Histopathology of Hybrid Tilapias Infected witha Biotype of Streptococcus iniae

Abstract A systemic streptococcal disease caused by a biotype of Streptococcus iniae affecting hybrids of Nile tilapia Tilapia nilotica × blue tilapia T. aurea at a commercial aquaculture facility in central Texas was investigated. Histological analysis revealed a general septicemia with the occurrence of cellular infiltration and numerous cocci in most organ systems. Observations included cellular infiltration of the eye, meningitis, meningeal granuloma, numerous foci of infection, and massive cellular infiltration in the kidneys. Bacteria were seen in the subcapsular capillaries of the liver, and some liver tissue was granulomatous. Splenic sinuses contained numerous cocci, and both epicarditis and myocarditis was observed in the heart tissue of some specimens. Cocci were free in the plasma of infected fish and often phagocytized by macrophages.