The Old and the New on Viral Diseases in Sturgeon

Leiomyoma Causing Disease in a Wild Adriatic Sturgeon (Acipenser naccarii , Bonaparte, 1836)

The Adriatic sturgeon, Acipenser naccarii, is an endemic species to the Adriatic Sea basin classified as critically endangered according to the IUCN Red List. Limited information is available regarding the health status and diseases affecting this species, particularly in the wild. In September 2023, a post-spawning female Adriatic sturgeon was found along the edges of the Adige River. The fish displayed abnormal swimming, cutaneous haemorrhages, extreme emaciation and died shortly after its finding. At necropsy, the stomach appeared enlarged with a firm texture and a severe restriction of the lumen due to the thickening of the gastric wall. A gastric leiomyoma was characterised by histology, and this represents the first description of such neoplasia in A. naccarii. In the intestine, a severe infestation with roundworm belonging to the phylum Acanthocephala was responsible for severe haemorrhagic enteritis. The parasites were identified as belonging to the species Leptorhynchoides polycristatus, providing the first description of such a parasite in Italy. Finally, genetic analyses were performed in order to assess genetic purity and relatedness with captive breeding stocks. The results showed that the individual did not cluster with any captive identified groups, supporting the likelihood of a pure and wild-origin Adriatic sturgeon.

Types and structural analysis of interferons from four kinds of sturgeon

Interferons (IFNs) play a significant role in the immune regulation of vertebrates, and three types (IFN-I, IFN-II, and IFN-IV) have been identified in bony fish. This study investigated the IFN genes in Acipenser sinensis, Acipenser ruthenus, and Acipenser oxyrinchus from the Acipenseridae, as well as in Polyodon spathula from the Polyodontidae. Using a series of bioinformatics methods, three IFN-I subgroups were identified: IFNb, IFNe, and IFNf. A conserved structure of scn4a/IFN-I/arhgap27 was found in the IFN-I gene cluster, and the IFN-I genes were tandemly arranged on the chromosomes in a 4C-2C pattern. Notably, unlike other bony fish, the sturgeon IFNf lacked the third cysteine residue, allowing for the formation of only one S-S bridge. Furthermore, IFN-II (IFNγ) and IFN-IV (IFNυ) were also identified in sturgeon, with IFN-IV being identified for the first time in sturgeon. Phylogenetic tree and sequence analyses revealed a high degree of homology among IFN genes across different families within the Acipenseriformes, suggesting that these genes have remained evolutionarily conserved. The systematic identification of sturgeon IFNs has provided resources for future studies on the antiviral immune response in sturgeon.

First report of ictavirus acipenseridallo2 (AciHV-2) in Danube sturgeons Acipenser gueldenstaedtii in Austria

Newly stocked Danube sturgeons Acipenser gueldenstaedtii developed cutaneous lesions and nearly 100% mortality over the course of 2 mo after introduction into an Austrian fish farm. Necropsy revealed cutaneous plaques and hemorrhages, and histological findings in skin, gills, spleen and kidney tissues showed cell-nucleus alterations consistent with infection by a herpesvirus. The presence of a herpesvirus was demonstrated by the visualization of numerous typical viral particles in different tissues by electron microscopy. A newly developed conventional PCR protocol, targeting a fragment of the viral DNA polymerase gene, further confirmed the presence of a virus related to the species Ictavirus acipenseridallo2 (formerly Acipenserid herpesvirus-2; AciHV-2) in the diseased fish. Amplification products were sequenced and showed 100% identity to the Siberian sturgeon herpesvirus (SbSHV) strain. This is the first report of herpesvirus detection in sturgeon in Austria and of SbSHV, a strain of AciHV-2, in Danube sturgeons.

