Small amoeboid cells, believed to be the infectious stage of Ichthyophonus sp., were observed in the bolus (stomach contents) and tunica propria (stomach wall) of Pacific staghorn sculpins and rainbow trout shortly after they ingested Ichthyophonus sp.–infected tissues. By 24–48 hr post-exposure (PE) the parasite morphed from the classically reported multinucleate thick walled schizonts to 2 distinct cell types, i.e., a larger multinucleate amoeboid cell surrounded by a narrow translucent zone and a smaller spherical cell surrounded by a “halo” and resembling a small schizont. Both cell types also appeared in the tunica propria, indicating that they had recently penetrated the columnar epithelium of the stomach. No Ichthyophonus sp. pseudo-hyphae (“germination tubes”) were observed in the bolus or penetrating the stomach wall. Simultaneously, Ichthyophonus sp. was isolated in vitro from aortic blood, which was consistently positive from 6 to 144 hr PE, then only intermittently for the next 4 wk. Small PAS-positive cells observed in blood cultures grew into colonies consisting of non-septate tubules (pseudo-hyphae) terminating in multinucleated knob-like apices similar to those seen in organ explant cultures. Organ explants were culture positive every day; however, typical Ichthyophonus sp. schizonts were not observed histologically until 20–25 days PE. From 20 to 60 days PE, schizont diameter increased from ≤25 μm to ≥82 μm. Based on the data presented herein, we are confident that we have resolved the life cycle of Ichthyophonus sp. within the piscivorous host.

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