Simulation studies of St. Louis encephalitis and West Nile viruses: the impact of bird mortality.

West Nile virus (WNv) has spread through much of the eastern United States following its introduction in 1999, and arrived in Florida in 2001. Prior to its arrival, we anticipated that its transmission cycle was likely to be similar to that of St. Louis encephalitis virus (SLEv). However, high levels of avian mortality have been reported for WNv in the northeastern United States, and it was unknown how this would impact the transmission dynamics of WNv. Simulation models were used to compare the two viruses by considering the impact of bird mortality on the transmission dynamics of arboviruses in south Florida. Transmission models without disease-induced mortality (SLEv) were compared with models including disease-induced mortality (WNv). Disease-induced mortality depressed transmission, eliminating epizootics in two of 14 simulations that were epizootic without the additional mortality. In both models, the most important factor in the likelihood of epizootics was mosquito population size; the mosquito mortality rate was also important. The additional avian mortality altered the factors most important in the size and timing of epizootics, although it did not always directly affect the outcome of the simulations. In some cases, low-level transmission occurred prior to the epizootic peak. When disease-induced avian mortality was included in the simulations, appreciable numbers of dead birds occurred prior to high levels of infection in mosquitoes. This has implications for the use of dead birds as a surveillance tool monitoring the spread and transmission of WNv.