In vivo exposure of susceptible and non-susceptible fish species to Myxobolus cerebralis actinospores reveals non-specific invasion behaviour.

Due to controversial previous results, it has been unclear whether Myxobolus cerebralis (Myxosporea, Myxozoa) is able to specifically choose a salmonid host by selective attachment and penetration. Using a novel in vivo tracking assay that employs fluorescence staining of actinospore sporoplasms, we demonstrate a lack of host specificity of M. cerebralis actinospores during their initial invasion reactions. Fish were experimentally exposed to stained actinospores that could be detected as emitted sporoplasms in and on the fish integument of skin, gills and fins. There were no significant differences in the number of actively emerging sporoplasms found on epithelial surfaces of a susceptible and resistant strain of rainbow trout and common carp after experimental exposure. Numbers of parasite attachment rates to carp and trout gill tissue were also assessed using quantitative real-time PCR (qPCR). This method demonstrated that actinospore reactivity rate was not affected by the staining procedure. An even higher number of parasite stages was detected in carp than in trout gills. Subsequently, the ability of carp to lower the infection severity of susceptible rainbow trout by trapping the parasites under natural conditions was also investigated. Myxospore load was significantly reduced in hosts infected with actinospore samples that were preincubated with live carp. These results indicate the possibility of biological disturbance to the life cycle of the parasite in the wild by interceptor fish species as one measure to prevent whirling disease.