Virulence of Aeromonas hydrophila and some other bacteria isolated from European eels Anguilla anguilla reared in fresh water

Studies were conducted to determine the pathogenicity for European eels Anguilla anguffla and rainbow trout Oncorhynchus mykiss of several stra~ns of the species Aeromonas hydrophila, A. jandaei, A. sobria, A. caviae. Plesiomonas shigelloides. Vibrio anguillarum, Pseudomonas fluorescens and Shewanella putrefaciens isolated from an eel farm. Virulence of 20 isolates obtained during epizootics from diseased eels, asymptomatic carriers and tank water and of 20 isolates obtained during the absence of disease from healthy eels and water was evaluated by 2 challenge methods: intraperitoneal injection (IP) and bath exposure. Of the isolates tested by IP, all strains of A. hydrophila. and those of A. jandaei mainly from epizootics, were pathogenic for eels (LD,, dose 105.4 to 107.5 cfu fish-'). Strains of these species caused an ulcerous disease by bath exposure to 10' to 10' cfu ml-'. In contrast, rainbow trout were less susceptible to infection caused by A. hydrophila and A. jandaei isolates, perhaps because of the lower water temperatures at which they were held during challenge. The epizootics in the eel populations occurred in spring and summer when water temperatures were 17 to 22 "C. The data indicate that A. hydrophila and A. jandaei are European eel pathogens and that infections may occur by waterborne transmission. Therefore, the presence of these bacteria in the farm environment probably constitutes a health hazard for eels.