Distribution and molecular identification of some causative agents of streptococcosis isolated from farmed rainbow trout (Oncorhynchus mykiss, Walbaum) in Iran

Over the past few years, the syndrome of streptococcosis has been associated with outbreaks in rainbow trout (Oncorhynchus mykiss, Walbaum) and caused significant economic losses in the aquaculture industry in Iran. The main purpose of this work was molecular identification of some causative agents of streptococcosis in rainbow trout. A total of 520 samples were collected from the head kidney of diseased fish (weight, 50_200g) in 72 farms of 8 provinces in Iran, during 2008 to 2009. Bacterial isolates representing morphology and biochemical profiles of Streptococcus spp. were further confirmed by polymerase chain reaction (PCR). DNA extraction was carried out from a single colony by using the extraction promega kit following the conditions described by the supplier. The PCR assay was developed based on the 16S rRNA and glucose kinase genes of Streptococcus spp. for the rapid and specific detection and identification of this pathogen from different sources. Approximately 40% of specimens were infected to Streptococcus spp. Consequently, five pathogenic species have been identified, including S. iniae in Fars province, S. faecium in Mazandaran province, S. agalactiae in Gilan and Mazandaran provinces, S. dysgalatiae in Lorestan, Kohgiluyeh and Boyerahmad, Gilan and Kermanshah provinces and S. uberis, which was common in all provinces (except Mazandaran and Lorestan). The dominant species (based on important species index) were S. uberis, S. dysgalactiae and S. agalactiae, respectively.