en fish disease fish disease پژوهشي Orginal research papers This study was performed in order to determine the antibacterial activity of oregano, <em>Origanum vulgare</em> extract (OVE) and its nano extract (NOVE) on <em>Streptococcus iniae</em> in rainbow trout <em>in vitro</em> and <em>in vivo</em>. Extraction was done under standard percolation method. Then OVE and NOVE were screened to identify their antimicrobial activity against <em>Streptococcus iniae</em> by using disk diffusion assay and challenge methods. Also, the MIC and MBC values for OVE and NOVE were determined by micro dilution method. Furthermore, the antimicrobial effect of OVE and NOVE were examined by feeding rainbow trout fingerlings, with <em>O. vulgare</em> extract and its nano extract and florfenicol as a positive control. Then, all fish were injected intraperitoneal with <em>S. iniae</em> suspension in 3&times;10⁸ CFU ml^-1 concentration. Mortality rate was investigated for next 14 days after injection; then the spleen of fish samples removed to determine bacteria total count (SBT). <em>In vitro</em> results showed that OVE and NOVE had antimicrobial activity which was higher in NOVE. The inhibition zone of NOVE (26.066 mm) was significantly greater than OVE (21.7mm) (<em>p</em><0.05). Also, the MIC and MBC values with high efficacy against <em>S. iniae</em> in NOVE treatment group were 0.25 and 0.5 mgmL respectively, which was lower than OVE treatment values (<em>p</em><0.05). <em>In vivo</em> results showed that the mortality rate and SBT was significantly lower (<em>p</em><0.05) in the fish fed with NOVE than OVE and the control group (<em>p</em><0.05). These results indicated that the <em>Origanum vulgare </em>extract and its nano extract supplement are satisfactory as a prophylactic measure against rainbow trout streptococcosis and also for fish health improvement. Oregano, Bacterial challenge, Streptococcus iniae, Rainbow trout 2454 2463 http://jifro.ir/browse.php?a_code=A-10-2404-1&slc_lang=en&sid=1 S.M.E. Fakharzadeh acipencer@gmail.com 100319475328460036397 100319475328460036397 No Iranian Fisheries Research Science Institute, Coldwater Fishes Research Center, Agriculture Research Education, and Extension Organization (AREEO), Tonekabon-Iran M. Haghighi masoud126@yahoo.com 100319475328460036398 100319475328460036398 Yes Iranian Fisheries Research Science Institute, Coldwater Fishes Research Center, Agriculture Research Education, and Extension Organization (AREEO), Tonekabon-Iran M. Sahrif Rohani mostafasharif@yahoo.com 100319475328460036399 100319475328460036399 No Iranian Fisheries Research Science Institute, Agriculture Research Education, and Extention Organization (AREEO), Tehran-Iran I. Sharifpour isharifpour@yahoo.com 100319475328460036400 100319475328460036400 No Iranian Fisheries Research Science Institute, Agriculture Research Education, and Extention Organization (AREEO), Tehran-Iran M. Hamidi acipencer@yahoo.com 100319475328460036401 100319475328460036401 No Zanjan University of Medical Sciencesand Health Services, School of Pharmacy Zanjan-Iran