Correlation between Leukocyte Numbers and Body Size of Rainbow Trout

Abstract

Immune cells in rainbow trout Oncorhynchus mykiss comprise granulocytes (neutrophils, basophils and eosinophils), macrophages/monocytes and lymphocytes (B- and T-cells). These cellular elements occur early during the ontogenetic development of trout and allow both innate and adaptive responses towards an antigen to be initiated even in fry. The number of leukocytes in individual fish at different developmental stages is likely to influence the capacity of the fish to respond simultaneously to several antigens (pathogens and vaccine components). This parameter may therefore be crucial for both wild and cultured fish and we show that the size of the leukocyte population increase exponentially with body size of rainbow trout. Four groups (5 fish/ group) of naive rainbow trout ( Oncorhynchus mykiss ) with a mean body weight of 2-4 g (group I), 4-6 g (group II), 25-30 g (group III), and 650-780 g (group IV) were investigated. The number of lymphocytes was generally higher in head kidney compared to blood and spleen but they dominated in all samples (blood, head kidney, and spleen) and their numbers increased exponentially with fish size. Percentages of lymphocytes in relation to neutrophils and macrophages were higher in spleen (98%-99%) compared to blood and head kidney in all groups. Fish fry is therefore equipped to respond specifically against one or a few vaccine antigens but the capacity to raise protective responses against a repertoire of pathogens may be limited until the larger fingerling stage has been reached. The implications for vaccination of early fry are discussed.