Diagnosis of piscine francisellosis in Largemouth Bass from a public display exhibit in north-central Florida, USA

OBJECTIVE

The Largemouth Bass Micropterus salmoides is an important freshwater fish that is native to the southeastern United States and is cultured for conservation, food, and for the sports fishing industry. Francisella orientalis is a globally distributed bacterial pathogen of warmwater fish species and is associated with granulomatous inflammation and high mortalities. Outbreaks of piscine francisellosis in the United States have been reported in only a few fish species. This study describes three case presentations of francisellosis in Largemouth Bass from a public display system in north-central Florida. Additionally, laboratory-controlled immersion challenges using an F. orientalis isolate from tilapia Oreochromis spp. evaluate susceptibility of Largemouth Bass fingerlings to F. orientalis infection and mortality through this exposure route.

METHODS

Necropsy, histologic examination, immunohistochemistry, bacterial recovery and culture, and quantitative polymerase chain reaction were used as diagnostic tools to evaluate both the affected display fish and the immersion-challenged fingerlings.

RESULT

Although the display fish and immersion-challenged fingerlings presented with nonspecific clinical signs, gross and histological changes were indicative of granulomatous disease. Immunohistochemical and molecular testing methods confirmed F. orientalis infection in affected fish.

CONCLUSION

The three case presentations described here mark the first reporting of naturally occurring piscine francisellosis in Largemouth Bass that were held in a public display exhibit. Additionally, causality was proven in the Largemouth Bass fingerlings through the immersion challenges. These findings demonstrate susceptibility through immersion-based exposure and assert that francisellosis should be considered among the list of differential diagnoses for Largemouth Bass with granulomatous disease.

Nocardia seriolae mediates liver granulomatous chronic inflammation in Micropterus salmoides through pyroptosis

Granulomatous diseases caused by Nocardia seriously endanger the health of cultured fish. These bacteria are widely distributed, but prevention and treatment methods are very limited. Chronic granulomatous inflammation is an important pathological feature of Nocardia infection. However, the molecular mechanisms of granuloma formation and chronic inflammation are still unclear. Constructing a granuloma infection model of Nocardia is the key to exploring the pathogenesis of the disease. In this study, we established a granuloma model in the liver of largemouth bass (Micropterus salmoides) and assessed the infection process of Nocardia seriolae at different concentrations by analysing relevant pathological features. By measuring the expression of pro-inflammatory cytokines, transcription factors and a pyroptosis-related protein, we revealed the close relationship between pyroptosis and chronic inflammation of granulomas. We further analysed the immunofluorescence results and the expression of pyroptosis-related protein of macrophage infected by N. seriolae and found that N. seriolae infection induced macrophage pyroptosis in vitro. These results were proved by flow cytometry analysis of infection experiment in vivo. Our results indicated that the pyroptosis effect may be the key to inducing chronic inflammation in the fish liver and further mediating granuloma formation. In this study, we explored the molecular mechanism underlying chronic inflammation of granulomas and developed research ideas for understanding the occurrence and development of granulomatous diseases in fish.

Potential virulence factors of Nocardia seriolae AHLQ20-01 based on whole-genome analysis and its pathogenicity to largemouth bass (Micropterus salmoides)

Nocardia seriolae is a major causative agent of fish nocardiosis that results in serious economic losses in the aquaculture industry. However, the virulence factors and pathogenic mechanisms of the bacterium are poorly understood. Here, a new N. seriolae strain AHLQ20-01 was isolated from the diseased Micropterus salmoides and identified by phenotypic examination combined with 16S rRNA sequencing. Subsequently, the potential virulence factors of the strain were analysed at genome level by whole-genome sequencing. The results showed that the whole-genome sequence derived from N. seriolae AHLQ20-01 circular chromosome contains 8,129,380 bp DNA with G + C content of 68.14%, and encompasses 7650 protein-coding genes, 114 pseudo-genes, 3 rRNAs, 66 tRNAs and 36 non-coding RNAs. More importantly, a total of 139 genes, which mainly involved in adhesion, invasion, resistance to oxidative and nitrosative stress, phagosome arresting, iron acquisition system, toxin production and bacterial secretion systems, were identified as core virulence-associated genes. Furthermore, the pathogenicity of N. seriolae AHLQ20-01 to M. salmoides was further investigated through experimental infection. It was found that the LD₅₀ value of the strain to M. salmoides was 9.3 × 10⁶  colony forming unit/fish. Histopathological examination demonstrated typical granuloma with varying sizes in the liver, head kidney, spleen and heart of the experimentally infected fish. Terminal deoxynucleotidyl transferase dUTP nick end labelling assay and 4',6-diamidino-2-phenylindole staining showed that there were distinctly more apoptotic cells in all the tested tissues in the infection group, but not in the control group. Together, these findings provide the foundation to further explore the pathogenic mechanism of N. seriolae, which might contribute to the prevention and treatment of fish nocardiosis.

