Infections of Edwardsiella tarda among Brook Trout in Quebec

Expanding the Spectrum of Diseases and Disease Associations Caused by Edwardsiella tarda and Related Species

The genus Edwardsiella, previously residing in the family Enterobacteriaceae and now a member of the family Hafniaceae, is currently composed of five species, although the taxonomy of this genus is still unsettled. The genus can primarily be divided into two pathogenic groups: E. tarda strains are responsible for almost all human infections, and two other species (E. ictaluri, E. piscicida) cause diseases in fish. Human infections predominate in subtropical habitats of the world and in specific geospatial regions with gastrointestinal disease, bloodborne infections, and wound infections, the most common clinical presentations in decreasing order. Gastroenteritis can present in many different forms and mimic other intestinal disturbances. Chronic gastroenteritis is not uncommon. Septicemia is primarily found in persons with comorbid conditions including malignancies and liver disease. Mortality rates range from 9% to 28%. Most human infections are linked to one of several risk factors associated with freshwater or marine environments such as seafood consumption. In contrast, edwardsiellosis in fish is caused by two other species, in particular E. ictaluri. Both E. ictaluri and E. piscicida can cause massive outbreaks of disease in aquaculture systems worldwide, including enteric septicemia in channel catfish and tilapia. Collectively, these species are increasingly being recognized as important pathogens in clinical and veterinary medicine. This article highlights and provides a current perspective on the taxonomy, microbiology, epidemiology, and pathogenicity of this increasingly important group.

Pathology and virulence of Edwardsiella tarda, Edwardsiella piscicida, and Edwardsiella anguillarum in channel (Ictalurus punctatus), blue (Ictalurus furcatus), and channel × blue hybrid catfish

In the mid-2010s, Edwardsiella tarda was reaffiliated into three discrete taxa (E. anguillarum, E. piscicida, and E. tarda), obscuring previous descriptions of E. tarda-induced pathology in fish. To clarify ambiguity regarding the pathology of E. tarda, E. piscicida, and E. anguillarum infections in US farm-raised catfish, channel catfish (Ictalurus punctatus), blue catfish (I. furcatus), and channel × blue catfish hybrids were challenged with comparable doses of each bacterium. The most severe pathology and mortality occurred in fish challenged with E. piscicida, supporting previous reports of increased pathogenicity in commercially important ictalurids, while E. anguillarum and E. tarda warrant only minimal concern. Acute pathologic lesions among bacterial species were predominantly necrotizing and characteristic of gram-negative sepsis but became progressively granulomatous over time. After 100 days, survivors were exposed to the approximate median lethal doses of E. piscicida and E. ictaluri, revealing some cross-protective effects among E. piscicida, E. anguillarum, and E. ictaluri. In contrast, no fish that survived E. tarda challenge demonstrated any protection against E. piscicida or E. ictaluri. This work supports reports of increased susceptibility of channel, blue, and hybrid catfish to E. piscicida, while highlighting potential cross-protective affects among fish associated Edwardsiella spp.

Temperature induces metabolic reprogramming in fish during bacterial infection

Water temperature elevation as a consequence of global warming results in increased incidence of bacterial disease, such as edwardsiellosis, in fish farming. Edwardsiellosis is caused by the bacterial pathogen Edwardsiella tarda and affects many farmed fish including flounder (Paralichthys olivaceus). Currently, the effect of temperature on the metabolic response of flounder to E. tarda infection is unclear. In this study, we found that compared to low temperature (15°C), high temperature (23°C) enhanced E. tarda dissemination in flounder tissues. To examine the impact of temperature on the metabolism of flounder induced by E. tarda, comparative metabolomics were performed, which identified a large number of metabolites responsive to E. tarda invasion and temperature alteration. During E. tarda infection, the metabolic profile induced by elevated temperature was mainly featured by extensively decreased amino acids and TCA intermediates such as succinate, a proven immune regulator. Further, 38 potential metabolite markers of temperature effect (MMTE) in association with bacterial infection were identified. When used as exogenous supplements, two of the MMTE, i.e., L-methionine and UDP-glucose, effectively upregulated the expression of pro-inflammatory cytokines and suppressed E. tarda infection in flounder leukocytes. Taken together, the results of this study indicate an important influence of temperature on the metabolism of flounder during bacterial infection, which eventually affects the survivability of the fish.

