Assessment of acute effects of Streptococcus faecium on some hematological and histopathological parameters in juveniles rainbow trout (Oncorhynchus mykiss)

Streptococcosis is one of the most important bacterial diseases in cold water fish because of its rapid outbreak specially in summer, relatively high morbidity and mortality, productive reduction and economic losses in aquaculture industry. However, it would be necessary to determine of 50% lethal dose (LD50) of bacteria (Streptococcus faecium) and studying its acute effects on hematological and histopathological parameters. For measuring LD50 five dilutions of bacteria were obtained by Macfarlane method and the fish were injected by intraperitoneal (IP). The experience was carried out in 6 treatments and 3 replicates. The fish were injected by LD50 dose and after 10 days, samples of blood and tissues including: Eyes, brain, kidney, liver and gills were collected. The injected fish showed darkening of body, exophthalmia, abdominal distension and prolapsed anal. The obtained results showed significant reduction (P