Two genotypes of the intestinal parasite Ceratonova shasta infect Oncorhynchus mykiss: genotype 0 results in a chronic infection with low mortality while genotype IIR causes disease with high mortality. We determined parasite load and the relative expression of six immune factors (IgT, IgM, IL-6, IL-8, IL-10, IFNG) in fish infected with either genotype over 29 days post-exposure. In genotype IIR infections the host responded with upregulation of inflammatory and regulatory cytokines. In contrast, genotype 0 infection did not elicit an inflammatory response and expression of IFNG and IL-10 was lower. Antibody expression was upregulated in both infections but appeared to have limited efficacy in the virulent genotype IIR infections. Histologically, in genotype 0 infections the parasite migrated through the tissue layers causing inflammation but minimal damage to the mucosal epithelium, which contrasts with the severe pathology found in genotype IIR infections.

Department of Biomedical Sciences 106 Dryden Hall Oregon State University Corvallis OR 97331 USA

Department of Microbiology 226 Nash Hall Oregon State University Corvallis OR 97331 USA

Department of Microbiology 226 Nash Hall Oregon State University Corvallis OR 97331 USA; Institute of Parasitology Biology Center of the Czech Academy of Sciences České Budějovice 37005 Czech Republic

Faculty of Marine Science and Technology Fukui Prefectural University Obama Fukui Japan

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