Characterization of Streptococcus iniae isolated from ornamental cyprinid fishes and development of challenge models

Abstract

Streptococcus iniae was isolated from red-tail black shark(RTB shark) Epalzeorhynchos bicolor, and rainbow shark, E. erythrurus (fam: Cyprinidae), obtained from a local freshwater ornamental fish farm in Hillsborough county (FL). Darkening of the skin and lethargy were the first signs observed in infected sharks. Hemorrhages were observed on the ventral side of the body, on the head, and at the base of the pelvic and pectoral fins. Exophthalmia was observed in a low percentage of fish (∼ 10%). Moribund fish demonstrated the characteristic spinning, swimming pattern observed during streptococcal infections. Histological analysis revealed leukocyte infiltration in the intestinal area, spleen, posterior kidney, and brain. Necrosis and tissue degeneration were observed in the same organs in addition to degeneration of the renal tubules. Gram positive cocci were isolated from the cultures of brain and kidney tissues. Bacterial identification was obtained with the BIOLOG MicroLog3 version 4.00 System (Biolog, Inc., Hayward, California) and with standard microbiological tests. S. iniae-free RTB and rainbow sharks were used for developing a challenge model with the isolated bacteria strain. The experiments were conducted in two separate recirculating systems each with 25 tanks, 38 L volume per tank. In all experiments, the water in the system was maintained at 25 ± 1 °C. For each treatment, there were 3 replicate tanks, each stocked with 25 fish. Tanks were randomly assigned to treatment or control groups. Sharks were challenged by intracoelomic injection of S. iniae, and mortality was recorded for 14 days. Moribund fish showed classical signs of streptococcosis and S. iniae was isolated from moribund and dead fish. Differences in susceptibility to S. iniae were observed between the two species. An injection of 1.5 × 10 4 colony forming units (CFUs)/fish caused mortality close to 70% in RTB shark, and a higher mortality rate in rainbow shark. Mortalities ceased in 11–12 days after bacterial challenge in RTB shark, and in 3–4 days in rainbow shark. In this report, the pathology of S. iniae infections in two ornamental freshwater sharks and the development of a challenge model are described.