Mixed bacterial infections in trout fish farms of the Leningrad region

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In recent years, outbreaks of mixed bacterial infections, similar to bacterial hemorrhagic septicemia, have become more frequent in the cage trout farms of the Leningrad Region. Previously, a similar course of the infectious process was observed mainly in closed water supply installations, especially for the first time years of their operation. High water temperatures play an important role in the development of the disease process (more than 20 °C), which last for a month or more, as well as high planting densities. In the period from 2021 to 2024, in the summer, a bacterial infection, the causative agents of which are Yersenia ruckeri and Flexibacter columnaris, is dangerous in cages. In May, and later during the summer, the presence of mixed bacterial infections caused by the development of opportunistic microflora was revealed — aeromonas (Aeromonas hydrophila, Aeromonas caviae), pseudomonas (Pseudomonas fluorescens, Pseudomonas putida, Pseudomonas chlororaphis) and myxobacteria (Flexibacter psyhrophila, Flexibacter columnaris). It should be noted the appearance of enterobacteria (Enterobacter spp.) as representatives of the secondary microflora, whose pathogenicity has been confirmed. The complex of aeromonas with flavobacteria is characterized by increased aggressiveness, which contributes to the development of serious septic infection. In one farm, two or three associations of pathogenic bacteria could be noted. A significant difficulty in detecting bacterial hemorrhagic septicemia is the selection of antimicrobial drugs. In this case, it is necessary to take into account the sensitivity to them of the main representatives of the association of pathogens. Otherwise, you can provoke anew outbreak of the disease. It was noted that mixed infections were often found in farms where fish purchased from three to four suppliers were grown on one fish farm. An extremely negative effect was given by stocking in the autumn with juveniles with low fatness and with a small amount of cavitary fat. Timely measures taken, including the normalization of conditions of detention and feeding, as well as medical and preventive measures, made it possible to suppress the development of infections and improve the epizootic state of kindergarten farms. In the future, it is recommended to avoid stocking one site with a large number of fish of different origin, and, accordingly, with different opportunistic microflora, the contact of which with each other leads to a deterioration in the epizootic state of farms. To normalize the in testinal microflora and increase the immune-physiological status, the use of probiotics can be proposed.