Susceptibility of common carp and sunfish to a strain of Francisella noatunensis subsp. orientalis in a challenge experiment

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.

Pathological and Tissue-Based Molecular Investigation of Granulomas in Cichlids Reared as Ornamental Fish

Simple Summary

The global ornamental fish trade has an estimated value of USD 15–30 billion per year and more than a 10% average annual growth. Despite their economic importance, the management of ornamental fish is challenged by a paucity of information, including data on the fish health status. Pathological and microbiological investigations were conducted on ornamental cichlids sampled during routine management activities held at an aquarium commercial facility, in order to evaluate the presence of granuloma in the organs. Cutibacterium acnes and Mycobacterium spp. were detected by molecular methods and immunohistochemistry. These bacteria represent potential zoonotic agents, and the advancement of their knowledge could significantly improve the management of ornamental fish and reduce the risk of exposure for people, such as hobbyists, fish handlers, aquarists, and dedicated personnel.

Abstract

Cichlids include hundreds of species with a high economic value for aquaculture. These fish are subjected to intensive trade and farming that expose them to the risk of infectious diseases. This work focuses on ornamental cichlids held in an aquarium commercial facility presenting emaciation, in order to evaluate the presence of lesions in fish skin and organs. The fish were sampled during routine management activities and subjected to pathological and molecular investigations. The presence of lymphocystis disease virus, typically associated with cutaneous nodular disease, was ruled out. Histologically, they presented granulomas in the spleen, sometimes extending to the other visceral organs. Bacterial heat-shock protein 65 PCR products were detected in tissues associated, in the majority of cases, with granulomas; molecular investigation identified Mycobacterium spp. in two cases and Cutibacterium acnes in seven cases. Immunoreactivity to anti-Mycobacterium and anti-C. acnes antibodies was detected within granulomas. The presence of C. acnes within granuloma is elucidated for the first time in fish; however, similarly to what is found in humans, this bacterium could be harmless in normal conditions, whereas other contributing factors would be required to trigger a granulomatogenous response. Further confirmation by bacterial culture, as well as using large-scale studies in more controlled situations, is needed.

Francisella and Antibodies

Immune responses to intracellular pathogens depend largely upon the activation of T helper type 1-dependent mechanisms. The contribution of B cells to establishing protective immunity has long been underestimated. Francisella tularensis, including a number of subspecies, provides a suitable model for the study of immune responses against intracellular bacterial pathogens. We previously demonstrated that Francisella infects B cells and activates B-cell subtypes to produce a number of cytokines and express the activation markers. Recently, we documented the early production of natural antibodies as a consequence of Francisella infection in mice. Here, we summarize current knowledge on the innate and acquired humoral immune responses initiated by Francisella infection and their relationships with the immune defense systems.

Updates on Selected Emerging Infectious Diseases of Ornamental Fish

Emerging infectious diseases of ornamental fish are discussed with special focus on clinical relevance, detection, and treatment, where applicable. Important emerging infectious diseases of fish include goldfish herpesvirus, koi herpesvirus, carp edema virus, Erysipelothrix, Edwardsiella ictaluri, Edwardseilla piscicida, and Francisella. Some diseases are more species or genus specific, but many emerging diseases do not seem to have a species preference and affect a variety of species worldwide. Proper husbandry and biosecurity with a disease detection plan for ornamental fish is essential to monitor and prevent future outbreaks.

Comparison of the pathogenicity of Francisella orientalis in Nile tilapia (Oreochromis niloticus), Asian seabass (Lates calcarifer) and largemouth bass (Micropterus salmoides) through experimental intraperitoneal infection

Francisella orientalis is a highly virulent, emerging bacterium that causes mass mortalities in tilapia. This pathogen also affects numerous other warm-water fish species, including three-line grunt, hybrid striped bass and various ornamental fish. This study sheds light on two new species of fish that are susceptible to F. orientalis. Asian seabass and largemouth bass showed variable levels of susceptibility in a bacterial challenge experiment. After intraperitoneally injected with a dose of 10⁶  CFU/fish, a total of 64.28% and 21.42% mortalities were obtained in Asian seabass and largemouth bass, respectively. Meanwhile, Nile tilapia showed acute mortality of 100%. All fish showed typical lesions of francisellosis, including multifocal granulomas in the spleen and head kidney. Immunohistochemical analysis revealed strong positive signals inside the granulomas of all fish. The bacterial recovery in solid media from infected fish was highest in Nile tilapia (85.71%), followed by Asian seabass (35.71%) and largemouth bass (21.42%). PCR results tested 100% positive for Nile tilapia, and 78.57% and 21.42% for Asian seabass and largemouth bass, respectively. In conclusion, Asian seabass and largemouth bass are susceptible to this pathogen, which warrants new management strategies when employing predation polyculture systems of these species with tilapia.

Susceptibility and immune responses after challenge with Flavobacterium columnare and Pseudomonas fluorescens in conventional and specific pathogen-free rare minnow (Gobiocypris rarus)

The susceptibility of fish from different culture environments to bacterial infection is not well known. The susceptibility and pathological changes of conventional (CV) and specific pathogen-free (SPF) rare minnow (Gobiocypris rarus) infected with two gram-negative bacteria, Flavobacterium columnare and Pseudomonas fluorescens are investigated. Rare minnows were intraperitoneally challenged with two bacterial species to first determine semi-lethal doses (LD₅₀), and then with the LD₅₀ dose, determine innate immune response. Infected rare minnows developed characteristic red bellies and then died. LD₅₀ doses of F. columnare and P. fluorescens were 4.586 × 10⁸ cfu/mL and 2.319 × 10¹⁰ cfu/mL for CV rare minnow, and 2.575 × 10⁸ cfu/mL and 1.935 × 10¹⁰ cfu/mL, respectively, for SPF rare minnow. The results of RT-PCR showed that the highest levels of toll-like receptor 3 (TLR3), interleukin-6 (IL-6), interferon-2 (IFN-2) and rare minnow Z-DNA binding protein kinase (GrPKZ) mRNA were noticed at 6-48 h post-infection (hpi). In addition, TLR3, IL-6 and IFN-2 in F. columnare challenged rare minnow were more highly expressed than those in P. fluorescens challenged rare minnow, whereas as opposed in the expression of GrPKZ mRNA. Stimulation of innate immune responses is closely related to bacterial virulence. SPF rare minnow might be more susceptible to these bacteria than CV rare minnow, possibly due to their clean environment and lack of resistance. We speculate that clean environment renders rare minnow more susceptible to bacterial infections.