Outcome of within-host competition demonstrates that parasite virulence doesn't equal success in a myxozoan model system

Within-host competition can affect outcomes of infections when parasites occupy the same niche. We investigated within-host competition and infection outcomes in Chinook salmon exposed to two genotypes of Ceratonova shasta (myxozoan parasite). We assessed i) virulence (host mortality, median days to death), ii) within-host competition (abundance in host), and iii) success (spore production, proportion of myxospore-producing hosts) following concurrent and sequential exposures to single or mixed-genotype treatments. In single treatments, genotype-I replicated faster, and caused higher and earlier host mortality (higher virulence) but genotype-II produced more myxospores (higher success). In mixed treatments, costs of competition were observed for both genotypes evidenced by reduced replication or myxospore production following concurrent exposures, but only the less-virulent genotype suffered costs of competition when hosts were exposed to genotypes sequentially. To understand potential host effects on competition outcomes, we characterized systemic (spleen) and local (intestine) cytokine and immunoglobulin expression in single and mixed infections. We observed delayed systemic and immunosuppressive responses to the virulent genotype (I), rapid, localized and non-suppressive responses to the less-virulent genotype (II), and a combination of responses to mixed-genotypes. Thus, competition outcomes favoring the virulent genotype may be partially explained by the localized response to genotype-II that facilitates myxospore production (success) offsetting the systemic response to genotype-I that results in early inflammation and immunosuppression (that increases onset of mortality). This evidence for different but simultaneous responses to each genotype suggests selection should favor the exclusion of the weaker competitor and the evolution of increased virulence in the stronger competitor because the outcome was generally more costly for the less-virulent genotype. With caveats, our results are relevant for understanding infection outcomes in commercially and ecologically important salmonids in C. shasta endemic regions where mixed infections are commonplace.

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Competition between two genotypes of Ceratonova shasta was asymmetric in Chinook salmon hosts.

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Genotype I was more virulent but genotype-II was more successful (produced more myxospores).

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Costs of competition differed between genotypes, may be mediated by host immune response.

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Host immune response to genotype-I was delayed systemic and immunosuppressive.

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Host immune response to genotype-II was rapid, localized and non-suppressive.

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

The recent identification of multiple genotypes of the salmonid parasite Ceratonova shasta with different virulence levels in Chinook Salmon Oncorhynchus tshawytscha suggests that it is 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 with fish only infected with the more virulent genotype. To test this hypothesis, 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 compared with fish exposed only to genotype I, suggesting that protection against C. shasta using a less-virulent genotype of the parasite does not occur.

Ceratomyxa shasta genotypes cause differential mortality in their salmonid hosts

Ceratomyxa shasta is a myxozoan parasite of salmonid fish. In natural communities, distinct genotypes of the parasite are associated with different salmonid hosts. To test the hypothesis that genotypes of C. shasta cause differential mortality, the polychaete host was experimentally infected with different parasite genotypes. Genotype I was obtained from Chinook salmon, Oncorhynchus tshawytscha, and genotype II from either coho salmon, O. kisutch, or rainbow trout, O. mykiss, We then challenged four salmonid strains: Chinook and coho salmon that occur in sympatry with the parasite and allopatric Chinook salmon and rainbow trout. Parasite genotype I caused mortality only in Chinook strains, although mortality in the allopatric strain also occurred from exposure to genotype II. A second experiment demonstrated that genotype II could be separated into two biotypes based on differential mortality in rainbow trout and coho salmon. These differential patterns of mortality as a result of infection by certain genotypes of C. shasta support field observations and suggest a co-evolutionary relationship between these parasites and their hosts.