Introduction
Streptococcus iniae is a well-recognized bacterial pathogen affecting a wide range of marine and freshwater fish species, causing significant economic losses in aquaculture worldwide (Austin & Austin, 2016). It has been widely reported in species such as tilapia and rainbow trout, where it causes septicemia and meningoencephalitis (Austin & Austin, 2016; Agnew & Barnes, 2007). In the Mediterranean aquaculture industry, European seabass (Dicentrarchus labrax) is one of the most commercially valuable species; however, reports of S. iniae infections in this host remain scarce. The emergence and spread of S. iniae have been associated with environmental stressors, including elevated water temperatures and high stocking densities (Mian et al., 2009). The present study describes the first confirmed occurrence of S. iniae infection in farmed European seabass in marine aquaculture facilities in East Greece, highlighting its clinical, microbiological, and epidemiological characteristics.
Materials and Methods
During the summer of 2025, increased mortality (8-10%) was observed in seabass populations from two marine cage farms located in South-East Aegean (Greece). Samples of moribund European seabass (average weight 90–500 g) were collected and subjected to clinical and laboratory examination. Gross pathological assessment was performed, followed by bacteriological analysis using standard culture techniques on blood agar. Isolated colonies were identified based on colony morphology, Gram staining, and biochemical characteristics, and further confirmed by molecular methods, including PCR targeting 16S rRNA followed by sequencing (Mata et al., 2004). The genome of the isolate was extracted and subsequently sequenced using both short- and long-read technologies followed by hybrid assembly. Antimicrobial susceptibility testing was performed using the disk diffusion method.
Results
Affected fish exhibited clinical signs consistent with streptococcosis, including anorexia, lethargy, erratic swimming behavior, ocular opacity and exophthalmia. External examination revealed redness of the head, anemia of the gills, petechial hemorrhages on the operculum and at the base of fins. Internally, ascites, splenomegaly, discoloration of the liver, enteritis with distended intestine hemorrhages, serous fluid within the lumen and congestion of visceral organs were observed. Pure bacterial cultures yielded Gram-positive, β-hemolytic, catalase-negative cocci arranged in chains. Biochemical profiling and genomic analysis confirmed the isolates as Streptococcus iniae. Antimicrobial susceptibility testing showed sensitivity to florfenicol and oxytetracycline. The outbreak coincided with elevated seawater temperatures (25–27°C), a known predisposing factor for streptococcal infections (Agnew & Barnes, 2007).
Discussion
This study represents the first documented case of S. iniae infection in farmed European seabass in Eastern Greece, expanding the known host range and geographical distribution of this pathogen. The clinical signs and pathological lesions observed are consistent with previous descriptions of streptococcosis in other fish species (Austin & Austin, 2016; Agnew & Barnes, 2007). The emergence of S. iniae in seabass raises concerns regarding its potential impact on Mediterranean aquaculture, particularly under conditions of elevated water temperatures, which may favor disease outbreaks. Early diagnosis through combined bacteriological and molecular methods proved essential for accurate identification. Genomic analyses provided insights regarding the virulence factors of the isolate and its phylogenetic position. Environmental conditions, particularly increased water temperature, likely played a critical role in the onset of the outbreak, as previously suggested (Mian et al., 2009). The observed antimicrobial resistance patterns emphasize the need for responsible antibiotic use (CLSI, 2020; Shoemaker et al., 2001). Preventive strategies, including vaccination, biosecurity measures, and improved farm management practices, should be considered to mitigate future outbreaks. Further epidemiological studies are required to assess the prevalence and impact of S. iniae in Mediterranean seabass aquaculture.
References
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