Fish coccidia: critical notes on life cycles, classification and pathogenicity

Abstract. The present knowledge of oocyst morphology, life cycles, classification and pathogenicity of eimeriid coccidia infecting fishes is reviewed. Compared with typical Eimeria species from higher vertebrates, fish coccidia, currently classified as all belonging to the same genus, differ in having, as a rule, a thin oocyst wall and endogenous sporulation. Some species possess a bivalved sporocyst, a structure and consequently an excystation mechanism, essentially different from that of the genus Eimeria. An analysis of the life cycles of more than 120 recorded species of fish coccidia has shown six different types of developmental patterns. The life cycles offish coccidia are compared with the life cycles of coccidia in general. None of the fish coccidia having a sporocyst with a Stieda body sporulates exogenously and therefore do not display a true eimerian developmental pattern. In view of the importance of sporocyst structure and developmental patterns in taxonomy, the revalidation of Labbé's genera Goussia and Crystallospora is proposed along with the establishment of a new genus, Epieimeria n.g. Most of the species offish coccidia are left within the genus Eimeria until new data make possible their correct classification. Detailed data on histopathological changes are available only in infections by two carp-infecting species, E. carpelli and E. subepithelialis and E. funduli infections in killifishes. The potential threat of coccidiosis in fishes reared under intensive conditions requires a thorough study of the pathogenicity of further species infecting commercially important hosts.