Fish & Shellfish Immunology (2000) 10, 47–59 Article No. fsim.1999·0229 Available online at http://www.idealibrary.com on Effects of sea lice (Lepeophtheirus salmonis Kröyer, 1837) infestation on macrophage functions in Atlantic salmon (Salmo salar L.) AHMED MUSTAFA 1 *, CHRIS MACWILLIAMS 1 ,NICOLE FERNANDEZ 1 , KELLY MATCHETT 1 ,GARY A. CONBOY 2 AND JOHN F. BURKA 1 1 Department of Anatomy and Physiology, and 2 Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, PEI CIA 4P3, Canada (Received 1 December 1998, accepted after revision 7 June 1999) Experiments were conducted to determine the e#ects of sea lice, Lepeophthei- rus salmonis, on non-specific defence mechanisms in Atlantic salmon, Salmo salar, by experimentally infesting hatchery-reared 1 and 2 year old post- smolts, S 1 and S 2 , with laboratory grown infective copepodids at moderate to high infection intensities ranging from 15–285 lice per fish. The e#ects of sea lice-induced stress were investigated by measuring the blood levels of cortisol and glucose as indicators of primary and secondary stress responses, and by changes in macrophage respiratory burst activity and phagocytosis as indi- cators of tertiary stress responses as well as non-specific defence mechanisms. Fish were sampled prior to sea lice infestation at day 0 and at days 3, 7, 14 and 21 post-infestation. Sea lice were at copepodid stage at day 3, at chalimus stages at days 7 and 14, and at pre-adult stage at day 21. Blood levels of cortisol and glucose were found to be significantly increased at day 21 in fish-infested with the highest levels. Macrophage respiratory burst and phago- cytic activities were found to be significantly decreased only at day 21. These results indicate that sea lice do not suppress host defence mechanisms during the earlier stages of infestation. They do have e#ects on the development of chronic stress and on the host non-specific defence mechanisms soon after the lice reach the pre-adult stage.  2000 Academic Press Key words: sea lice, Atlantic salmon, stress, macrophage, respiratory burst, phagocytosis. I. Introduction The term sea lice refers to several species of marine ectoparasitic copepods of the genera Lepeophtheirus and Caligus that commonly infest salmonids. Of these, Lepeophtheirus salmonis is responsible for serious disease outbreaks and high economic losses to salmon farmers throughout the northern hemi- sphere. Pre-adult and adult sea lice browse on the surface of salmon, eating *Corresponding author 47 1050–4648/00/010047+13 $35.00/0  2000 Academic Press