Species in the genus Streptococcus belong to the Order Lactobacillales (lactic acid bacteria) and are Gram-positive, spherical or ovoid, non-spore-forming, non-motile and facultative anaerobic bacteria. They have been isolated from water and from the gastrointestinal (GI) tracts of various animals, such as humans, cattle, chickens, dogs, cats, hamsters, mice, monkeys, nutria, camels, horses, sheep, goat, bottlenose dolphins, fish, frogs and seals [43,44,45,46]. Streptococcosis is the general term for a variety of diseases caused by members of the genus Streptococcus. In fish, streptococcosis is mostly reported for infections with S. iniae, S. agalactiae and S. dysgalactiae [47,48,49]. S. iniae is one of the leading fish pathogens in freshwater and saltwater aquaculture species, especially in warmer regions. The bacterium is β-hemolytic on 5% sheep blood agar, but it cannot be grouped by the Lancefield antigen method typically used to categorize Streptococcus species. S. agalactiae is β-hemolytic and carries the Lancefield group B antigen. S. dysgalactiae is mostly non-beta hemolytic, with the Lancefield group C antigen.

Lethargy, loss of appetite, skin discoloration, exophthalmia/corneal opacity, abdominal distention and abnormal behavioral swimming are the most common clinical signs of streptococcosis in infected fish [9,47,50,51,52,53,54,55,56,57,58]. These clinical signs are not pathognomonic because they are not distinct from lactococcosis caused by Lactococcus garvieae, at least in some high susceptible species such as rainbow trout (Oncorhynchus mykiss) [59]. Other occasional macroscopical findings such as skin and fin hemorrhage, dorsal rigidity, vertebral deformity, tachypnoea and subcutaneous edema with ulceration are also reported in various degrees, but mostly in fish infected with S. iniae, S. agalactiae and S. dysgalactiae. The size and severity of the clinical signs are, however, varied and are dependent on a range of factors including fish species and size, bacterial virulence and health management criteria, particularly water temperature and dissolved oxygen (e.g., [9,47,60]). Internally, affected fish may show ascitic fluid in the abdominal cavity; the enlargement of liver and spleen; fibrinous pericarditis and peritonitis, hemorrhages in tissues of the brain, retrobulbar region, intestines and liver; and the congestion of the spleen and kidney in various degrees [50,56,58,61,62]. The clinical presentations are, however, known to be more severe in fish infected with capsulated strains of S. iniae and S. agalactiae, particularly in susceptible fish species such as rainbow trout and tilapia (Oreochromis niloticus).

The observation of intracellular bacteria in various external and internal organs are a clear sign of generalized bacterial septicemia in infected fish, and the most common histopathological findings due to S. iniae, S. agalactiae and S. dysgalactiae have been reported in tissues of the brain, heart and eyes of affected fish with marked pericarditis, choroiditis and meningitis [47,52,55,56,62]. Affected fish can, however, develop various other pathological findings, including keratitis, hemorrhagic or granulomatous meningoencephalitis, interstitial nephritis, branchitis, splenitis, ophthalmitis, choroiditis, hepatitis, gastritis, enteritis, pancreatitis, peritonitis, skeletal muscle myositis and fasciitis, and ulcerative and hyperemic dermatitis, as well as granulomatous reactions and inflammatory responses [47,50,56,61]. In addition, other tissues including liver, kidney, spleen, heart and gill may be affected, showing necrosis and hemorrhage [9,52,55,56,62,63]. Little information is available, however, on the pathology of other streptococcal infections in fish. Macrophage infiltration in kidney, liver and muscle, focal necrosis in muscle fibers in freshwater fish infected with S. parauberis [57] and large numbers of vacuoles in the brain matrix of fish infected with S. uberis have been reported [58], but no microscopic pathology data are available for S. phocae infections in fish [53,64].

