Streptococcus iniae and Lactococcus garvieae Infections in Farmed Fish: Detection and Antimicrobial Stewardship

Introduction

Streptococcus iniae and Lactococcus garvieae are Gram-positive cocci that cause significant economic losses in global aquaculture, particularly in warm-water and temperate species such as tilapia (Oreochromis spp.) and rainbow trout (Oncorhynchus mykiss). Both pathogens induce acute to subacute septicemias with meningoencephalitis and ocular involvement, often referred to as streptococcosis-like syndromes. S. iniae was first isolated from Amazon freshwater dolphins and later recognized as a major pathogen of cultured fish in Asia, the Middle East, and the Americas [1, 2]. L. garvieae, historically known as Enterococcus seriolicida, is the causative agent of lactococcosis in marine and freshwater fish, with outbreaks reported in Japan, Europe, and Australia [3, 4]. Coinfections with other bacterial agents, such as Aeromonas hydrophila, can complicate the clinical picture [5]. Effective detection and prudent antimicrobial use are critical for limiting mortality and preventing the emergence of resistant strains. This article reviews the clinical presentation, diagnostic modalities, antimicrobial resistance trends, and stewardship frameworks for these two pathogens in farmed fish, with emphasis on tilapia and trout production systems.

Clinical Signs in Tilapia and Trout

Streptococcus iniae in Tilapia

Tilapia infected with S. iniae exhibit acute mortality within a few days of exposure, with cumulative losses often exceeding 50% in untreated populations [6]. External signs include bilateral exophthalmia, corneal opacity, and periocular hemorrhage. Affected fish display erratic swimming, spiraling, and lethargy before death. Internal necropsy findings typically reveal splenomegaly, renomegaly, petechial hemorrhages on the liver and visceral adipose tissue, and meningeal congestion [7, 8]. Histopathologically, the bacterium induces a granulomatous to pyogranulomatous meningoencephalitis with extensive infiltration of macrophages and neutrophils in the brain parenchyma [9].

Lactococcus garvieae in Rainbow Trout

In rainbow trout, L. garvieae infection presents as a hyperacute septicemia with high mortality at water temperatures above 15 degrees Celsius [10]. Fish show uni- or bilateral exophthalmia, darkening of the skin, and ascites. Hemorrhagic enteritis, splenic infarcts, and liquefactive necrosis of the liver are common on gross examination [11]. The pathogen has a tropism for the central nervous system, causing a non-suppurative meningitis characterized by perivascular cuffing of mononuclear cells [12]. Chronic or survivor fish may develop ocular calcification and vertebral deformities [13].

Comparative Clinical Features

The clinical similarity between S. iniae and L. garvieae infections necessitates laboratory differentiation. Both pathogens cause exophthalmia and erratic swimming, but L. garvieae tends to produce more pronounced hemorrhagic enteritis and hepatic necrosis, whereas S. iniae is associated with a higher frequency of corneal ulceration [14]. A summary of distinguishing features is presented in Table 1.

Table 1. Comparative clinical and pathological features of S. iniae and L. garvieae infections in tilapia and trout.

Pathogen Biology and Host Interactions

S. iniae is a beta-hemolytic, Lancefield group B Streptococcus that possesses a polysaccharide capsule as a key virulence factor. The capsule impedes phagocytosis and complement deposition, allowing the bacterium to survive in the bloodstream [15]. Additional virulence determinants include streptolysin S (encoded by the sag operon), which causes hemolysis and cytotoxicity, and the phosphoglucomutase enzyme required for capsule biosynthesis [16]. L. garvieae is a non-motile, catalase-negative coccus that produces alpha-hemolysis on sheep blood agar. Its virulence repertoire includes a capsule, the surface adhesion protein GAPDH, and a hemolysin similar to streptolysin O [17, 18]. Both bacteria resist killing by the alternative complement pathway of serum, a trait that correlates with their ability to cause systemic disease [19].

Host susceptibility is influenced by stress, water quality, and stocking density. High stocking densities promote horizontal transmission through the water column and by cohabitation of carrier fish [20]. The bacterium gains entry through the gills, gastrointestinal tract, or skin abrasions. Once in the bloodstream, it disseminates to the brain, eye, and kidney, where it triggers an innate immune response dominated by macrophage and neutrophil infiltration [21].

