Clinical signs
External signs:
Abnormal behaviour: Due to the tropism of the bacteria for the central nervous system, swirling behaviour, lethargy, bent bodies and disorientated fish are commonly observed. Eye lesions: Sick fish often have eye lesions such as endophthalmia or exophthalmia. Unilateral or bilateral opacification of the eye can also be associated with the previous signs and eye haemorrhages are common. However, not all fish affected with streptococcus necessarily have eye lesions.
Abscesses: In Streptococcus-infected fish, it is common to find 2- to 3-mm abscesses symmetrically positioned on the inferior jaw. In general, these abscesses quickly burst and become haemorrhagic ulcers which do not heal. Bigger abscesses of approximately 5 mm can also be observed at the base of the pectoral fins. The base of the tail is a common site for large abscesses (10-20 cm). These abscesses contain purulent material. When fish survive a Streptococcus infection, these abscesses usually remain and are often found at the time of processing.
Skin haemorrhages: Streptococcosis causes external haemorrhages. In general, these multi-focal pin-point haemorrhages are seen around the mouth or at the base of the fins. Sometimes, it is possible to observe a reddish pigmentation around the anus or on the genital papilla.
Ascites: Presence of abdominal fluid is common during acute outbreaks of streptococcosis. This ascites is often seen in association with a protruding anus.