Effects of host density on furunculosis epidemics determined by the simple SIR model

To determine the transmission dynamics of furunculosis, Chinook salmon, Oncorhynchus tshawytscha held at various densities were challenged with a single infectious fish. Data from the in vivo experiments were compared to predicted values from a susceptible--infected--removed (SIR) model. Seven different densities were tested including 9.13, 4.56, 0.72, 0.36, 0.19, 0.06 and 0.03 fish/L and each produced different transmission coefficients (beta) of 0.01, 0.019, 0.0051, 0.0076, 0.0001, 0.0005 and 0.00, respectively. Furunculosis related mortality rates decreased as density of the host decreased. Mortality rates at the highest fish densities where disease specific mortality (DSM) was observed were 0.42, 0.44, 0.18 and 0.02 infected animals per day. The natural mortality rate also followed a similar pattern with 0.005, 0.005, 0.001, 0.005, 0.0001, 0.000 and 0.000 surviving animals per day. The results indicated that the transmission coefficient and the DSM of furunculosis was dependent on host density. It is possible that fish behavioral changes at the lower densities caused the reproductive rate of disease to be persistent even at very low densities. In summary, the simple SIR model suggested that low mortality does not always mean low prevalence of furunculosis in a given population. This is the first study evaluating host density as a risk factor for fish diseases using experimental furunculosis data.