Lack of Protection following Re-Exposure of Chinook Salmon to Ceratonova shasta (Myxozoa)

This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by American Fisheries Society and published by Taylor & Francis. It can be found at: http://www.tandfonline.com/toc/uahh20/currentThe recent identification of multiple genotypes of the salmonid parasite Ceratonova shasta with different virulence levels in Chinook salmon (Oncorhynchus tshawytscha) suggests that it may be possible to immunize fish against subsequent infection and disease. We hypothesized that exposure of Chinook salmon to the less virulent parasite genotype (II) prior to the more virulent parasite genotype (I) would decrease disease and/or result in fewer mature parasites compared to fish only infected with the more virulent genotype. To test this, fish were challenged in a combination of field and laboratory exposures and we measured infection prevalence, percent morbidity, and mature parasite production. Neither mortality nor mature parasite production were reduced when fish were exposed to genotype II prior to genotype I as compared with fish exposed only to genotype I, suggesting that protection using a less virulent parasite genotype does not occur