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Streptococcus spp

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Eye diseases in aquarium fish caused by Streptococcus spp
Blisters on the mouth of a Tilapia caused by S. agalactiae
Blistered abscess on the tail of a Tilapia caused by S. agalactiae

Streptococcus spp are a Gram-positive bacteria which cause diseases such as 'Bloat', 'Dropsy' and 'Pop-eye' in many cultured marine and freshwater fish worldwide.

Several streptococcal species are involved, and the relationship between aetiological agents, isolated from the different fish has not been fully determined. Most recently, Streptococcus agalactiae has become an important pathogen of tilapia in Asia and the Americas. It is this pathogen that is responsible for much of the mortality in Thai tilapia culture in recent years.


Streptococcal disease in fish was first reported in 1957, affecting cultured rainbow trout in Japan[1]. Since then, numerous other species of fish have been found susceptible to infection, including salmon, mullet, golden shiner, pinfish, eel, sea trout, tilapia, sturgeon, and striped bass[2]. Strep has also been isolated from a variety of ornamental fish, including rainbow sharks, red-tailed black sharks[3], rosey barbs, danios, some cichlids including Venustus (Nimbochromis "Haplochromis" venustus) and Pelvicachromis spp, and several species of tetras.

Clinical signs

Strep infections in fish can cause high mortality rates (> 50%) over a period of 3 to 7 days. Some outbreaks, however, are more chronic in nature and mortalities may extend over a period of several weeks, with only a few fish dying each day. A typical history suggesting that Strep may be the cause of disease in a group of fish might include reports of abnormal swimming behavior, often described as spiraling or spinning. Anytime fish are observed behaving in an unusual manner, Streptococcus spp should be considered as one of the possible causes. However, not all infected fish show abnormal behavior.

In acute infection, signs develop within 3–4 days, gross ascites occurs, as well as dermal lesions. Exophthalmus is characteristic of the chronic stage, which may be clinically manifested or subclinical. The chronic condition does not occur in silver carp. Clinical symptoms are characteristically of the neuro-motor type; erratic swimming, swimming in circles and lack of control of the pectoral fins, leaving fish in an erect position.

Bacteria are readily isolated from the brain, optic nerve and fatty tissue around the eyes, but only from the viscera in a late chronic condition.


A tentative diagnosis of Streptococcus spp can be made from the history and clinical signs, necropsy findings, and identification of Gram-positive bacteria from stains of impressions (produced by blotting sections of fresh tissues onto a glass slide) from the brain, spleen, kidney, or liver. Strep should be highly suspected if fish exhibit abnormal swimming behavior, pop-eye, hemorrhages, and rapid and severe mortalities, and Gram-positive cocci are found in brain, kidney, and/or other organs.

Isolated bacteria are Gram-positive, non-sporulating, facultatively anaerobic chains forming cocci and catalase negative. They were able to grow at pH 9.6, but not at 10°C nor at 45°C, nor in the presence of 40% (v/v) bile salts or the presence of 6.5% NaCI (w/v). S. difficile and S. shiloi have been isolated from cultured tilapia (Oreochromis hybrids), in Israel. Isolates from Israeli grey mullet and tilapia from Japan were identified as S. difficile, while isolates from Taiwanese tilapia and trout corresponded to S. shiloi[4].

The brain seems to be the primary site of infection. In the acute state the ascitis fluid is fibrinous and contains bacteria-laden macrophages, all vital organs become heavily infected and mortality becomes massive (50–60% of the stock). Epizootic infection, in a variety of estuarine fish, caused clinical signs of erratic swimming, whirling motion and gross pathological signs of haemmorhagic lesions on the body, exophthalamus, corneal opacity, some macroscopic changes in the liver and spleen, and ascites (dropsy) with mucoid inflammation of the gut.

Histopathological changes in the heart result from acute pericarditis, with aggregation of bacteria-laden macrophages and accumulation of fibrinous exudate in the outer layer of the heart muscle. In the brain, meningoencephalitis is evident by accumulation of bacteria-laden macrophages in the meninges. In the liver, multiple necrotic areas develop, holding bacteria-laden macrophages and sometimes with a formation of granulomata. Proliferative changes have also been reported to occur in the spleen, with the kidney containing free bacteria, and granulomatous inflammation also occurring in the ovaries and the testes[5][6].


Broad-spectrum antimicrobials are usually required to control outbreaks in aquarium fish.

Food has been implicated as a source of infection in maricultured fish[7], but it is unlikely to be relevant in cases of tilapia fed on dry and pelleted feeds[8].

The affected population should be isolated from all others. If possible, have dedicated equipment (nets, siphon hoses) for these fish, and be sure to follow each use with an antibacterial dip such as benzalkonium chloride or Virkon®.

Immunostimulants added to the feed, such as beta-glucans and nucleotides, have been shown to increase survival for infected redtail black shark (ornamental fish) populations[9]. Vaccines may be useful for facilities that have continual or cyclic outbreaks of Streptococcosis. Autogenous vaccines (vaccines developed for a specific facility, targeting a specific bacteria isolated from a disease outbreak at that facility) have been shown to be effective under certain conditions. Commercial vaccines may also be available for use within the next few years.


  1. Hoshina, T., Sano, T. & Morimoto, Y. (1958) A Streptococcus pathogenic to fish. Journal of Tokyo University of Fisheries 44:57-68
  2. Inglis, V., Roberts, R.J. & Bromage, N.R. (1993) Chapter 12: Streptococcal infections. In Bacterial Diseases of Fish, Halsted Press, John Wiley & Sons, Inc., NY. pp:196-97
  3. Russo, R., H. Mitchell, & R. P. E. Yanong. (2006) Characterization of Streptococcus iniae isolated from ornamental cyprinid fishes and development of challenge models. Aquaculture 256:105-110
  4. Eldar, A., Bejerano, J. & Bercovier, H., (1994) Streptococcus shiloi and Streptococcus difficile: two new streptococcal species causing a meningoencephalitis in fish. Curr Microbiol 28:139–143
  5. Miyazaki, T., Kubota, S.K. & Miyashita, T., (1984) A histopathological study of Pseudomonas fluorescens infection in tilapia. Fish Pathol 19:161–166
  6. Hubbert, R.M., (1989) Bacterial diseases in warmwater aquaculture. In: Shilo, M. & Sarig, S. (eds.) Fish Culture in Warm Water Systems: Problems and Trends. CRC Press, Boca Raton, Florida. pp:179–194
  7. Taniguchi, M., (1982) Experiments on preoral inoculation via food to induce yellowtail streptococcosis. Bull Jap Soc Sci Fish 48:1721–1723
  8. Hubbert, R.M., (1989) Bacterial diseases in warmwater aquaculture. In: Shilo, M. & Sarig, S. (eds.) Fish Culture in Warm Water Systems: Problems and Trends. CRC Press, Boca Raton, Florida. pp:179–194
  9. Russo, R., H. Mitchell, & R. P. E. Yanong. (2006) Characterization of Streptococcus iniae isolated from ornamental cyprinid fishes and development of challenge models. Aquaculture 256:105-110