Bacterial infections are a serious problem for commercial farms of carp (Cyprinus carpio L.) in Poland. Given the absence of efficient vaccines, it is crucial to search for new agents enhancing the non-specific immune response of the fish. In the present study, immunomodulating effects of lipopolysaccharide extracted from the virulent (LPS-P) and non-virulent (LPS-NP) strains of Aeromonas hydrophila were studied. For that purpose, different concentrations of LPS-P and LPS-NP (25, 50, and 75 µg/100 g body weight) were administered to test animals through intraperitoneal injection. The non-specific immune response of the fish were studied at days 7, 14, 21, 28, and 60 after vaccination. Immunostimulated fish showed an increase in total leukocyte count, in the population of monocytes and neutrophils, and in immature cell count in the whole blood. Phagocytic activity and the production of reactive oxygen species were also elevated after vaccination. Moreover, vaccinated fish showed a significant increase in serum lysozyme. The results of the present study show that LPS-NP has a greater immunostimulatory effect than LPS-P at doses of 50 and 75 µg/100 g body weight. In addition, 7 days after vaccination with LPS-NP (50 µg/100 g body weight), fish were challenged with the virulent strain of A. hydrophila. The relative percentage of survival in groups 1 and 2 was 58.82% and 76. 47%, respectively, which indicates that the administration of LPS-NP makes C. carpio more resistant to infection by A. hydrophila
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