Histopathological features of infection by Streptococcus iniae in Persian sturgeon, Acipenser persicus

The present study evaluated the susceptibility of juvenile Persian sturgeon, Acipenser persicus, to Streptococcus iniae. Fish were intraperitoneally injected with 4.7 × 104 CFU fish-1. The control fish received sterile normal saline (PBS) intraperitoneally. The fish were monitored for clinical signs, and mortalities were recoded daily for two weeks post-challenge. Moribund and/or freshly dead fish were immediately removed for biopsy and/or necropsy, gross evaluation, and histopathological examinations, and the cause of mortality was confirmed by re-isolation of the S. iniae from kidney or spleen using standard microbiological tests. Mortality rates were in ranged from 20.83 to 83.33% within 48 to 120 h post challenge. Hepatocyte vacuolization, focal necrosis and melano-macrophage aggregation were dominant histological findings in affected fish. Severe meningitis with massive inflammatory cells infiltration and vacuolation were also detected in the brain layers. In addition, the kidney tissues exhibited severe necrosis, diffused tubular degeneration, hyaline exudates accumulation in tubular lumens, shrinkage of glomeruli, dilation of Bowman’s space, cellular infiltration and interstitial tissue necrosis. Hyperemia and inflammatory cells infiltration in the base of primary lamella were the most common histopathological signs of gills. In the eye sections, congestion, hemorrhage and degeneration of the retinal layers were also observable. These histopathological findings show that juvenile Persian sturgeon is a highly susceptible species to S. iniae thus, raising a high risk of streptococcosis outbreaks in sturgeon aquaculture either in freshwater or marine environments.