Koi herpesvirus (KHV) has infected farmed common carp in Poland clinically and asymptomatically since 2004. The role of non-carp species as vectors of virus transmission is well known except for in the case of KHV. The aim was to better understand this virus’ infection and transmission pathways in common carp, looking at the potential vector role of fishes kept with them. Eight species were experimentally infected with KHV by immersion in a suspension at 20°C ±1 and transferred to a tank after 45 minutes. Specimens were euthanised at intervals up to 56 days post infection (dpi) and tissue was examined for KHV DNA. Surviving infected fishes were introduced at intervals, each time into a separate tank, to naïve common carp for experimental infection. These were observed daily for symptoms, sacrificed along with controls after three months, and dissected to provide tissue samples. Also fish from 14 species collected from a farm with a history of KHV were sampled from 3 to 22 months after disease was confirmed. Organ sections from single fish were collected in a single tube. Viral DNA was detected in tench and roach samples up to 49 dpi, but in three-spined stickleback and stone maroko samples only up to 14 dpi. Transmission of KHV to naïve carp occurred after cohabitation. KHV DNA was detected in three fish species three months after the farm outbreak. We confirmed that grass and Prussian carp, tench, roach, and brown bullhead can transfer the virus to naïve common carp.

Introduction: Immune-potentiating functions of Lactobacillus plantarum strains in the common carp were evaluated. Material and Methods: Fourteen days of feeding fish dry diet supplemented with the bacteria provided parameters of nonspecific humoral immunity (lysozyme, ceruloplasmin, γ-globulin, total protein levels, and serum bactericidal activity) and cellular immunity (pinocytosis, respiratory burst activity, and potential killing activity of organ phagocytes), as well as the proliferative response of organ lymphocytes stimulated with mitogens. The resistance of fish to infection with Aeromonas hydrophila was also determined. Results: Dietary supplementation with L. plantarum had a substantial influence on the activity of organ phagocytes, especially the potential killing activity of head kidney cells. A significant increase in the proliferative activity of LPS-stimulated B lymphocytes and in the levels of γ-globulins and total protein was observed. The supplemented diet conveyed higher resistance than the control diet as the cumulative fish mortalities after infection with A. hydrophila were 65% and 85%, respectively. Conclusion: The results indicate that dietary supplementation with L. plantarum stimulates the antibacterial resistance of common carp and may reinforce defence against bacterial infections, but further studies need to be conducted.