Invasion of Ceratomyxa shasta (Myxozoa) and comparison of migration to the intestine between susceptible and resistant fish hosts

The myxozoan parasite Ceratomyxa shasta infects salmonids causing ceratomyxosis, a disease elicited by proliferation of the parasite in the intestine. This parasite is endemic to the Pacific Northwest of North America and salmon and trout strains from endemic river basins show increased resistance to the parasite. It has been suggested that these resistant fish (i) exclude the parasite at the site of invasion and/or (ii) prevent establishment in the intestine. Using parasites pre-labeled with a fluorescent stain, carboxyfluorescein succinimidyl diacetate (CFSE), the gills were identified as the site of attachment of C. shasta in a susceptible fish strain. In situ hybridization (ISH) of histological sections was then used to describe the invasion of the parasites in the gill filaments. To investigate differences in the progress of infection between resistant and susceptible fish, a C. shasta-susceptible strain of rainbow trout (Oncorhynchus mykiss) and a C. shasta-resistant strain of Chinook salmon (Oncorhynchus tshawytscha) were sampled at consecutive time points following exposure at an endemic site. Using ISH in both species, the parasite was observed to migrate from the gill epithelium into the gill blood vessels where replication and release of parasite stages occurred. Quantitative PCR verified entry of the parasite into the blood. Parasite levels in blood increased 4 days p.i. and remained at a consistent level until the second week when parasite abundance increased further and coincided with host mortality. The timing of parasite replication and migration to the intestine were similar for both fish species. The field exposure dose was unexpectedly high and apparently overwhelmed the Chinook salmon’s defenses, as no evidence of resistance to parasite penetration into the gills or prevention of parasite establishment in the intestine was observed.