Detection Methods

Culture and Phenotypic Identification

Conventional bacteriological culture remains the first-line diagnostic approach. Kidney, brain, and ascitic fluid are sampled aseptically and plated onto tryptic soy agar supplemented with 5% sheep blood or selective media such as Columbia colistin-nalidixic acid agar [22]. S. iniae produces small, beta-hemolytic colonies after 24–48 hours at 28–30 degrees Celsius. L. garvieae forms alpha-hemolytic colonies that are indistinguishable from enterococci without biochemical testing. Presumptive identification relies on Gram staining (positive cocci in chains), catalase negativity, and the absence of growth in 6.5% sodium chloride for S. iniae, whereas L. garvieae is salt-tolerant [23]. Commercial biochemical strips, such as the API 20 Strep system, can differentiate the two; however, misidentification with other streptococci or enterococci is possible [24].

Molecular Diagnostics

Polymerase chain reaction (PCR) assays offer higher sensitivity and specificity than culture, particularly for subclinically infected or antibiotic-treated fish. Species-specific PCRs targeting the 16S rRNA gene or the lactate oxidase gene (lctO) are widely used for S. iniae [25]. For L. garvieae, PCR primers directed at the 16S–23S rRNA intergenic spacer region or the hemolysin gene provide reliable identification [26]. Quantitative real-time PCR (qPCR) with hydrolysis probes enables quantification of bacterial load in tissues and water samples [27]. Multiplex PCR panels that simultaneously detect S. iniae, L. garvieae, S. agalactiae, and Aeromonas hydrophila have been developed and validated in tilapia and trout populations [28]. Loop-mediated isothermal amplification (LAMP) assays suitable for field deployment have also been reported [29].

The application of high-throughput sequencing to aquaculture diagnostics, while not yet routine, offers a culture-independent approach for outbreak investigation and monitoring of antimicrobial resistance genes [30]. Metagenomic workflows can detect coinfections with parasitic agents such as Ichthyophthirius multifiliis, which may exacerbate bacterial disease (see Ichthyophthirius multifiliis (Ich) in Freshwater Aquaculture: Rapid Detection and Integrated Control).

Serological Methods

Enzyme-linked immunosorbent assays (ELISA) have been developed to detect antibodies against S. iniae and L. garvieae in fish sera [31]. For a discussion of ELISA principles in veterinary diagnostics, refer to Enzyme-Linked Immunosorbent Assay (ELISA) for Feline Leukemia Virus. However, seroconversion is slow in poikilotherms, and antibody titers may not correlate with protective immunity [32]. Antigen-capture ELISA using monoclonal antibodies against capsular polysaccharides has been explored for direct detection of S. iniae in tissue homogenates, with reported sensitivity comparable to PCR [33].

Antimicrobial Susceptibility Testing

Antimicrobial stewardship depends on accurate in vitro susceptibility data. Minimum inhibitory concentration (MIC) determination by broth microdilution or disk diffusion following Clinical and Laboratory Standards Institute (CLSI) guidelines for aquatic bacteria is recommended [34]. For S. iniae, breakpoints for oxytetracycline, florfenicol, and amoxicillin have been proposed. L. garvieae isolates often exhibit elevated MICs to macrolides and tetracyclines, particularly in regions with a long history of medicated feeds [35]. Acquired resistance genes such as tet(M), erm(B), and aadE have been characterized in both species [36, 37].

Antimicrobial Resistance Landscape

Resistance to commonly used antimicrobials in aquaculture has increased over the past two decades. A systematic review of S. iniae isolates from tilapia farms in Southeast Asia reported that resistance to oxytetracycline exceeded 60%, while florfenicol resistance remained below 10% [38]. For L. garvieae, erythromycin resistance rates have reached 40% in trout farms in Southern Europe [39]. The primary drivers of resistance are subtherapeutic dosing, prolonged treatment courses, and the use of metaphylactic medicated feeds without prior susceptibility testing [40]. Co-selection of resistance genes on mobile genetic elements is a growing concern; for example, the vanB operon conferring vancomycin resistance has been detected in L. garvieae strains from clinical and environmental samples [41].

Surveillance programs that integrate phenotypic and genotypic data are essential for detecting emerging threats. The principles of genomic surveillance applied in other livestock sectors (e.g., Porcine Reproductive and Respiratory Syndrome Coinfections with Bacterial Pathogens in Swine) can be adapted to aquaculture systems.

Vaccine Development

Several vaccine formulations have been evaluated against S. iniae and L. garvieae, including inactivated whole-cell bacterins, live attenuated strains, and recombinant subunit vaccines. Bacterins based on formalin-killed S. iniae serotype I confer moderate protection (relative percent survival 60–75%) in tilapia when administered by intraperitoneal injection [42]. For L. garvieae, oil-adjuvanted bacterins have shown efficacy in rainbow trout, with protection lasting up to six months [43]. The major drawback of injection vaccines is the labor cost and stress associated with handling individual fish, particularly in large-scale pond production.

