Streptococcosis in Tilapia and Warmwater Fish: A Disease Guide

Streptococcosis, caused by Streptococcus agalactiae and Streptococcus iniae, causes meningoencephalitis and high mortality in tilapia. Vaccination is the most effective control.

Overview

Streptococcosis is one of the most serious bacterial diseases of farmed tilapia and other warmwater fish. It is caused by gram-positive cocci, principally Streptococcus agalactiae (Group B Streptococcus) and Streptococcus iniae. The disease causes septicaemia and meningoencephalitis and drives high mortality and severe economic losses, particularly during hot weather when water temperatures are high.

Affected species and zoonotic risk

Nile tilapia (Oreochromis niloticus) is a primary host, and Streptococcus iniae has been reported in many other fish species, including rainbow trout, amberjack, red drum, and barramundi. Streptococcus iniae is also zoonotic: most human infections are linked to injuries sustained while handling live or fresh infected fish, and it can cause bacteraemic cellulitis as well as more serious infections such as endocarditis and meningitis, especially in older or immunocompromised people. Fish handlers should treat wounds and use appropriate protection.

Clinical signs

Erratic, spiral, or whirling swimming and loss of balance
C-shaped body curvature and dorsal rigidity
Exophthalmia, often unilateral, with corneal opacity
Haemorrhages around the eyes, operculum, and base of the fins
Lethargy, loss of appetite, and abdominal distension with ascites
Rapid death following the onset of nervous signs

Transmission and risk factors

The bacteria spread horizontally through water and contact with infected or carrier fish, and outbreaks are strongly promoted by high water temperature, high stocking density, poor water quality, and stress. These conditions are common in intensive warmwater tilapia culture, which is why streptococcosis is a recurring problem in the industry.

Diagnosis

Diagnosis is confirmed by bacterial culture and species identification, supported by PCR. Because nervous signs can resemble other systemic infections, laboratory identification of the specific Streptococcus species is important for selecting control measures, including the correct vaccine.

Treatment, control and prevention

Vaccination is the most effective measure for preventing and controlling streptococcosis, and both commercial and autogenous vaccines are used; a single dose can provide protection lasting for an extended period after vaccination. Antibiotics may be used to manage active outbreaks but should be used judiciously to limit resistance.

Vaccinate stock against the relevant Streptococcus species where vaccines are available
Reduce stocking density and maintain good water quality and dissolved oxygen
Avoid temperature and handling stress, especially during hot weather
Quarantine new fish and remove carriers and mortalities promptly
Use antibiotics only under veterinary direction, based on sensitivity testing, with proper withdrawal periods