Interactions Between Parasitic Ciliates and Their Hosts: Ichthyophthirius multifiliis and Cryptocaryon irritans as Examples

Among all known parasitic ciliates, Ichthyophthirius multifiliis and Cryptocaryon irritans are probably the two best studied “model parasites,” which cause the white spot diseases in freshwater and marine fish, respectively. These two parasites are not phylogenetically closely related, but share very similar life cycle and pathological manifestations. Both parasites invade and feed in the epithelial layer of the skin and gill, and trigger both innate and adaptive immune responses in fish hosts, locally and systemically. The parasite-host interactions not only help add to our knowledge of fish immunity, but also shed light on research regarding other less well-studied ciliate parasites. Future research directions are also discussed in this chapter.