Immune response of fishes to ciliates

Abstract

Ciliates are highly evolved protists comprising a phylum of diverse species, many of which are opportunistic or obligate parasites. Ciliates parasitic to fish consist of salt and freshwater forms with endo- or ectoparasitic modes of infection. Some of the more commonly encountered genera include Chilodonella, Brooklynella, Ophryoglenina, Ichthyophthirius, Cryptocaryon, Uronema, Tetrahymena, Epistylus, and Trichodina. Species range from obligate parasites and commensals to opportunistic, facultative forms. Some parasitic ciliates are highly pathogenic and fishes in closed environments such as aquaria and farm ponds are particularly susceptible to high mortalities. Nevertheless, fish have evolved an immune system capable of mounting an effective protective response against parasite challenge. Much of the experimental research on immunity against ciliates has been carried out with Ichthyophthirius multifiliis, on obligate parasite that invades surface epithelia of virtually all freshwater fish species. Interest in the immune response against I. multifiliis stems from the fact that convalescent fish become resistant to subsequent challenge (suggesting the possibility of immunoprophylaxis), and the need to curtail severe losses caused by this parasite in intensively farmed fishes. Furthermore, I. multifiliis has proven to be a useful experimental model because it is amenable to study under laboratory conditions. In this review cellular and humoral factors involved in both innate and acquired immunity against ciliates are covered and include natural killer cells, phagocytic cells, and antibody responses. Current ideas on the mechanisms of antibody-mediated cutaneous immunity against I. multifiliis are discussed and approaches toward the development of vaccines against this and other ciliate parasites are presented.