Objective In late July 2025, an outbreak of acute mortality occurred in cultured Siberian hybrid sturgeon (Acipenser baerii ♀×A. schrenckii ♂) in Ya’an, Sichuan Province, characterized by extensive cutaneous and visceral hemorrhages. This study aims to identify the etiological agents and characterize the histopathological lesions to provide a scientific basis for clinical diagnosis and disease control of A. baerii ♀×A. schrenckii ♂.

Methods One moribund A. baerii ♀×A. schrenckii ♂ exhibiting typical clinical signs was randomly collected from each of the six affected ponds (n=6, S1-S6). Tissues from the liver, spleen, kidney, and brain were sampled for bacterial isolation, viral polymerase chain reaction (PCR) detection, and histopathological examination. Molecular biological identification of the isolated strains was performed using 16S rDNA sequence analysis. Simultaneously, the major capsid protein (MCP) gene of acipenserid herpesvirus 3 (AciHV-3) was amplified and sequenced for phylogenetic analysis. Antimicrobial susceptibility testing was conducted on the recovered bacterial isolates.

Results Viral detection confirmed that all six samples were positive for AciHV-3; phylogenetic analysis showed that the MCP gene sequences clustered closely with reference strains within a single clade. Regarding bacterial isolation, morphologically identical Gram-positive cocci were recovered from four of the six samples (4/6). The 16S rDNA sequence analysis of the representative strain (GenBank accession no. PX427207) showed 99% identity with that of Streptococcus iniae in GenBank. In the phylogenetic tree constructed based on 16S rDNA sequences, the isolate clustered into a single clade with S. iniae. Therefore, the isolate was eventually identified as Streptococcus iniae. Antimicrobial susceptibility testing (AST) revealed significant variation among the isolated strains. Of the eight antimicrobials tested, only cefotaxime exhibited potent inhibitory activity against all isolates. Susceptibility to florfenicol, doxycycline, enrofloxacin, and ciprofloxacin fluctuated markedly across strains, with the majority being intermediate or resistant. Furthermore, all isolates demonstrated complete resistance to amoxicillin, sulfamethoxazole/trimethoprim, and neomycin sulfate, exhibiting a pronounced multidrug resistance (MDR) phenotype. Histopathological analysis revealed extensive necrosis, hemorrhage, and inflammatory infiltration across multiple organs.

Conclusion The study confirms the co-detection of AciHV-3 and S. iniae in cultured sturgeon in Sichuan. Given the severe and consistent histopathological damage, these findings suggest that the high-mortality outbreak was likely driven by the synergy of AciHV-3 and S. iniae. This research provides essential data for the clinical monitoring and management of AciHV-3.