation by regulating pro- and anti-inflammatory factors. These findings suggest VC as a promising therapeutic for SDDV infection.

A mass mortality event of Asian seabass Lates calcarifer juveniles occurred in Zhuhai, the main Asian seabass cultured area in mainland China. The fish samples were pooled for pathogen identification and both high viral loads of ISKNV and NNV were detected by real-time microfluidic quantitative PCR. Immunohistochemistry and immunofluorescence showed that strong ISKNV signals were examined in spleen and liver, while strong NNV signals were detected in brain and eye. The multi-tissue homogenates were inoculated into MFF-1 cell and SSN-1 cell, respectively. After several viral passages, both ISKNV and NNV were purely isolated from each other, and designated as ASB-ISKNV-23 and ASB-NNV-23, respectively. The whole genome sequences of ASB-ISKNV-23 and ASB-NNV-23 were determined and annotated. The result showed that ASB-ISKNV-23 and ASB-NNV-23 are composed of 112,236 bp and 4,538 bp, respectively. Phylogeny analysis showed that ASB-ISKNV-23 belongs to ISKNV-II sub-genotype and ASB-NNV-23 belongs to RGNNV genotype. Collectedly, coinfection of ISKNV-II and RGNNV were firstly documented in mass mortality of Asian seabass in mainland China. Importantly, both ISKNV-II and RGNNV were purely isolated using two different permissive cell lines. Our study provides useful information for better understanding the complex pathogenesis regarding the coinfection with ISKNV and NNV in farmed fish.