Mortality in farmed European eel (Anguilla anguilla) in Italy due to Streptococcus iniae

Abstract

Background Streptococcal infections are one of the main causes of fish disease. During the last decade, Streptococcus iniae has become one of the most important aquatic pathogens worldwide, causing high losses in marine and freshwater finfish. Clinical signs in farmed fish include loss of appetite, lethargy and grouping at the bottom of the tank. Gross changes comprise darkening of the skin and haemorrhage at the basis of fins and opercula. To date, S. iniae has been isolated from several wild and farmed fish species but never in the European eel (Anguilla anguilla). In Europe, eel production from aquaculture is around 4500 tonnes and Italy is the third largest producer. This communication represents the first report of an outbreak of S. iniae infection in European eels. Case presentation The outbreak occurred at an eel farm in northern Italy between May 2021 and September 2021. The outbreak caused about 2% mortality per month, resulting in the loss of about 10% of the farmed fish. The diseased eels showed apathy, lethargy, inactivity and inappetence. In July 2021, three eels were necropsied. Necropsy revealed skin and branchial hyperaemia, a few skin ulcers, and diffuse peritoneal congestion with a few haemorrhagic-like spot lesions. Swab samples for bacteriology were taken from the kidneys, liver, spleen, and brain. Additionally, four eels were opened and swap samples as above were taken. All the investigated eels were found dead. Bacteriological examination revealed growth of Streptococcus spp. from all samples. Identification of S. iniae was done by biochemical characterization, the API20STREP microsystem, 16S rDNA sequencing, and MALDI-TOF. Antimicrobial therapy (oxytetracycline and erythromycin) was ineffective. Conclusions This is the first report of S. iniae infection in the European eel. Although this may be an isolated outbreak, it is of concern due to the losses associated with this pathogen in fish worldwide and because the European eel is an endangered species. Due to the difficulties of controlling the disease with antimicrobials, it is advisable to plan other effective control measures, such as improving water quality and the environmental conditions, reducing fish density, improving biosecurity, and by using immunostimulants and, when possible, vaccines.

Biofilm formation and auto-aggregation abilities of novel targeted aqua-probiotics

“…Fish vibriosis is widespread in the seas and brackish waters, affecting both marine and coastal freshwater fish of various species, including salmon, cod, eel, herring, perch, and flounder [71]. Along with fish vibriosis, Aeromonas septicemia, edwardsiellosis, columnaris and streptococcus, as well as a number of other diseases, are largely responsible for economic losses in aquaculture production [72][73][74][75][76][77][78][79][80][81][82][83][84].…”

Comparative genomics analysis of Streptococcus iniae isolated from Trachinotus ovatus: novel insight into antimicrobial resistance and virulence differentiation

“…The most critical VAF validated in S.iniae including polysaccharide capsular ( cps ), phosphoglucomutase ( pgmA ), M-like protein ( simA ), beta-hemolysin/cytolysin ( cyl ), C5a peptidase ( scp ). CPS of S.iniae is a major virulence factor that provides resistance to the bactericidal activity of phagocytes and stimulates prolonged inflammatory responses, which including cpsA , cpsB , cpsC , cpsD , cpsE , cpsG , cpsJ [ 45 , 46 ]. The streptococcal CpsA protein was reported to associated with important virulence determinants, including cell wall processing [ 47 ], polysaccharide synthesis [ 48 ], and reaction to antimicrobial stress [ 49 ].…”

Immune Defenses in European Eels (Anguilla anguilla) Post Exposure to Formalized Killed Bacterin of Vibrio vulnificus

“…climate change, migration, pollution, and other factors that negatively impact wild populations of elvers and adults (Aschonitis et al, 2017;Pirollo et al, 2023).…”