Immunomodulation And Disease Resistance In Myxobolus cerebralis Infected Rainbow Trout     Christine L. Densmore1, Christopher A. Ottinger1, Vicki S. Blazer1, and Luke R. Iwanowicz2   1USGS, Leetown Science Center, National Fish Health Research Laboratory,  11700 Leetown Road, Kearneysville, WV; 2Department of Environmental Sciences, Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA     Populations of rainbow trout (Oncorhynchus mykiss) infected with Myxobolus cerebralis, the myxosporean that causes whirling disease, may experience increased mortality levels when exposed to various environmental stressors. In this study, we examined rainbow trout for indications of altered immune function and disease resistance ten to twelve months after M. cerebralis infection. The function of anterior kidney leucocytes was evaluated in sham-exposed and M. cerebralis exposed fish. In addition, both uninfected and M. cerebralis infected trout, were bath-exposed to a bacterial pathogen, Yersinia ruckeri. Macrophage function as measured by bactericidal activity was significantly greater in the M. cerebralis infected fish.  In contrast, lymphocyte mitogen responses of the M. cerebralis infected fish were lower relative to control fish.  Slightly higher mortality levels attributed to Y. ruckeri were observed among the M. cerebralis infected fish compared to controls. Two weeks after Y. ruckeri exposure, survivors were cultured for the organism, and the percentage of fish testing positive was again just slightly higher for the M. cerebralis infected fish compared to controls. Our findings indicate some degree of immunomodulation among rainbow trout infected with M. cerebralis.