Genetic characterization of heterologous Edwardsiella piscicida isolates from diverse fish hosts and virulence assessment in a Chinook salmon Oncorhynchus tshawytscha model

Edwardsiella piscicida is an emergent global fish pathogen with a wide host range, although host associations driving genetic diversity remain unclear. This study investigated the genetic and virulence diversity of 37 E. piscicida isolates recovered from 10 fish species in North America. Multilocus sequence analysis (MLSA) was conducted using concatenated alignments of the gyrB, pgi and phoU sequences. MLSA clustered the tested isolates into six discrete clades. In light of recent disease outbreaks in cultured salmonids, the virulence of each clade was evaluated in Chinook salmon Oncorhynchus tshawytscha fingerlings following intracoelomic challenge of ~10⁶  CFU/fish. Challenged and control fish were monitored for 21d, and microbiological and histological examination was performed on dead and surviving fish. Peak mortality occurred 3-5 days post-challenge (dpc) regardless of isolate or genetic group. Edwardsiella piscicida was recovered from all moribund and dead animals. At 21 dpc, fish challenged with isolates from clades II, III and IV presented cumulative mortality ≥83.3%, whereas isolates from clade I, V and VI resulted in cumulative mortality ≤71.4%. This study suggests an underlying genetic basis for strain virulence and potential host associations. Further investigations using other fish models and variable challenge conditions are warranted.

Systemic Edwardsiella tarda infection in a Western African lungfish (Protopterus annectens) with cytologic observation of heterophil projections

This report describes a case of systemic bacterial infection caused by Edwardsiella tarda in a Western African lungfish (Protopterus annectens) exposed to poor environmental and husbandry conditions. The fish presented with a large, external ulcerative lesion and died 2 weeks after developing anorexia. Histological evaluation revealed multifocal areas of necrosis and heterophilic and histiocytic inflammation throughout multiple tissues. Gram stain identified small numbers of intra- and extracellular monomorphic Gram-negative 1 to 2 μm rod-shaped bacilli. Cytology of lung granuloma, kidney and testes imprints identified heterophilic inflammation with phagocytosis of small monomorphic bacilli and some heterophils exhibiting cytoplasmic projections indicative of heterophil extracellular traps (HETs). Initial phenotypic analysis of isolates from coelomic fluid cultures identified E. tarda. Subsequent molecular analysis of spleen, liver and intestine DNA using an E. tarda-specific endpoint PCR assay targeting the bacterial fimbrial subunit yielded a 115 bp band. Sequencing and BLASTN search revealed the sequence was identical (76/76) to E. tarda strain FL95-01 (GenBank acc. CP011359) and displayed 93% sequence identity (66/71) to Edwardsiella hoshinae strain ATCC 35051 (GenBank acc. CP011359). This is the first report of systemic edwardsiellosis in a lungfish with concurrent cytologically identified structures suggestive of HETs.

Pathogenesis of and strategies for preventing Edwardsiella tarda infection in fish

Edwardsiella tarda is one of the serious fish pathogens, infecting both cultured and wild fish species. Research on edwardsiellosis has revealed that E. tarda has a broad host range and geographic distribution, and contains important virulence factors that enhance bacterial survival and pathogenesis in hosts. Although recent progress in edwardsiellosis research has enabled the development of numerous, highly effective vaccine candidates, these efforts have not been translated into a commercialized vaccine. The present review aims to provide an overview of the identification, pathology, diagnosis and virulence factors of E. tarda in fish, and describe recent strategies for developing vaccines against edwardsiellosis. The hope is that this presentation will be useful not only from the standpoint of understanding the pathogenesis of E. tarda, but also from the perspective of facilitating the development of effective vaccines.

Edwardsiellosis in fish: a brief review

Edwardsiellosis is one of the most important bacterial diseases in fish. Scientific work on this disease started more than forty years ago and numerous workers around the world are continually adding to the knowledge of the disease. In spite of this, not a single article that reviews the enormous scientific data thus generated is available in the English language. This article briefly discusses some of the recent research on edwardsiellosis, describing the pathogen's interaction with the host and environment, its pathogenesis and pathology as well as diagnostic, preventive and control measures.