Effects of Ceratomyxa shasta dose on a susceptible strain of rainbow trout and comparatively resistant Chinook and coho salmon.

Ceratomyxa shasta infects salmon and trout, causing ceratomyxosis, a disease characterized by parasite proliferation in the intestine and death. We used laboratory challenges to investigate the infective dose for 3 fish species: a susceptible strain of rainbow trout Oncorhynchus mykiss and comparatively resistant Chinook O. tshawytscha and coho salmon O. kisutch. For susceptible rainbow trout, we determined the outcome of infection under conditions of varying parasite dose, fish size, and parasite concentration. A single actinospore was sufficient to cause a lethal infection in susceptible rainbow trout. The mean days to death (MDD) did not significantly decrease among doses causing 100% prevalence, indicating a minimum time required for parasites to replicate to a fatal level. When dose was constant, but delivered in a higher parasite concentration, higher infection prevalence and mortality resulted. One actinospore fish(-1) caused 57% infection and mortality in fish challenged in 0.5 1 of water, whereas 10 spores fish(-1) resulted in an average of 49% infection and mortality in 1 l challenges. This effect is most likely due to a higher encounter rate in the smaller water volume. Neither infection prevalence nor MDD was significantly different between large trout (84.9 g) and small trout (6.3 g). Chinook salmon did not become infected even when challenged with 5000 actinospores. One fatal infection occurred in coho salmon challenged with 1000 actinospores. This study confirms that even low doses of C. shasta cause severe infection in highly susceptible fish, describes the dose response on MDD, and demonstrates that parasite concentration influences infection prevalence.