Seasonal Changes of Phagocytic Activity Enhanced by an Acoholic Mint Extract in Cultured Rainbow Trout

  Objective: We hypothesized that leukocyte numbers and their in vitro phagocytic activity in farmed rainbow trout are subject to negative influence of extreme temperatures for fish and that mint extract could alleviate these effects. Materials and Methods: Blood samples from farmed rainbow trout collected during winter and summer were subjected to leukocyte subpopulation counts (%) and carbon particle inclusion test to estimate the in vitro phagocytic activity after 15 and 30 min of incubation at 4 and 22°C, with or without alcoholic mint extract (ln). Results and Discussion: There was a significant increase in lymphocytes (p<0.05) and monocytes (p<0.01) in summer versus winter, while the heterophils were dominant during winter (p<0.01). In vitro spontaneous phagocytosis increased with the seasonal increase in temperature (0.425±0.094, winter and 0.835±0.102, summer, p<0.05) and during the first reading period (0 to 15 min, p<0.01). The mint extract increased phagocytosis for the overall reading period from 2x (winter, 0.281±0.221) up to 5x (summer, 1.032±0.221) (p<0.01). Conclusion: The positive activity of the alcoholic mint extract during phagocytosis, more intense during summer than during winter, proved to be a temperature driven process, plant active principles playing a dual stimulating role for both monocytes and heterophiles.