Transmission to Eels, Portals of Entry, and Putative Reservoirs of Vibrio vulnificus Serovar E (Biotype 2)

Abstract

Vibrio vulnificus serovar E (formerly biotype 2) is the etiologic agent that is responsible for the main infectious disease affecting farmed eels. Although the pathogen can theoretically use water as a vehicle for disease transmission, it has not been isolated from tank water during epizootics to date. In this work, the mode of transmission of the disease to healthy eels, the portals of entry of the pathogen into fish, and their putative reservoirs have been investigated by means of laboratory and field experiments. Results of the experiments of direct and indirect host-to-host transmission, patch contact challenges, and oral-anal intubations suggest that water is the prime vehicle for disease transmission and that gills are the main portals of entry into the eel body. The pathogen mixed with food can also come into the fish through the gastrointestinal tract and develop the disease. These conclusions were supported by field data obtained during a natural outbreak in which we were able to isolate this microorganism from tank water for the first time. The examination of some survivors from experimental infections by indirect immunofluorescence and scanning electron microscopy showed that V. vulnificus serovar E formed a biofilm-like structure on the eel skin surface. In vitro assays demonstrated that the ability of the pathogen to colonize both hydrophilic and hydrophobic surfaces was inhibited by glucose. The capacity to form biofilms on eel surface could constitute a strategy for surviving between epizootics or outbreaks, and coated survivors could act as reservoirs for the disease.Vibrio vulnificus serovar E (formerly biotype 2) is a primary pathogen for eels and a secondary pathogen for humans (2, 35). As a human pathogen, this serovar probably behaves like the biotype 1 of the species, causing sporadic diseases and outbreaks in immunocompromised hosts (27,36). As an eel pathogen, this serovar causes a primary septicemia, named vibriosis, that affects captured eels maintained in farms, occasionally resulting in economic losses (5,7,8,13,18). The incidence of the vibriosis in natural populations of wild eels is unknown. In farms, the disease can suddenly appear and cause high mortality rates (7,8,13,18). After antibiotic treatment, the disease usually disappears and reappears as recurrent outbreaks that are often associated with stress factors such as changes in pH and nitrite levels (R. Barrera, personal communication). The onset of a new outbreak can be delayed by lowering water salinity, which partially inhibits the pathogen's ability to survive and spread (3, 21). However, the origin of the infection, the mode of transmission, and the reservoir between outbreaks or epizootics have yet to be determined.It has been suggested that eel-virulent strains, like the avirulent ones (biotype 1), are natural inhabitants of aquatic ecosystems (3, 21). This hypothesis is mainly based on laboratory results that demonstrate the ability of eel-virulent isolates to survive in artificial seawater microcosms for years (21) a...

The Fish Pathogen Vibrio vulnificus Biotype 2: Epidemiology, Phylogeny, and Virulence Factors Involved in Warm-Water Vibriosis

“…To find out how the pathogen is transmitted to new hosts and how it colonizes and invades fish, direct and indirect host-to-host transmission experiments, patch contact challenges, and oral-anal intubation experiments were performed on the eel (25,38). The results clearly suggest that water is the prime vehicle for the transmission of both modalities of vibriosis, and that nonserovar E strains are specifically adapted to be infective in freshwater.…”

“…The physico-chemical parameters for optimal eel production are water temperature around 24°C, water salinity around 1%, and high density in tanks (i.e., for eels weighing 50 g, densities of 100 to 150 kg/m 2 ) (71). These conditions are favorable for V. vulnificus survival and warm-water vibriosis transmission (25,53). The most accepted hypothesis is that eels become infected with Vibrio in coastal waters, mainly estuarine waters, and carry vibrios to the aquaculture installations when they are captured.…”

“…Other susceptible hosts are derbio (Trachinotus ovatus) (21), tilapia (Oreochromis sp) (22), trout (Oncorhynchus mykiss) (23), and shrimp (Penaeus vannamei) (24). Warm-water vibriosis in its acute form is also a primary septicemia, but in this case, it is triggered irrespectively of the pathogen's route of entry (gills, intestine, skin injury) and/or the host's immune status (25). This means that V. vulnificus behaves as an opportunistic (or secondary) pathogen in humans, and as a primary one in fish.…”

Phenotypic and genotypic characterization of a new fish-virulent Vibrio vulnificus serovar that lacks potential to infect humans

“…Human vibriosis occurs after eating contaminated seafood or exposing open wounds to seawater (Veenstra et al, 1992;Strom & Paranjpye, 2000), and fish vibriosis after immersion in contaminated water or contact with diseased animals or carriers (Amaro et al, 1995;Marco-Noales et al, 2001;Valiente & Amaro, 2006). The species has been subdivided into three biotypes on the basis of differences in biochemical properties, such as indole production and cellobiose fermentation, as well as serological and genetic traits and host range (Tison et al, 1982;Bisharat et al, 1999Bisharat et al, , 2007.…”

INVESTIGATION OF GENETIC VARIABILITY IN THE EUROPEAN EEL (ANGUILLA ANGUILLA(L.)) IN LITHUANIA USING MICROSATELLITE DNA MARKERS

“…Therefore, Lithuanian fishery in the coastal waters and the Curonian Lagoon depends on natural recruitment primarily, while restocking is still not very essential. Furthermore, besides other eel restocking problems, such as spread of parasites or infections (Madsen et al 2000;Hansen et al 2001;Marco-Noales et al 2001;Kirk et al 2002;Shin 2004), eel restocking can modify genetic variability and even reduce reproductive success if genetic differentiation in A. anguilla populations exists. This study presents results of the initial investigation into the A. anguilla population in Lithuania using microsatellite DNA markers.…”