Investigation of Mortality Events Reveals Coinfections and Identification of Infectious Spleen and Kidney Necrosis Virus in Red Hybrid Tilapia in Thailand.

Infectious diseases have become a major challenge in global tilapia aquaculture. This study investigated disease outbreaks affecting red hybrid tilapia (Oreochromis spp.) by collecting 80 pooled samples from 36 farms between 2022 and 2024. Although the majority of samples tested positive for tilapia lake virus (TiLV), infectious spleen and kidney necrosis virus (ISKNV) was detected in 10% of the cases, specifically in fry and fingerlings originating from river-based cage systems. All the ISKNV-positive cases involved coinfections with TiLV, bacteria, and ectoparasites. Megalocytes with basophilic intracytoplasmic inclusion bodies were detected along with hemosiderin deposits in the anterior kidneys and spleens of the ISKNV-positive fish. ISKNV was confirmed by PCR and in situ hybridization. The virus was successfully isolated in red hybrid tilapia brain (RHTiB) cells, showing cytopathic effects within 2-5 days postinfection. Phylogenetic analysis of the major capsid protein gene clustered the Thai isolate within ISKNV Clade 1, related to other Asian strains. Our findings highlight the need to include ISKNV in routine diagnostic surveillance and the importance of implementing integrated health management strategies focused on the early life stages of fish in intensive tilapia production systems.