Oral and immersion routes are being actively pursued. A live attenuated S. iniae strain with a deletion in the aroA gene provided robust protection and cross-protection against heterologous isolates [44]. Recombinant vaccines expressing the S. iniae surface antigen siMA or the L. garvieae GAPDH protein have elicited specific antibody responses and reduced mortality in challenge trials [45]. DNA vaccines encoding the L. garvieae hemolysin gene have also been tested but with variable efficacy [46].

Despite these advances, commercial vaccines remain limited to a few licensed products, and autogenous vaccines are frequently used as a stopgap measure [47]. The economic threshold for vaccination depends on farm size, disease prevalence, and the cost of alternative control measures.

Antimicrobial Stewardship Strategies

Stewardship in aquaculture must consider the aquatic environment, where antimicrobial residues can persist in sediment and select for resistance in non-target bacteria [48]. Key principles include:

• Diagnosis before treatment: Confirm the pathogen by culture or PCR before using antimicrobials. Empiric therapy should be reserved for acute outbreaks with high mortality.
• Susceptibility-guided therapy: Select antimicrobials based on recent MIC data from the farm or region. Avoid drugs with known high resistance rates.
• Dose optimization: Calculate dose based on fish biomass and feeding rate; ensure adequate water solubility and stability of the medicated feed. Avoid underdosing.
• Withdrawal periods: Adhere to regulatory withdrawal times to prevent tissue residues. Educate farmers on record keeping.
• Rotational use: Alternate between antimicrobial classes (e.g., florfenicol and amoxicillin) in consecutive treatments to reduce selection pressure.
• Alternative interventions: Integrate probiotics, prebiotics, and immunostimulants (e.g., beta-glucans) to reduce reliance on antimicrobials [49].

A diagnostic-driven decision tool is presented in Figure 1.

graph TD
    A[Acute mortality, exophthalmia, erratic swimming], > B[Post-mortem sampling: brain, kidney, ascites]
    B, > C{Gram stain & catalase test}
    C, >|Gram+ cocci, catalase-| D[Blood agar culture]
    D, > E{Colony hemolysis}
    E, >|Beta-hemolysis| F[Presumptive S. iniae]
    E, >|Alpha-hemolysis, salt-tolerant| G[Presumptive L. garvieae]
    F, > H[Species-specific PCR / qPCR]
    G, > H
    H, > I[Confirmation of etiology]
    I, > J{Antimicrobial susceptibility testing}
    J, > K[Select antimicrobial based on MIC]
    K, > L[Medicated feed therapy with dose optimization]
    L, > M[Monitor mortality and retreat if needed]
    M, > N[Implement biosecurity: reduce stocking density, improve water quality]
    N, > O[Vaccinate survivors or at-risk cohorts if recurrent]
    O, > P[Long-term stewardship: rotate drug classes, record use, surveillance]

Figure 1. Diagnostic workflow and antimicrobial stewardship algorithm for suspected streptococcosis/lactococcosis in farmed tilapia and trout.

Integration of stewardship with broader health management is essential. Coinfestation with parasitic agents such as Sea Lice (Lepeophtheirus salmonis) Infestations in Farmed Salmon can increase bacterial susceptibility, and concurrent control measures should be applied. Similarly, environmental stressors must be minimized through optimal feeding, aeration, and ammonia management.

Conclusions

Streptococcus iniae and Lactococcus garvieae remain formidable pathogens in tilapia and trout aquaculture. Accurate detection requires a combination of culture, biochemical tests, and molecular methods, with qPCR and LAMP offering field-deployable options. Antimicrobial resistance is escalating, driven by imprudent use and horizontal gene transfer. Vaccine development has progressed, but coverage and delivery methods need improvement. A stewardship framework that integrates rapid diagnostics, susceptibility testing, dose optimization, and alternative interventions is essential for sustainable disease control. Future efforts should focus on genomic surveillance of resistance determinants and the development of multivalent vaccines that protect against both S. iniae and L. garvieae.

References

[1] Pier GB, Madin SH. Streptococcus iniae sp. nov., a beta-hemolytic streptococcus isolated from an Amazon freshwater dolphin. Int J Syst Bacteriol 26, 545-553.

[2] Eldar A, Bejerano Y, Bercovier H. Streptococcus shiloi and Streptococcus difficile: two new streptococcal species causing a meningoencephalitis in fish. Curr Microbiol 28, 139-143.

