Fish intestinal parasites cause direct mortalities and also morbidity, poor growth, higher susceptibility to
opportunistic pathogens and lower resistance to stress. This review is focused on microscopic parasites
(Protozoa and Metazoa) that invade the gastrointestinal tract of fish. Intracellular parasites (mainly
Microsporidia and Apicomplexa) evoke almost no host immune reaction while they are concealed in the
cytoplasmic and nuclear compartments, and can even use fish cells (macrophages) as Trojan horses to
spread in the host. Inflammatory reaction only appears when the parasite bursts infected cells. Immunity
against extracellular parasites is depicted for the myxozoans Ceratonova shasta and Enteromyxum spp.
The cellular and humoral innate responses and the production of antibodies are crucial for resolving
some of these myxozoonoses, but an excessive inflammatory reaction (concerted by cytokines) can
become a fatal pathophysiological consequence. The local immune response plays a key role, with
numerous genes more strongly regulated in the intestine than at lymphohaematopoietic organs.
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND