Granulocyte responses to experimental injection of live and formalin-killed bacteria in carp (Cyprinus carpio)

The objective of the study was to investigate the dynamics of changes in number of granulocytes in bacterial infections of carp (Cyprinus carpio). Carp were inoculated with non-pathogenic or pathogenic bacteria and changes in type I (neutrophils) and type II granulocyte (basophils/eosinophils) counts in kidney, circulating blood and peritoneal cavity were assessed. After the injection of non-pathogenic bacteria (Escherichia coli), the number of type I and II cells in blood increased after 6–12 h, but retuned to the control level after 24–48 h. In contrast, after the injection of pathogenic bacteria (Aeromonas hydrophila), the number of type I cells initially increased followed by an increase in the number of type II cells. The peak counts of type I and II cells were at 12 and 24 h after the injection, respectively. When the fish were given serial-injections of formalin-killed bacteria at 12-h intervals, the type II cells also predominantly increased and remained at high levels, following the peak count of type I cells.