Vibrios Associated with Mortality in Cultured Plaice Pleuronectes platessa Fry

Abstract

Fifty two bacterial strains, identified as Vibrio spp., were isolated from diseased plaice fry. The most numerous group comprised V. anguillarum (26/52), of which 3 isolates belonged to serogroup O2a, 16 corresponded to serogroup O18, and 7 isolates were non-typeable. All serogroup O18 isolates had identical ribotype patterns. Fourteen isolates were identified as V. splendidus biotype I (n = 11) or V. splendidus-like (n = 3). Seven isolates were V. fluvialis, representing the first isolation of this species in Denmark and the first description of V. fluvialis associated with diseased fish. All V. fluvialis isolates had identical ribotype patterns, indicating the presence of a single clone. The last 5 isolates belonged to 2 different, unidentified Vibrio species (n=2 and 3, respectively). Although all isolates were recovered from diseased plaice fry, their exact role as pathogens for the fry is as yet uncertain. Selected isolates were tested for virulence to salmon and turbot. When injected into juvenile salmonid fish, the recorded LD₅₀ values were higher than 10⁶, indicating that their virulence was relatively low. However, virulence seemed to deteriorate upon subculturing, and therefore, the strains may have been more virulent upon primary isolation from the plaice fry.

Sammendrag

I alt 52 bakteriestammer blev isoleret fra syge rødspætte-yngel og identificeret som Vibrio spp. Vibrio anguillarum var den mest talrige gruppe omfattende 26 ud af de 52 isolater. Af disse 26 V. anguillarum tilhørte de 3 serogruppe O2a, mens 16 tilhørte serogruppe O18, og 7 isolater var ikke-typbare. Alle serogruppe O18 isolater havde identisk ribotype mønster. Fjorten isolater blev identificeret som V. splendidus biotype I (n=11) eller V. splendidus-lignende (n=3). Syv isolater tilhørte V. fluvialis, og er således de første isolater af denne art i Danmark og tillige den første beskrivelse af V. fluvialis associeret med syge fisk. Alle V. fluvialis isolates havde identisk ribotype profil, hvilket indikerede tilstedeværelsen af en enkelt klon. De sidste 5 isolater tilhørte 2 forskellige, hidtil uidentificerede Vibrio arter (n=2 henholdsvis 3). Selvom alle bakterier blev isoleret fra syge rødspætte-yngel, er deres præcise rolle som patogener stadig noget usikker. Ved challengeforsøg blev juvenile laks og pighvar injiceret med forskellige doser af udvalgte isolater, men de fundne LD₅₀ værdier fandtes at være højere end 10⁶, hvilket antyder, at de er relativt lidt virulente. Imidlertid aftog virulensen tilsyneladende ved subkultivering, og isolaterne kan således have været mere virulente ved isoleringen fra rødspætteyngelen.