Infection by the parasite Myxobolus bejeranoi (Cnidaria: Myxozoa) is triggered by temperature elevation resulting in immune suppression of its fish host

Myxozoans are microscopic obligate parasites that belong to the phylum Cnidaria, alongside free-living corals, sea anemones, jellyfish and hydrozoans. With over 2200 species, myxozoans can cause severe ecological and economic effects in both freshwater and marine environments. Myxozoans have complex life cycles, involving transmission between alternate vertebrate (mostly fish) and invertebrate (mostly worms) hosts via waterborne spores. Recently, the myxozoan Myxobolus bejeranoi was found to infect the gills of cultured hybrid tilapia at more than 50% prevalence, leading to high mortality rates. We characterized the temporal progression of M. bejeranoi infection and the immune response of the host fish by performing transcriptomic analyses of both the sporulation site and the immune organs. We found that the parasite employed an immune evasion strategy of a thorough shutdown of the immune response, including deactivation of secreted cytokines and of a transcription factor responsible for T helper cell differentiation. The consequence is an immune-deprived fish, which is expected to be highly susceptible to other opportunistic pathogens. Interestingly, parasite-host interaction was temperature dependent, providing additional support to the notion that temperature is the primary driver of myxozoan disease dynamics. These findings suggest that M. bejeranoi is a highly efficient parasite that can disable the defense mechanisms of its fish host. However, because of the complexity of myxozoan life cycles, predicting the effects of global warming on myxozoan disease is difficult. In the talk, additional examples of the effect of climate change on myxozoans around the world will be discussed.