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We investigated intra-specific variation in the response of salmon to infection with the myxozoan Ceratomyxa shasta by comparing the progress of parasite infection and measures of host immune response in susceptible and resistant Chinook salmon Oncorhynchus tshawytscha at days 12, 25 and 90 post exposure. There were no differences in invasion of the gills indicating that resistance does not occur at the site of entry. In the intestine on day 12, infection intensity and Igþ cell numbers were higher in susceptible than resistant fish, but histological examination at that timepoint showed more severe inflammation in resistant fish. This suggests a role for the immune response in resistant fish that eliminates some parasites prior to or soon after reaching the intestine. Susceptible fish had a higher IFNg, IL-6 and IL-10 response at day 12, but all died of fatal enteronecrosis by day 25. The greatest fold change in IFNg expression was detected at day 25 in resistant Chinook. In addition, the number of Igþ cells in resistant Chinook also increased by day 25. By day 90, resistant Chinook had resolved the inflammation, cytokine expression had decreased and Igþ cell numbers were similar to uninfected controls. Thus, it appears that the susceptible strain was incapable of containing or eliminating C. shasta but resistant fish: 1) reduced infection intensity during early intestinal infection, 2) elicited an effective inflammatory
response in the intestine that eliminated C. shasta, 3) resolved the inflammation and recovered from infection.
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