Respiratory burst activity of head kidney leucocytes at 4 days post-injection of P. vulgaris . Five groups (7 fish/group) of fish were I.P. injected with 0, 100, 300 and 500 μg/100 g of fish, and FCA. Data represent the mean + S.D. (n = 7). Statistical differences (p < 0.05) between groups are indicated by different letters over the bar.