Multiple cutaneous nerve sheath tumours with myxoid differentiation in farmed Russian sturgeons (Acipenser gueldenstaedtii, Brandt and Ratzeburg 1833)

Sturgeon species are well-suited for aquaculture because of their favourable characteristics, including robustness, suitability for farming in facilities unsuitable for other fish species, and adaptability to diverse farming conditions. The Russian sturgeon (Acipenser gueldenstaedtii, Brandt and Ratzeburg 1833) is one of the most prominent farmed species; however, like other aquaculture species, it is susceptible to significant losses from bacterial and viral diseases. Beyond infectious causes, there are few reports documenting conditions that produce cutaneous masses in Russian sturgeons. This study presents a multidisciplinary investigation of six farmed Russian sturgeons exhibiting discrete, multiple cutaneous masses. Bacteriological analysis of tissue samples revealed the presence of Morganella morganii and Aeromonas veronii biovar sobria, identified as opportunistic bacteria. Virological assays targeting the principal viruses affecting sturgeon, Acipenser iridovirus and Acipenser herpesvirus, yielded negative results. Ultrastructural analysis with direct negative staining revealed no evidence of biological agents. Histologically, the dermal masses were well-demarcated, expansile, and moderately cellular, consisting of spindle-to-stellate neoplastic cells that were multifocally periodic acid-Schiff-positive and embedded in abundant alcianophilic ground substance. Immunohistochemistry with the S-100 antibody confirmed cytoplasmic staining of the neoplastic cells. A final diagnosis of cutaneous nerve sheath tumour with myxoid differentiation was made, replicating findings from a similar tumour in rainbow trout. To the best of our knowledge, this represents the first description of multiple cutaneous nerve sheath tumours in sturgeon species. The potential factors contributing to the development of this neoplastic condition are discussed.

Effects of Citrobacter freundii on sturgeon: Insights from skin mucosal immunology and microbiota

Skin mucus analysis has recently been used as a non-invasive method to evaluate for fish welfare. The present research study was conducted to examine the skin mucosal immunity and skin microbiota profiles of sturgeons infected with Citrobacter freundii. Our histology results showed that the thickness of the epidermal layer of skin remained thinner, and the number of mucous cells was significantly decreased in sturgeons after infection (p < 0.05). Total protein, alanine aminotransferase, aspartate aminotransferase, superoxide dismutase, and creatine kinase levels in the mucus showed biphasic pattern (decrease and then increase). Lactate dehydrogenase, lysozyme, and acid phosphatase activities in the mucus showed an increasing trend after infection. Furthermore, 16S rRNA sequencing also revealed that C. freundii infection also affected the diversity and community structure of the skin mucus microbiota. An increase in microbial diversity (p > 0.05) and a decrease in microbial abundance (p < 0.05) after infection were noted. The predominant bacterial phyla in the skin mucus were Proteobacteria, Fusobacteria, Bacteroidetes, Firmicutes, and Actinobacteria. Specifically, the relative abundance of Fusobacteria increased after infection. The predominant bacterial genera in the skin mucus were Cetobacterium, Pelomonas, Bradyrhizobium, Flavobacterium, and Pseudomonas. The relative abundance of Cetobacterium, Pseudomonas, and Flavobacterium increased after infection. Our current research findings will provide new insights into the theoretical basis for future research studies exploring the mechanism of sturgeon infection with C. freundii.

Mechanism of Chinese sturgeon IFN-γ inhibition on Mycobacterium marinum (Acipenser sinensis)

IFN-γ plays a crucial role in both innate and adaptive immune responses and is a typical Th1 cytokine that promotes Th1 response and activates macrophages. When macrophages were incubated with IFN-γ, their phagocytosis ratio against Mycobacterium marinum increased significantly, as observed under fluorescence microscopy. The macrophages engulfed a large number of M. marinum. The proliferative ability of macrophages treated with IFN-γ was significantly weaker on the 4th and 7th day after phagocytosis and subsequent re-infection with marine chlamydia (P < 0.001). This suggests that IFN-γ enhances the phagocytosis and killing ability of macrophages against M. marinum. IFN-γ protein also significantly increased the production of reactive oxygen species (H2O2) and nitric oxide (NO) by macrophages. Additionally, the expression levels of toll-like receptor 2 (tlr2) and caspase 8 (casp8) were significantly higher in macrophages after IFN-γ incubation compared to direct infection after 12 h of M. marinum stimulation. Apoptosis was also observed to a higher degree in IFN-γ incubated macrophage. Moreover, mRNA expression of major histocompatibility complex (MHC) molecules produced by macrophages after IFN-γ incubation was significantly higher than direct infection. This indicates that IFN-γ enhances antigen presentation by upregulating MHC expression. It also upregulates tlr2 and casp8 expression through the TLR2 signaling pathway to induce apoptosis in macrophages. The pro-inflammatory cytokine showed an initial increase followed by a decline, suggesting that IFN-γ enhances the immune response of macrophages against M. marinum infection. On the other hand, the anti-inflammatory cytokine showed a delayed increase, significantly reducing the expression of pro-inflammatory cytokines. The expression of both cytokines balanced each other and together regulated the inflammatory reaction against M. marinum infection.

