Transmission of the parasite Ichthyophonus hoferi in cultured rainbow trout and comparison of epidemic models.

The epidemic process of the parasite Ichthyophonus hoferi in cultured rainbow trout Oncorhynchus mykiss was quantitatively estimated by both the cohabitation experiment and two standard models (the Kermarck-McKendrick model and the Reed-Frost model). For analysis of the parasite transmission by cohabitation, fish in two replicate tanks were exposed to 1, 5, or 10 infected fish, and daily mortality was counted for 102 d. Despite simple experiments for artificial exposure to the pathogen, the daily estimate of dead fish in the Kermarck-McKendrick model did not fit the observed number of dead fish in the experiment. In contrast, when the longest possible incubation period (generation time) was assumed to be 51 d in the Reed-Frost model, the estimated number of dead fish in discrete generations was close to the observed number of dead fish. If the time unit was 51 d, the estimated mortalities in the generation-based Kermarck-McKendrick model were significantly correlated with observed mortalities. These results suggest that the deterministic aspects of the epidemic process of the parasite can be quantitatively demonstrated on a 51-d timescale or longer, whereas transmission on a daily timescale is uncertain.