The Biology of Fish Coccidia

This chapter discusses the biology of fish coccidia. The oocyst and, particularly, the structure of its contained sporocysts, are important features in differentiating the genera and species of coccidia in fishes. Except for three genera, Cryptosporidium, Octosporella, and Isospora, the fish coccidia have oocysts containing four sporocysts, each with two sporozoites (tetrasporocystic and dizoic). As with coccidia from other groups of vertebrates, the gut is often the favoured site for development, but fish coccidia are remarkable for their extra intestinal development. Regarding the life cycles of fish coccidia, their sequential development is similar to that observed in coccidia from other vertebrates. In the life cycles of fish coccidia, transmission is of two main types. The simpler method is by direct transmission involving faecal contamination. The second mode of transmission is indirect and involves an invertebrate host. However, some fish coccidia employ both methods. Both the direct and indirect mode of transmission are described in detail.