The mechanism of pathogenesis and virulence factors involved in the disease caused by streptococcal species in affected fish is not yet fully understood. After the colonization and multiplication of the external (skin, fin, gills or nares) or gastrointestinal tissues, the bacteria invade internal tissues and blood, causing a generalized bacteremia followed by a septicemic condition induced by bacterial toxins. In fish infected with S. iniae, infection of the central nervous system (CNS) causing meningitis can occur through the entrance of bacteria via the blood circulation system or by contaminated monocytes/phagocytes with bacterial cells, and the incidence of CNS infection was correlated with the bacterial concentration in the blood and the duration of the bacteremia [65]. As some fish infected with S. iniae can carry the pathogen asymptomatically with no clinical signs [66], further research required to understand the mechanism of pathogenesis in more detail.

Several virulence factors are reported in Streptococcus species, but these are mostly detected in strains isolated from terrestrial animals. In fish, most virulence factors have been reported for S. iniae, S. agalactiae and S. dysgalactiae, and scarce data are available for other streptococcal species. The capsular polysaccharides are thought to be one of the most important virulence factors, inducing resistance to phagocytosis and the host humoral immune responses [10,47,67,68,69]. The survival of pathogens at the intracellular stage can facilitate the progression from a local to a systemic infection [70,71], and virulent isolates expressing a completed polysaccharide capsule are more resistant to phagocytosis than other strains [69]. Some non-capsulated strains [67] are more virulent to fish, however, suggesting that intra-phagocytic survival may not be a primary mechanism of disease establishment in fish; thus, further investigations are required.

The enhancement of the apoptosis of infected cells may also assist disease establishment, as it can cause cell death without the release of cellular components, resulting in the suppression of the host inflammation responses [47]. Some strains of S. iniae, such as serotype II, have capsules with more surface antigens, which can present additional anti-phagocytic properties [67,72]. A cell surface Fc binding factor, which blocks the binding and activation of complement cascade, has also been demonstrated to be part of S. iniae pathogenesis [73]. Further, M-like protein for cell adhesion, phosphoglucomutase for sugar metabolism, streptolysin S (SLS) for the synthesis of the SLS structural peptide and the SLS modification protein, peptidoglycan deacetylase for peptidoglycan acetyl modification, cell envelope proteinase for the synthesis of IL-8-cleaving cysteine protease and SivR/S for the two-component transcriptional regulation system encoded by different genes (i.e., simA, pgmA, sagA–sagI, agA, agB, PDI, cepI and SivR/S) have been confirmed as virulence factors of S. iniae strains [67,69,74,75,76,77,78,79,80]. Furthermore, the C5a peptidase [78] and fibrinogen binding protein [81], and recently two novel virulence factors, an extracellular nuclease and a secreted nucleotidase, probably with enzymatic activities, have been identified in strains of S. iniae that were involved in the experimental infection of zebrafish [80].

Several virulence factors have been identified in pathogenic S. agalactiae strains, including pore-forming toxins [β-hemolysin/cytolysin, CAMP factor (a protein B that enlarges the area of hemolysis formed by the β-hemolysin elaborated from the bacterium)], factors for immune evasion (sialic acid capsular polysaccharide, C5a peptidase, serine protease), superoxide dismutase, D-alanylated lipoteichoic acid, adhesins, hyaluronate lyase, and methionine transport regulator [82,83,84]. Most of these factors, however, have been studied in strains recovered from infected terrestrial animals, and thus virulence factors in strains infecting fish require further research. Despite the transmission of the pathogen from mother to newborn being an important risk factor of infection and disease progression for S. agalactiae in humans [85], no data are available on the vertical transmission of this pathogen in susceptible oviparous or viviparous fish species.

In S. dysgalactiae, the M-like protein is the most extensively studied virulence factor. This protein can opsonize both adherence to and entrance into host cells [86,87] and aids in immune evasion by inhibiting phagocytic activity and inactivating the complement cascade [87]. Adhesins encoded by different bacterial genes (gfba, fnB, fbBA, fnBB, lmb and gapC) are known to mediate binding to fibronectin [88,89,90,91], and the gfba gene can also assist bacterial entry into host endothelial cells and intracellular persistence [92,93]. Most adhesins, however, are recognized in strains of S. dysgalactiae isolated from affected human and other warm-blooded animals. In addition, protein G, a known virulent factor in S. dysgalactiae strains, can bind with circulating immunoglobulins and, hence, interfere with the host humoral immune response [94]. Furthermore, several toxins and secreted enzymes, including the hemolysins, streptolysin O and SLS [95,96] and superantigen speG [97], the streptokinase enzyme that enables the hydrolysis of fibrin and aids in bacterial spreading through tissues [87], have been identified in virulent isolates of S. dysgalactiae.