[3] Kusuda R, Kawai K, Salati F, et al. Enterococcus seriolicida sp. nov., a fish pathogen. Int J Syst Bacteriol 41, 406-409.

[4] Vendrell D, Balcázar JL, Ruiz-Zarzuela I, et al. Lactococcus garvieae in fish: a review. Comp Immunol Microbiol Infect Dis 29, 177-190.

[5] Janda JM, Abbott SL. Historical perspective on the genus Aeromonas. Clin Microbiol Rev 20, 35-59.

[6] Shoemaker CA, Klesius PH, Evans JJ. Prevalence of Streptococcus iniae in tilapia, hybrid striped bass, and channel catfish on commercial fish farms in the United States. J Aquat Anim Health 13, 103-110.

[7] Chang PH, Plumb JA. Histopathology of experimental Streptococcus sp. infection in tilapia (Oreochromis niloticus). J Fish Dis 19, 227-234.

[8] Bromage ES, Thomas A, Owens L. Streptococcus iniae, a bacterial infection in barramundi Lates calcarifer. Dis Aquat Organ 36, 177-181.

[9] Neely MN, Pfeifer JD, Caparon M. Streptococcus zoonotic pathogens: characterization of an extracellular protease in Streptococcus iniae. Infect Immun 70, 4489-4496.

[10] Eldar A, Ghittino C. Lactococcus garvieae outbreaks in trout farms in Italy. Vet Rec 142, 571-572.

[11] Austin B, Austin DA. Bacterial Fish Pathogens: Disease of Farmed and Wild Fish. Springer.

[12] Ooyama T, Kikuchi Y, Sakai M, et al. Pathological changes in rainbow trout experimentally infected with Lactococcus garvieae. Fish Pathol 34, 89-93.

[13] Schmidtke LM, Carson J. Lactococcus garvieae infection in rainbow trout: clinical and pathological findings. Aust Vet J 76, 250-254.

[14] Evans JJ, Klesius PH, Shoemaker CA. Streptococcus iniae and Streptococcus agalactiae: comparative pathology in tilapia. J Fish Dis 29, 201-209.

[15] Locke JB, Colvin KM, Varki N, et al. The Streptococcus iniae polysaccharide capsule is a major virulence factor. Infect Immun 75, 1466-1475.

[16] Fuller JD, Camus AC, Duncan CL, et al. Identification of a streptolysin S-associated gene cluster and its role in the pathogenesis of Streptococcus iniae disease. Infect Immun 70, 5730-5739.

[17] Kawai K, Liu Y, Ohnishi K, et al. A hemolysin produced by Lactococcus garvieae. Fish Pathol 34, 125-130.

[18] Watanabe Y, Aoki T. Molecular characterization of a surface layer protein from Lactococcus garvieae. Microbiol Immunol 46, 499-506.

[19] Barnes AC, Horne MT, Ellis AE. Streptococcus iniae resists complement-mediated killing by rainbow trout serum. Fish Shellfish Immunol 7, 383-394.

[20] Muzquiz JL, Royo FM, Ortega C, et al. Influence of stocking density on the transmission of Streptococcus iniae in tilapia. J Fish Dis 27, 283-286.

[21] Gallage S, Katagiri T, Endo M, et al. Innate immune responses in tilapia ovary cells infected with Streptococcus iniae. Dev Comp Immunol 55, 101-108.

[22] Nho SW, Shin GW, Park SB, et al. Phenotypic and genotypic characteristics of Streptococcus iniae isolated from cultured fish in Korea. J Fish Dis 34, 427-435.

[23] Facklam R, Elliott JA. Identification, classification, and clinical relevance of catalase-negative, gram-positive cocci. Clin Microbiol Rev 8, 479-495.

[24] Mata AI, Gibello A, Casamayor A, et al. Multiplex PCR assay for detection of bacterial pathogens associated with warm-water streptococcosis in fish. Appl Environ Microbiol 70, 3183-3187.

[25] Matsuura Y, Terashima S, Miyazaki T. Specific detection of Streptococcus iniae using PCR targeting the lactate oxidase gene. Fish Pathol 40, 41-43.

[26] Zamora-Sillero J, Ghanmi K, Esteban MA, et al. Development of a PCR method for detection of Lactococcus garvieae in fish. J Appl Microbiol 108, 762-770.

[27] Keeling SE, Brosnahan CL, Johnson R, et al. Quantitative real-time PCR for detection and enumeration of Streptococcus iniae in fish tissues. J Microbiol Methods 75, 474-480.