The recombinant subunit vaccine encapsulated by alginate-chitosan microsphere enhances the immune effect against Micropterus salmoides rhabdovirus

The disease caused by Micropterus salmoides rhabdovirus (MSRV) has brought substantial economic losses to the largemouth bass aquaculture industry in China. Vaccination was considered as a potential way to prevent and control this disease. As a kind of sustained and controlled release system, alginate and chitosan microspheres (SA-CS) are widely used in the development of oral vaccination for fish. Here, we prepared a king of alginate-chitosan composite microsphere to encapsulate the second segment of MSRV glycoprotein (G2 protein) and then evaluated the immune effect of the microsphere vaccine on largemouth bass. Largemouth bass were vaccinated via intragastric immunization by different treatments (PBS, SA-CS, G2 and SA-CS-G2). The results showed that a stronger immune response including serum antibody levels, immune-related physiological indexes (acid phosphatase, alkaline phosphatase, superoxide dismutase and total antioxidant capacity) and the expression of immune-related gene (IgM、IL-8、IL-1β、CD4、TGF-β、TNF-α) can be induced obviously with SA-CS-G2 groups compared with G2 groups when fish were vaccinated. Furthermore, fish were injected with a lethal dose of MSRV after immunization for 28 days, and the highest relative percentage survival (54.8%) was observed in SA-CS-G2 group (40 μg per fish), which is significantly higher than that of G2 group (25.8%). This study showed that alginate-chitosan microspheres as the vaccine carrier can effectively improve the immune effect of oral vaccination and induce better immune protection effect against MSRV infection.

Projected shifts in fish species dominance in Wisconsin lakes under climate change

Temperate lakes may contain both coolwater fish species such as walleye (Sander vitreus) and warmwater fish species such as largemouth bass (Micropterus salmoides). Recent declining walleye and increasing largemouth bass populations have raised questions regarding the future trajectories and management actions for these species. We developed a thermodynamic model of water temperatures driven by downscaled climate data and lake-specific characteristics to estimate daily water temperature profiles for 2148 lakes in Wisconsin, US, under contemporary (1989-2014) and future (2040-2064 and 2065-2089) conditions. We correlated contemporary walleye recruitment and largemouth bass relative abundance to modeled water temperature, lake morphometry, and lake productivity, and projected lake-specific changes in each species under future climate conditions. Walleye recruitment success was negatively related and largemouth bass abundance was positively related to water temperature degree days. Both species exhibited a threshold response at the same degree day value, albeit in opposite directions. Degree days were predicted to increase in the future, although the magnitude of increase varied among lakes, time periods, and global circulation models (GCMs). Under future conditions, we predicted a loss of walleye recruitment in 33-75% of lakes where recruitment is currently supported and a 27-60% increase in the number of lakes suitable for high largemouth bass abundance. The percentage of lakes capable of supporting abundant largemouth bass but failed walleye recruitment was predicted to increase from 58% in contemporary conditions to 86% by mid-century and to 91% of lakes by late century, based on median projections across GCMs. Conversely, the percentage of lakes with successful walleye recruitment and low largemouth bass abundance was predicted to decline from 9% of lakes in contemporary conditions to only 1% of lakes in both future periods. Importantly, we identify up to 85 resilient lakes predicted to continue to support natural walleye recruitment. Management resources could target preserving these resilient walleye populations.