Metamorphosis of Ichthyophonus Schizonts Transiting the Gastrointestinal Tract of Experimentally Exposed Rainbow Trout

Other than the initial infectious cell, schizonts are the only stage of the parasite Ichthyophonus sp. that has been identified in the tissues of a living host, and they are known to initiate new infections when ingested by a suitable host. However, after feeding Ichthyophonus-infected tissue to Rainbow Trout Oncorhynchus mykiss, we observed that once infection was initiated, some schizonts proceeded to develop into several other morphologic forms indistinguishable from those previously described from recently deceased hosts, decomposing infected corpses, and in vitro culture. It appeared that not all schizonts participated in the infection process; some initiated infection, as expected, while others passed into the intestines, where they morphed into multiple cell types (e.g., schizonts, some with partially digested or ruptured capsules, ameboid plasmodia, merozoites, hyphenated cells, and empty capsules). Some of these cells were viable when cultured, but none was infectious to naive Rainbow Trout when administered by gavage. We posit that (1) not all tissue schizonts are programmed to perform the same function or (2) not all respond similarly to their environment. After consumption by a piscivore, those schizonts that do not initiate an infection do not die but rather metamorphose into different cell types as they transit the gastrointestinal tract and are ultimately released back into the aquatic environment through defecation. The fate of these cells after exiting the host is presently unknown, but they likely represent a segment of the Ichthyophonus life cycle.