Several potential virulence factors, including hyaluronic acid capsule, hyaluronidase, uberis factor, antiphagocytic factors (capsule, neutrophil toxin, M-like protein and R-like protein), plasminogen activator/streptokinase factor, surface dehydrogenase protein, CAMP factor, lactoferrin binding protein and surface adhesion molecule, have been identified in both S. uberis and S. parauberis in warm-blooded animals [98,99,100], but these factors have never been studied in strains isolated from diseased fish. Some strains of S. parauberis carrying capsuled polysaccharide genes have recovered from diseased fish [101], but the role of other virulent factors in the pathogenicity of these species needs further study.

The capacity to form biofilms has been reported for S. iniae, S. dysgalactiae, S. uberis and S. parauberis. Biofilms can facilitate the survival and proliferation of bacteria in hostile environments, such as aquaculture recirculation bio-filtration systems [99,102,103], probably due to the bacterial extracellular production.

Virulence factors of S. phocae are mostly studied in marine mammals and rarely in fish species. Strains of S. phocae with antiphagocytic capsule ability are identified in experimental infections of Atlantic salmon (Salmo salar) [53,104,105]. In another study by González-Contreras et al. [106], cell-surface-related properties, including capsule detection, adhesion and hydrophobicity to fish mucus and cell lines, biofilm formation in skin mucus and serum resistance, were demonstrated in S. phocae isolates responsible for outbreaks in Atlantic salmon. More detailed studies of these properties are, however, required, as no mortalities or histopathological findings were seen in the fish injected with extracellular products. Other virulence factors, including fibronectin-binding proteins, the toxin SLS, genes encoding for a capsule [107] and the ability of the bacterium to invade fish and mammalian cell lines were also detected as part of S. phocae pathogenesis in aquatic animals, but these studies have mostly been in marine mammalians [104].

Despite the wide spread of infection in aquaculture, there are no recent estimates of annual losses by streptococcal pathogens in the industry [84]. The annual estimated losses caused by Streptococcosis were 150 and 250 million USD in 2000 and 2008, respectively [15,105,106]. In Iran, rainbow trout production in freshwater is remarkably high (about 180000 tons), but the annual loss through streptococcosis is estimated at around 30%, due to high water temperature in summer and poor health management [10,60,107,108]. Many fish species in freshwater, estuarine and marine environments are susceptible to S. iniae, and rainbow trout, yellow tail (Seriola quinqueradiata), Asian seabass (Lates calcarifer) and Nile tilapia (Oreochromis niloticus) are highly susceptible species (e.g., [9,47,50,105,109,110]).

Streptococcus agalactiae is a globally emerging fish pathogen causing huge economic losses in many freshwater and saltwater species [44]. The bacterium is reported in rainbow trout, seabream, tilapia, yellowtail, several species of catfish and mullet, croaker (Micropogonius undulatus), killfish (Menhaden spp.) and silver pomfret (Pampus argenteus) [44,51,52,111,112,113,114,115].

The first outbreak by the α-hemolytic Lancefield group C S. dysgalactiae subsp. dysgalactiae was reported in vaccinated and non-vaccinated farmed amberjack/yellowtail in Japanese fish farms [116]. Later, the pathogen was detected in kingfish (Seriola lalandi), grey mullet (Mugil cephalus), basket mullet (Liza alata), cobia (Rachycentron canadum), hybrid red tilapia (Oreochromis sp.), pompano (Trachinotus blochii), white spotted snapper (Lutjanus stellatus), Amur sturgeon (Acipenser schrenckii), golden pomfret (Trachinotus ovatus) and Nile tilapia from Brazil, Japan, China, Malaysia, Indonesia, Taiwan and Iran [28,54,117,118,119].

