Comparative pathological study of tilapia naturally infected with Streptococcus agalactiae and virulence gene profiling of isolated strains

We isolated 14, 4 and 2 strains of Streptococcus agalactiae from naturally infected adult and juvenile tilapia as well as tilapia naturally carrying S. agalactiae, respectively. The clinical signs and anatomy changes of adult tilapia were as follows: erratic swimming, congestion of brain, eyeballs, gills and fins, exophthalmia, corneal opacity and swelling of visceral organs, which were characterized by histopathological changes with tubular hyaline degeneration, meningitis and epicarditis. The clinical symptoms of juvenile tilapia were not obvious, but some of the internal organs showed swelling and congestion, characterized by main histopathological features of hemorrhage of spleen vascular area, degenerated renal tubular epithelial cells and milder inflammatory response in brain tissue. In addition, eosinophil infiltration was found in the lamina propria of tilapia and in the liver of juvenile fish. It was observed that S. agalactiae proliferated in the brain and heart of adult fish and in the liver of juvenile tilapia, respectively. The clinical symptoms and histological lesions in tilapia carrying S. agalactiae were not obvious. The results of PCR detection show that all the S. agalactiae strains had the same virulence gene profiles, but there were significant differences in pathological damages among adult fish, juvenile fish and tilapia carrying S. agalactiae.