Since the basis of host specificity in Myxozoa, i.e. the differential disposition and extinction of erroneously penetrated myxozoan infective stages in non-susceptible fish hosts, remains puzzling, we aimed to explore the role of the innate immune system in this issue. In a comparative incubation challenge of actinospore sporoplasms of the freshwater parasite species Myxobolus cerebralis, Henneguya nuesslini and Myxobolus pseudodispar to isolates of host and non-host muci and blood sera, we measured cellular disintegration proportions and times by means of a double staining viability assay utilizing fluorescent dyes. After their activation, emerging primary and secondary sporoplasm cells were evaluated microscopically for physical integrity and onset of cell death due to exposure. Impairment by any mucus used was not detected up to 100 min of exposure. All parasites showed significantly increased cellular breakdown in non-susceptible host serum compared to the respective substrates from susceptible host fish. Except for M. cerebralis, the serum of the susceptible host was considerably less effective over time. In this species, both the primary and the secondary cells were affected in much shorter times than in the other two representatives. Inhibition of protease activity did not affect carp serum effect on M. cerebralis stages. We suggest the active components to be complement or complement induced factors since heat inactivation and withdrawal of bivalent metal ions lowered serum activity significantly. The study marks the first in vitro viability challenge of activated myxozoan transmission stages with teleost derived immune factors.
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