Winter Kills in Farmed European Seabass (Dicentrarchus labrax): Co- infected with Shewanella putrefaciens and Aeromonas veronii

Aeromonas and Shewanella species are opportunistic pathogens and often cause serious diseases in farmed fish.  In the present study, persistent natural mortality was observed in El Manzala intensive marine fish farm stocked with European seabass (Dicentrarchus labrax), Damietta, Egypt during the course of winterkills from early December to mid-January. The temperature of the well and fish ponds during episodic die-offs ranged from 13 to 17°C. The diseased European seabass exhibited ulcer, fin hemorrhage, tail fin rot, red and swollen inflamed vents, abdominal distension and hemorrhage, with approximately 30% cumulative mortality. Parasitological examination of diseased fish revealed only mild infestation with Trichodina species in the gills. Furthermore, bacteriological examination of diseased fish resulted in the identification of Shewanella putrefaciens and Aeromonas veronii isolates. These clinical isolates were presumptively identified on the basis of phenotypic characteristics and API 20NE biochemical profiles, followed by 16S rRNA gene sequencing and BLAST alignments. Additionally, the analysis of the neighbor-joining phylogenetic tree supported the above-mentioned identification. RNA polymerase B subunit (rpoB) was considered a proper tight taxonomic tool to differentiate between Aeromonas species. Therefore, Aeromonas isolates were confirmed as Aeromonas veronii based on rpoB housekeeping gene. In addition, antimicrobial susceptibility testing revealed that Shewanella putrefaciens isolates were resistant to oxytetracycline, doxycycline, trimethoprim/sulfamethoxazole and ampicillin, while Aeromonas veronii were susceptible to oxytetracycline, doxycycline and trimethoprim/ sulfamethoxazole. Both isolates were susceptible to florfenicol which was chosen to control these bacterial co-infections in farmed European seabass. This is the first report of Shewanella putrefaciens and Aeromonas veronii natural co-infections in intensively stocked European seabass.