Antiparasitic immune mechanisms in teleost fish: A two-edged sword?

Numerous investigations during the last few decades have clearly demonstrat ed the existence of a wide array of immune mechanisms in teleosts. An incre asing amount of evidence indicates that both non-specific and specific fact ors in the humoral and cellular fish immune systems are well developed. How ever, a successful co-evolution of the host and its parasite necessitates t hat the latter develop evading mechanisms in order to avoid extinction. Som e parasites have even understood to benefit from the well developed antipar asitic armament in fish epithelia. Thus, a range of parasites are exploitin g the antiparasitic response mechanisms of the host to optimize host-findin g, invasion and survival in the host. Such interactions between hosts and p arasites are phylogenetically old. These dynamic interactions between hosts and invaders have been observed in various branches of fish parasitology. Experiments with protists such as Ichthyophthirius multifiliis and monogene ans such as Gyrodactylus deujavini indicate that parasites are activated by factors in host mucus (first line of defence) and actively search for epit helia rich in mucous cells. Some monogeneans, cestodes, digeneans, acanthoc ephalans and crustaceans are able to resist pronounced cellular host reacti ons which even will improve the attachment of the parasites to the host tis sue. Despite these evading mechanisms in the parasites it is possible to us e immunoprophylactic measures to control infections. Some parasites are ind eed actively rejected by their potential hosts provided these are effective ly immunized at certain early points before infection.