[28] Griffin MJ, Wise DJ, Khoo LH, et al. Development of a multiplex PCR for the detection of Streptococcus iniae and Streptococcus agalactiae in tilapia. J Vet Diagn Invest 23, 435-442.

[29] Suebsing R, Kiatpathomchai W, Srisuk C, et al. Detection of Streptococcus iniae by LAMP. J Fish Dis 32, 319-326.

[30] Bayliss SC, Verner-Jeffreys DW, Bartie KL, et al. Metagenomic analysis of aquaculture ponds reveals antimicrobial resistance gene diversity and abundance. Front Microbiol 8, 1765.

[31] Klesius PH, Shoemaker CA, Evans JJ. Development and validation of an enzyme-linked immunosorbent assay for the detection of antibodies to Streptococcus iniae in fish. J Fish Dis 23, 35-43.

[32] Pasnik DJ, Evans JJ, Klesius PH. Duration of protective antibodies and the correlation with protection in tilapia immunized against Streptococcus iniae. Fish Shellfish Immunol 20, 254-260.

[33] Chen D, Li Y, Lu C, et al. Development of a monoclonal antibody-based antigen-capture ELISA for detection of Streptococcus iniae. J Fish Dis 33, 417-423.

[34] Clinical and Laboratory Standards Institute. Methods for Antimicrobial Susceptibility Testing of Aquatic Bacteria. CLSI Document VET04.

[35] Mian GF, Godoy DT, Leal CAG, et al. Antimicrobial resistance of Lactococcus garvieae isolated from Brazilian fish. Aquaculture 294, 142-146.

[36] Aoki T, Kitao T, Kawai K, et al. Drug resistance in a strain of Streptococcus iniae isolated from cultured fish. Fish Pathol 28, 147-150.

[37] Chang PH, Sun JR, Kuo ST, et al. Antimicrobial susceptibility and plasmid profiles of Lactococcus garvieae isolates from fish in Taiwan. Dis Aquat Organ 54, 157-162.

[38] Shen GM, Shi CY, Fan C, et al. Prevalence of antimicrobial resistance in Streptococcus iniae from tilapia farms in China. Aquaculture 348-349, 50-55.

[39] Toranzo AE, Magariños B, Romalde JL. A review of the main bacterial fish diseases in mariculture systems. Aquaculture 246, 37-61.

[40] Cabello FC. Heavy use of prophylactic antibiotics in aquaculture: a growing problem for human and animal health and for the environment. Environ Microbiol 8, 1137-1144.

[41] Lara-Vergara J, Ibacache-Quiroga C, Moreno-Switt AI, et al. Detection of vancomycin resistance genes in Lactococcus garvieae from Chilean salmon farms. J Fish Dis 42, 131-138.

[42] Suanyuk N, Kong F, Ko D, et al. Efficacy of a killed Streptococcus iniae vaccine in tilapia. J Fish Dis 33, 353-360.

[43] Brudeseth BE, Wiulsrød R, Fredriksen BN, et al. Status and future perspectives of vaccines for industrialised fin-fish farming. Fish Shellfish Immunol 35, 1759-1768.

[44] Buchanan JT, Stannard JA, Lauth X, et al. Streptococcus iniae phosphoglucomutase is a virulence factor and a target for vaccine development. Infect Immun 73, 6935-6944.

[45] Sun Y, Liu C, Sun L, et al. A recombinant protein vaccine against Lactococcus garvieae infection in rainbow trout. Fish Shellfish Immunol 31, 883-889.

[46] Sheikhzadeh N, Heidari B, Baghban M, et al. DNA vaccine encoding hemolysin of Lactococcus garvieae induces protective immunity in rainbow trout. Vaccine 30, 4272-4277.

[47] Markvart K, Lorenzen E, Lorenzen N, et al. Autogenous vaccination against Streptococcus iniae in Danish trout farms. Vet Immunol Immunopathol 158, 87-93.

[48] Burridge L, Weis JS, Cabello F, et al. Chemical use in salmon aquaculture: a review of current practices and possible environmental effects. Aquaculture 306, 7-23.

[49] Lara-Flores M, Olvera-Novoa MA, Guzmán-Méndez BE, et al. Use of probiotics in fish feeds: a review. Aquaculture 377-380, 20-28.

[50] Watts JEM, Schreier HJ, Lanska L, et al. The rising tide of antimicrobial resistance in aquaculture: sources, sinks and solutions. Mar Drugs 15, 158.