Journal of Fish Diseases 22 4 277 283

Clinical signs associated with L. salmonae infection in Pacific salmon (Oncorhynchus tshawytscha (Walbaum)) arise when the parasite enters sporogony and forms xenoparasitic complexes (xenomas) within the gill. A thermal unit (TU) model, devised to predict the timing of xenoma formation, under conditions in which water temperature changed during the course of infection was evaluated. In vivo trials with juvenile trout showed that the model [TU for xenoma onset = (days post exposure) x ( degrees C above 7 degrees C)] accurately predicted the onset time for xenomas when fish were moved from either 11 or 5 degrees C to 15 degrees C at various intervals after exposure. It is suggest that the TU model may allow aquaculturists to predict disease onset. However, the model failed when fish were moved from 15 to 5 degrees C at intervals after exposure. This finding suggests that the temperature-constrained phase of the life-cycle of L. salmonae occurs early on after the spore is ingested by the fish.