Multifactorial Causes of Chronic Mortality in Juvenile Sturgeon (Huso huso)

Simple Summary Sturgeons are species of biological and economic importance. Among them, the beluga sturgeon (Huso huso, Linnaeus, 1758) is the largest freshwater fish and can reach a maximum weight of 1000 kg. Over 80% of existing sturgeon species are endangered, vulnerable, or on the brink of extinction because of their late sexual maturity and long periods between spawning in the wild. Many species of sturgeon are being reared to increase natural populations or produced for consumption as human food. Despite considerable global interest in sturgeon aquaculture, there is still a paucity of information on nutrient requirements and utilization and even their general nutrition. The development of sturgeon aquaculture has been accompanied by an increase in disease outbreaks, representing a serious problem. This paper presents an investigation of chronic mortality outbreak. The clinical presentation of diseased sturgeons, which is suggestive of a neurologic condition, was further investigated using various laboratory analyses that excluded the presence of the main viral agents of sturgeons while confirming the presence of both bacteria and mycetes. The pathological findings point out the severe myopathy, depletion of lymphohematopoietic tissue, and signs of degeneration consistent with a suboptimal environmental state. Results from chemical analyses also suggest nutritional imbalance. This investigation focused on an episode of chronic mortality observed in juvenile Huso huso sturgeons. The examined subjects underwent pathological, microbiological, molecular, and chemical investigations. Grossly severe body shape deformities, epaxial muscle softening, and multifocal ulcerative dermatitis were the main observed findings. The more constant histopathologic findings were moderate to severe rarefaction and disorganization of the lymphohematopoietic lymphoid tissues, myofiber degeneration, atrophy and interstitial edema of skeletal epaxial muscles, and degeneration and atrophy of the gangliar neurons close to the myofibers. Chemical investigations showed a lower selenium concentration in affected animals, suggesting nutritional myopathy. Other manifestations were nephrocalcinosis and splenic vessel wall hyalinosis. Septicemia due to bacteria such as Aeromonas veronii, Shewanella putrefaciens, Citrobacter freundii, Chryseobacterium sp., and pigmented hyphae were found. No major sturgeon viral pathogens were detected by classical methods. Next-generation sequencing (NGS) analysis confirmed the absence of viral pathogens, with the exception of herpesvirus, at the order level; also, the presence of Aeromonas veronii and Shewanella putrefaciens was confirmed at the family level by the metagenomic classification of NGS data. In the absence of a primary yet undetected biological cause, it is supposed that environmental stressors, including nutritional imbalances, may have led to immune system impairment, facilitating the entry of opportunistic bacteria and mycotic hyphae.