Streptococcosis - Fish

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Also known as: Red Boil Disease

Caused by: Streptococcus iniae


Streptooccosis in fish is most commonly caused by the gram positive, facultative anaerobe bacterium, Streptococcus iniae. It does not fit any Lancefield serotype groups but is beta-haemolytic on blood agar. Other species may be involved including S. faecalis, E. seriolicida, S. faecium, S. difficile, S. dysgalactiae and S. agalactiae.

S. iniae has also been identified as an emerging human pathogen producing soft tissue infections in people.[1] The risk is mainly to direct handlers of infected fish and is not thought to be food-borne.


Korea, Japan, Israel, Iran, Bahrain, South Africa, USA, Australia, Italy and Spain.


There are reports of streptococcosis in marine, brackish and freshwater fish. It can also affect dolphins.

Infection appears during the Summer in periods of warmer water temperatures.

Other predisposing factors include high stocking density and stressors.

Clinical Signs

S. iniae causes meningoencephalitis, systemic septicaemia and subsequent mortalities in fish, particularly cultured species.

Affected fish display bilateral exopthalmos and distension of the eyes giving their heads a strange shape. Internal and external haemorrhages may be grossly visible and skin is often darkened. Fish are lethargic, anorexic, stop feeding and they swim listlessly.


Diagnosis is usually based upon clinical signs and histopathological findings.

The bacterium grows very well on brain-heart infusion agar, nutrient agar, blood agar and Todd-Hewitt agar. It can then be biochemically characterised by fluorescent antibody testing (FA) or PCR.

Histopathologically, meningitis and panopthalmitis with cellular infiltration of the eye is evident.

On necropsy, exopthalmia is evident, petechiation common on the inside of the opercula and congestion of the pectoral and caudal fins and mouth. Ascites, hepatomegaly, splenomegaly and organ congestion are often noted.

Clinical signs and pathology are very similar to those of enterococcal infection.


Amoxycillin, Erythromycin and Oxytetracycline can be used but resistance is a problem with all except erythromycin.

Vaccination can also be used as a treatment.


A vaccine, administered intraperitoneally is available, derived from formalin killed whole cells but provides only short-term protection for 6 months.

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  1. Fuller, J. D., Bast, D. J., Nizet, V., Low, D. E., de Azavedo, J. C (2001) Streptococcus iniae virulence is associated with a distinct genetic profile. Infection and Immunity, 69

Salati, F. 2011. Enterococcus seriolicida and Streptococcus spp. (S. iniae, S. agalactiae and S. dysgalactiae). In: Fish Diseases and Disorders Volume 3: Viral, Bacterial and Fungal Infections (eds. P.T.K. Woo and D.W. Bruno), CABI, Walingford, UK, pp 375-396.


This article was originally sourced from The Animal Health & Production Compendium (AHPC) published online by CABI during the OVAL Project.

The datasheet was accessed on 13 July 2011.

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