Fish & Shellfish Immunology (2000) 10, 451–463 doi:10.1006/fsim.1999.0260 Available online at http://www.idealibrary.com on Experimental exposure of rainbow trout Oncorhynchus mykiss (Walbaum) to the infective stages of the sea louse Lepeophtheirus salmonis (Krøyer) influences the physiological response to an acute stressor N. M. RUANE 1 *, D. T. NOLAN 1 , J. ROTLLANT 2 , J. COSTELLOE 3 AND S. E. WENDELAAR BONGA 1 1 Department of Animal Physiology, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands, 2 Unit of Animal Physiology, Department of Cell Biology and Physiology, Universitat Autonoma de Barcelona, 08193 Cerdanyola, Spain and 3 Aqua-Fact International Services, 12 Kilkerrin Park, Liosbaun, Galway, Ireland (Received 27 July 1999, accepted after revision 20 December 1999) The influence of infection with the juvenile stages of the sea louse, Lepeophtheirus salmonis (Krøyer) on the response of rainbow trout Oncorhynchus mykiss (Walbaum) to a net confinement protocol was investi- gated. The experiment consisted of two groups of seawater-adapted rainbow trout, one which was exposed to a total of 4000 nauplii/copepodid stages of L. salmonis 30, 25 and 14 days prior to confinement. Confinement elicited a greater stress response in the lice-exposed fish, than in the controls, as seen by higher plasma cortisol and glucose levels. A reduced spleen somatic index in exposed fish following 6 h confinement coincided with increased erythrocyte and lymphocyte numbers in the blood. Circulating lymphocyte numbers were significantly reduced in both groups 24 h post-confinement, when a lower alternative complement activity was recorded in control fish. Prior to confine- ment, lice-exposed fish had an elevated serum lysozyme activity and reduced oxygen radical production by blood leukocytes. Following confinement, lysozyme activity was gradually reduced in lice-exposed trout. During confine- ment, oxygen radical production decreased in control fish and increased in infested fish. Overall, transient exposure to juvenile lice altered the response to a second stressor, which has implications for management procedures of L. salmonis exposed fish.  2000 Academic Press Key words: Oncorhynchus mykiss, crustacean ectoparasites, Lepeophtheirus salmonis, confinement, stress response, immunomodulation, haematology. I. Introduction The current awareness of the importance of reducing stress in farmed fish has resulted in the application of research knowledge to many aquacultural *Current address: Dept Fish Culture & Fisheries, University of Wageningen, Marijkeweg 40, 6709 PG Wageningen, The Netherlands. E-mail: Neil.Ruane@alg.venv.wau.nl 451 1050–4648/00/050451+13 $35.00/0  2000 Academic Press