Survey on viability of Streptococcus iniae in frozen rainbow trout (Oncorhynchus mykiss) following experimental infection

Streptococcus iniae is one of the most important agents that causes high mortality and losses in rainbow trout farms in Iran and the world. These bacteria can affects the consumers and people who deal with aquaculture via affected fish. So, viability of this bacterium in frozen fish and fish product can be useful for health care of consumers. A total of 90 rainbow trout, with average weight of 50 ± 3 g, were supplied from a fish farm and were transferred to six 200- liter tanks (15 fish per tank). The tanks were divided in two groups as control and treatment. The treatment group fish were experimentally challenged to Streptococcus iniae at dose of 3.6 × 105 cell/fish via intraperitoneal injection. Control group fish just received 0.1 ml of normal saline (0.9 % NaCl). In treatment group after 120 hours 91% of the fish were died and then mortality remained constant up to 14 days post challenge. Moribund and dead fish of treatment group and fish of control group were frozen and then kidney, liver and brain of fresh dead and frozen fish were used for bacterial culture after 8, 14, 20, 26, 30, 34, 36 and 38 months post freezing, to investigate the viability of these bacteria in different organs at freezing temperature (-20˚C). Streptococcus iniae were re-isolated from all of fresh and frozen tissue specimens within 38 month after freezing. All bacterium species confirmed as Streptococcus iniae using polymerase chain reaction (PCR) method. According to this study this bacterium as a zoonisis can be survived at least 38 month in freezing temperature (at -20˚C) in fish products, so the potential pathogenicity of this bacterium should be concerned in affected area as well as imported frozen fish.