BACKGROUND: Infectious diseases and microbial antibiotic resistance are the major problems of fish farming industry annually causing remarkable losses. Apart from the economic losses caused by these infections, some of these agents are zoonotic and may be transmitted to humans.OBJECTIVES: This study was aimed to identify the common causative agents of infections in rainbow trout farms and to determine their antibiotic resistance toward some common antibiotics.METHODS: Sampling was performed during a nine-month period between March and December 2021 by visiting and inspecting rainbow trout farms and the affected fish with disease symptoms were obtained from the farmed fish in Mazandaran, Lorestan, Chaharmahal and Bakhtiari and Zanjan provinces. Bacterial culture was undertaken from anterior kidney or spleen organs and the isolated bacterial strains were identified by phenotyping, biochemical and molecular assays. Antibiotic resistance pattern was evaluated by disk diffusion method (DDM) and minimum inhibition concentration against erythromycin, oxytetracycline, florfenicol, enrofloxacin and nitrofurantoin.RESULTS: Seventy-four bacterial isolates of Gram-positive cocci or Gram-negative coccobacilli were isolated. In phenotyping, biochemical and molecular (PCR) assays Lactococcus garvieae (12 isolates, 16.2 %), Aeromonas hydrophila (9 isolates, 12.2 %), Streptococcus iniae (17 isolates, 23 %), Streptococcus agalactiae (20 isolates, 27 %), and Yersinia ruckeri (16 isolates, 21.7 %) were identified. The majority of these isolates were obtained from the fish farms in Mazandaran province. Erythromycin and oxytetracycline with 87.8 % resistance were antibiotics with the highest resistance, while enrofloxacin with 24.3 % resistance revealed the lowest level of resistance. Antibiotic resistance rates for florfenicol and nitrofurantoin were also 43.2 % and 44.4 %, respectively. The highest antibiotic resistance was detected in the bacterial isolates of Lactococcus garvieae, Aeromonas hydrophila, Streptococcus iniae, Streptococcus agalactiae and Yersinia ruckeri, respectively.CONCLUSIONS: This study shows that the spread of streptococcosis, lactococcosis, yersiniasis and Aeromonas septicemia and their frequent treatments has led to an increase in antibiotic resistance, especially against commonly used drugs such as erythromycin and oxytetracycline.

Inclusion of high-ionic strength buffers helped us to develop a sandwich ELISA to detect hemorrhagic septicemia virus (HSV) in cell culture and infected trout tissue extracts. For maximal sensitivity of 0.1 to 0.2 ng/well/100 microliter or about 10 to 50 TCID50/well/100 microliter, trout extracts were diluted 1:1 and assayed for the earliest synthesized nucleoprotein N. Simultaneous binding of the N protein from HSV in the sample to the wells coated with monoclonal antibody (2D5 against the N protein) and to the peroxidase-labeled monoclonal antibody (2C9 against the N protein) proceeded during a 2-hour incubation at 20 to 22 C (room temperature). The response was linear between 6 to 60 ng/well of purified virus. Monoclonal antibodies used were noncompetitive with each other and reacted with F1, F2, 23.75, and 5 Spanish isolates of HSV, but not with infectious hematopoietic necrosis or infectious pancreatic necrosis viruses. Tissue specimens with low content of HSV virus may now be assayed directly without use of cell culture, rapidly, and with high precision, during the acute phase of the disease in salmonid fishes.