Detection of Acipenser European Iridovirus (AcIV-E) in Sturgeon Farms in Northern Italy between 2021-2023

Sturgeon farming is rapidly expanding in Europe, where Italy ranks first in farmed caviar production. A major threat to sturgeon health in captivity is infection with Acipenser European Iridovirus (AcIV-E), a viral disease definitively identified in 2016. Here we present data on the occurrence of AcIV-E in 482 sturgeons (age ≤ 12 months, species of the genus Acipenser and the species Huso huso) collected from sturgeon farms in northern Italy between January 2021 and December 2023. The health status of each specimen was determined by necroscopy and virological assay. Virological analysis was performed on gill samples and real-time PCR specific to the MCP gene of the iridovirus viral capsid. Molecular analysis revealed positivity to the virus in 204 samples (42.68% of the total), while anatomopathological examination of nearly all fish with positive real-time PCR disclosed swollen abdomen, hepatic steatosis, splenomegaly, and increased gill volume. Two challenges to timely diagnosis are the absence of pathognomonic symptoms and the inability to isolate the virus on cell monolayers. Continuous and widespread health monitoring is therefore crucial for disease management and to effectively control spread of the virus.

Functional characterization of group Ⅱ interferon, IFNf in the acipenseriform fish, Chinese sturgeon (Acipenser sinensis)

Teleost fish possess a diversity of type Ⅰ interferons (IFNs) repertoire, which play a crucial role in antiviral and antimicrobial immune responses. In our previous study, IFNe1-3 and IFNb were identified and cloned from Chinese sturgeon (Acipenser sinensis), an acipenseriform fish. However, the absence of Chinese sturgeon genome data has left the question of whether there are other type Ⅰ IFN members in this species unresolved. In this study, we have identified and characterized a novel IFN, IFNf in Chinese sturgeon (AsIFNf). Bioinformatics analysis revealed that the AsIFNf contains a unique disulfide bond (2 cysteines) located in the second exon and fifth exon region, distinguishing it from other reported teleost type I IFNs. Meanwhile, qPCR results showed that AsIFNf mRNA was detectable in all examined tissues and up-regulated in the spleen or kidney in response to poly I: C, Citrobacter freundii, and Spring Viremia of Carp Virus (SVCV), but not by LPS. Furthermore, compared to recombinant AsIFNe2 protein (rAsIFNe2), rAsIFNf exhibited a stronger protective effect on Chinese sturgeon fin cells against SVCV and also induced higher expression of antiviral genes Mx and viperin. Importantly, AsIFNf displayed characteristics similar to antimicrobial peptides (AMPs) with a positive charge and demonstrated a broad spectrum of antimicrobial activity in vitro. These findings provide a theoretical foundation for understanding the primitive structure and function of interferon, as well as deepening our comprehension of the innate immune system and disease defense in the endangered Chinese sturgeon.

Innate immune and chronic heat stress responses in sturgeons: Advances and insights from studies on Russian sturgeons

Chronic stress deteriorates the immune function of fish, thereby increasing their vulnerability to infections. However, the molecular and cellular mechanisms underlying stress-mediated immunosuppression and infection susceptibility in fish remain largely unknown. Understanding these mechanisms will contribute to improving fish welfare and their farm production. Herein, we review the challenges of sturgeon aquaculture in subtropical countries, where current climate change has giving rise to significant temperature increments during summer. This leads to the exposure of fish to stressful conditions during these months. Chronic heat stress deserves attention considering the rapid warming rate of the planet. It is already affecting wild fish populations, with disastrous consequences for sturgeons, which are one of the most endangered fish species in the world. In this context, we discuss the most recent advances through the studies on the effects of chronic heat stress on the innate immune components of sturgeons. To this end, we summarise the findings of studies focusing on the aquaculture of Russian sturgeons and observations made on other Acipenser species. Special attention is given to acute-phase proteins, as they might be valuable biomarkers of heat stress and infection, with applicability in monitoring the fish health status in farms.

