Streptococcosis caused by Streptococcus iniae in farmed rainbow trout (Oncorhynchus mykiss) in Iran: Biophysical characteristics and pathogenesis

Outbreaks of disease in cultured rainbow trout were observed in two farms in southwest Iran. The affected fish showed erratic swimming, dark discoloration, haemorrhages on the base of fins and exophthalmia with cataract; mortality varied from 20% to 40%. Internally, there were pale and/or haemorrhagic livers, congestion of spleen and kidney and occasionally ascites in the peritoneal cavity. Neither external nor intestinal parasites were found. Gram-positive haemolytic cocci were recovered from the diseased fish. The bacteria were identified as Streptococcus iniae by morphological, physiological and biochemical characterization. Using specific S. iniae primer sequences to the 16s rRNA gene revealed a 513bp fragment amplified by PCR. Intraperitonal injection of the recovered isolates in rainbow trout of 100-150 g caused 100% mortality within one week post-challenge at 17-19 degree C. This is the first report of S. iniae associated with rainbow trout disease in the Middle East.