The first study reporting streptococcosis infection by S. parauberis (formerly known as S. uberis type II) [120] in fish was revealed by Domeénech et al. [121]. Subsequently, the disease was reported in several species including turbot (Scophthalmus maximus), olive flounder, sea bass (Sebastes ventricosus), striped bass (Morone saxatilis) and ram cichlid (Mikrogeophagus ramirezi) [57,122,123,124,125]. More recently, S. parauberis classified as serotype III has been reported as the cause of streptococcosis in different turbot farms in China, and the isolates are different from those that infect flounder (Paralichthys olivaceus) in Japan and South Korea but similar to strains in Spain and the USA [101].

S. uberis is an important causative agent of bovine mastitis worldwide. Although [58] documented the first report of disease outbreak by S. uberis in mandarin fish (Siniperca chuatsi) in China, the first isolation and characterization of this streptococcal species from fish was reported by [119] in Iranian commercial rainbow trout farms. S. uberis was isolated from the gills of healthy fish [126], and thus, the report by Pourgholam et al. [119] is in doubt because they did not assess the pathogenicity of the isolated strains.

S. phocae subsp. salmonis was first isolated from clinical specimens from harbor seal (Phocae vitulina) by Skaar et al. [127] before being isolated from diseased Atlantic salmon cage-farmed in Chile in the summer in 1999, with a reported mortality up to 25% [53,64]. In addition to fish, S. phocae has been recognized as an important pathogen of marine mammals, gray seal (Halichoerus grypus), ringed seal (Phoca hispida), Cape fur seal (Arctocephalus pusillus pusillus), southern sea otters (Enhydra lutris nereis), harbor porpoise (Phocoena phocoena) and other cetaceans, causing pneumonia or respiratory infection [128,129,130,131]. This pathogen has also been associated with urogenital neoplasia in Steller sea lions (Eumatopias jubatus) and skin abscesses in southern sea otters (Enhydra lutris nereis) [132,133], and Taurisano et al. [46] suggested that S. phocae is a serious disease in marine mammals.

Some streptococci species are serious zoonotic pathogens, with S. iniae causing bacteremia, cellulitis, meningitis and osteomyelitis [134], neonatal meningitis, sepsis and pneumonia caused by S. agalactiae [135,136] and bacteremia, lower limb cellulitis and meningitis caused by both S. dysgalactiae subsp. equisimilus and S. dysgalactiae subsp. dysgalactiae [137,138,139,140,141,142]. There are also reports of S. uberis in humans, although the accuracy of this identification is arguable [143]. A problem with streptococcal infections in aquaculture is that, in some outbreaks, the infected fish exhibit no clinical signs prior to death, and the mortalities are mostly due to bacterial septicemia that can involve the brain and nervous system [47,144]. In these cases, the consumption of infected fish, which appear clinically normal, can seriously affect public health.

The immune system of aquatic animals can inevitably be suppressed by various stressors, which increases the animal’s susceptibility to pathogenic agents [47,145]. Stress and stressors, therefore play a significant role in the initiation and development of streptococcal infections. Streptococcal infections are highly stress-dependent and occur in farmed fish exposed to sub-optimal water quality parameters such as sudden fluctuations in temperature or salinity, high alkalinity (pH > 8), low dissolved oxygen concentration and increases in NH₃ and NO₂. Overfeeding, overstocking and overhandling can also cause outbreaks of streptococcosis with high cumulative mortality [108,146,147,148,149,150]. Mortality caused by streptococcal infections can be reduced by pathogen-free stock/larvae, separate water supplies for culture systems, reducing over-manipulation or transportation, the quarantining of newly arrived fish, reducing overcrowding, avoiding overfeeding, frequently removing dying and dead fish, and keeping excellent sanitary conditions [105]. These preventive precautions can, however, be exceedingly difficult and expensive to implement, as streptococcal agents are quite common in aquatic environments. Due to the formation of granulomatous reactions in different organs of affected fish [151], antibiotic therapy of streptococcal infections is unsuccessful [152]. Treatment by antibiotics may also increase water pollution through frequent drug administration and the release of excess chemical substances into the farm environments, causing further stress of fish. Additionally, frequent antibiotic therapy can increase the withholding period for fish carcasses, and this may interfere with the farm production scheme.