An Outbreak of Aeromonas salmonicida in Juvenile Siberian Sturgeons (Acipenser baerii)

Aeromonas salmonicida is one of the major threats to world aquaculture, causing fish furunculosis and high mortality rates in cultured fish, particularly salmonids. Although Aeromonas spp. is a thoroughly studied pathogen, little is known regarding aeromoniasis in sturgeons. After a mortality outbreak, four juvenile sturgeons (Acipenser baerii) were submitted for autopsy and tissue samples were collected for histopathological and microbiological studies. The external examination revealed size heterogenicity, skin hyperpigmentation and reduced body condition of sturgeons. Within the abdominal cavity, mild hepatomegaly and splenomegaly were observed, as well as generalized organic congestion. Histology revealed severe multifocal haemorrhagic and ulcerative dermatitis, mainly localized in the dorsal and latero-ventral areas of fish. The histological study also showed moderate to severe inflammation of gills and organic lesions compatible with septicaemia. Bacterial isolates were identified as Aeromonas salmonicida subsp. salmonicida using MALDI-TOF MS and PCR. Overall, the lesions first described here are consistent with those previously reported in other cultured fish species and contribute to a better understanding of the pathogenesis of Aeromonas salmonicida subsp. salmonicida in the Siberian sturgeon, aside from providing new diagnostic tools for bacterial diseases impacting the fast-growing industry of caviar.

Susceptibility of Pallid Sturgeon to viral hemorrhagic septicemia virus genotype IVb

OBJECTIVE

Viral hemorrhagic septicemia virus (VHSV) is an aquatic rhabdovirus causing severe disease in freshwater and saltwater fish species. The susceptibility of endangered Pallid Sturgeon Scaphirhynchus albus to VHSV genotype IVb (VHSV-IVb) infection was investigated.

METHODS

An in vitro assessment using two Pallid Sturgeon cell lines derived from skin and spleen tissue and in vivo evaluation of juvenile Pallid Sturgeon after exposure to VHSV-IVb were performed.

RESULT

Plaque assay and RT-PCR results confirmed VHSV-IVb replication in Pallid Sturgeon cell lines. Sturgeon were also susceptible to VHSV-IVb infection after immersion and injection exposures during laboratory experiments. However, after widespread mortality occurred in all treatment groups, including negative control fish, it was determined that the Pallid Sturgeon stock fish were infected with Missouri River sturgeon iridovirus (MRSIV) prior to experimental challenge. Nevertheless, mortalities were equal or higher among VHSV-exposed fish than among negative controls (MRSIV infected), and histopathological assessments indicated reduced hematopoietic cells in spleen and kidney tissues and hemorrhage in the gastrointestinal organs only in fish from the VHSV treatment.

CONCLUSION

These results indicate that Pallid Sturgeon is a susceptible host for VHSV-IVb, but the degree of pathogenicity was confounded by the underlying MRSIV infection. Research comparing susceptibility of specific pathogen-free and MRSIV-infected fish to VHSV-IVb is needed to accurately assess the vulnerability of Pallid Sturgeon to VHSV-IVb.

First Isolation of a Herpesvirus (Family Alloherpesviridae) from Great Lakes Lake Sturgeon (Acipenser fulvescens)

The lake sturgeon (Acipenser fulvescens; LST) is the only native sturgeon species in the Great Lakes (GL), but due to multiple factors, their current populations are estimated to be <1% of historical abundances. Little is known about infectious diseases affecting GL-LST in hatchery and wild settings. Therefore, a two-year disease surveillance study was undertaken, resulting in the detection and first in vitro isolation of a herpesvirus from grossly apparent cutaneous lesions in wild adult LST inhabiting two GL watersheds (Erie and Huron). Histological and ultrastructural examination of lesions revealed proliferative epidermitis associated with herpesvirus-like virions. A virus with identical ultrastructural characteristics was recovered from cells inoculated with lesion tissues. Partial DNA polymerase gene sequencing placed the virus within the Family Alloherpesviridae, with high similarity to a lake sturgeon herpesvirus (LSHV) from Wisconsin, USA. Genomic comparisons revealed ~84% Average Nucleotide Identity between the two isolates, leading to the proposed classification of LSHV-1 (Wisconsin) and LSHV-2 (Michigan) for the two viruses. When naïve juvenile LST were immersion-exposed to LSHV-2, severe disease and ~33% mortality occurred, with virus re-isolated from representative skin lesions, fulfilling Rivers’ postulates. Results collectively show LSHV-2 is associated with epithelial changes in wild adult LST, disease and mortality in juvenile LST, and is a potential threat to GL-LST conservation.

The conservation paradox of critically endangered fish species: Trading alien sturgeons versus native sturgeon reintroduction in the Rhine-Meuse river delta

Sturgeons rank among the most endangered vertebrates in the world. Yet, the dwindling of wild sturgeon populations stands in stark contrast to their thriving status in aquaculture. Moreover, through the exotic pet trade, sturgeons are introduced outside their natural ranges where they may compete and hybridize with native species and transmit parasites and diseases. Here, we present an in-depth inventory of alien sturgeons in the delta of the rivers Rhine and Meuse, because several countries consider reintroduction of the native, critically endangered European sturgeon (Acipenser sturio). Our study is based on (a) an inventory of the industry of sturgeon cultivation; (b) reports on spread of alien sturgeons; (c) an analysis of pathways for introduction and spread; and (d) a risk assessment using the Harmonia⁺ protocol. In total, 11 alien Acipenseriformes (sturgeons and paddlefishes) were traded across an intricate network of >1000 distribution points in the Netherlands, Belgium, and Germany. Circa 2500 alien sturgeons were reported from 53 angling ponds and 64 other lakes and ponds, whereas circa 500 alien sturgeons were reported widespread across hydrologically connected waters. Species that posed the highest risk of introduction, establishment and spread are Siberian sturgeon (A. baerii), Russian sturgeon (A. gueldenstaedtii) and Sterlet (A. ruthenus). We recommend to implement stringent trade regulations and practical solutions to prevent spread of alien sturgeons. Measures must preferably be taken at the spatial scale of river basins.

Emerging Viral Pathogens in Sturgeon Aquaculture in Poland: Focus on Herpesviruses and Mimivirus Detection

Recently, Poland has become a leading producer of sturgeon meat and caviar in Europe and is one of the largest in the world. The growing importance of this branch of aquaculture means that diseases of these fish, especially viral ones, are becoming the object of interest for ichthyopathologists. In recent years, there have been increasing reports of health problems in the dynamically developing sturgeon farming. The greatest risk appears to be emerging infectious diseases that are caused by viruses and that can become a serious threat to the development of the aquaculture industry and the success of sturgeon restitution programs undertaken in many European countries, including Poland. In this paper, an attempt was made to determine the spread of the two most important groups of viruses in Polish sturgeon farming: These include the herpesviruses and sturgeon nucleocytoplasmic large DNA viruses (sNCLDV), in particular, mimiviruses. In the years 2016–2020, 136 samples from nine farms were collected and tested by using the WSSK-1 cell line, PCR and Real Time PCR methods. All results were negative for herpesviruses. Out of the samples, 26% of the samples have been tested positive for mimiviruses. Sanger sequencing of mimiviruses demonstrated their affiliation with AciV-E. The sequence characterization confirmed the presence of both V1 and V2 lineages in Polish fish facilities, but variant V2 seems to be more widespread, as is observed in other European countries.

Phaeohyphomycosis due to Veronaea botryosa in cultured white sturgeon (Acipenser transmontanus Richardson) from California USA during 2006 to 2015

Infection with Veronaea botryosa can result in rare cutaneous or disseminated, granulomatous to pyogranulomatous phaeohyphomycosis in humans, although disease due to the fungus has also been reported in non-mammalian vertebrates. This report documents disease due to V. botryosa in captive, juvenile to subadult or young adult white sturgeon (Acipenser transmontanus Richardson) from California USA and complements a previous report of the disease in captive Siberian sturgeon (Acipenser baerii) from Florida USA. Pathological examinations revealed granulomatous to pyogranulomatous inflammation of multiple organs. Isolates of the fungal agent were phenotypically consistent with V. botryosa, and molecular analyses of the D1/D2 region of the fungal 28S rRNA gene and the internal transcribed spacer (ITS) region located between the fungal 18S and 28S rRNA genes confirmed the aetiologic agent as V. botryosa. The disease in captive sturgeon results in a considerable economic encumbrance to the producer due to the loss of the cumulative financial resources invested in the production of older subadult to young adult sturgeon.

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

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.