Virulence of vector-borne pathogens. A stochastic automata model of perpetuation.

To determine how virulence may be perpetuated in populations of vector-borne pathogens, we simulated their fitness in a stochastic simulation based on cellular automata. Thereby, directly transmissible pathogens that differed in virulence were permitted to compete for hosts with similarly virulent pathogens that could infect hosts remotely because they were vector-borne. Fitness was defined as the proportion of the host population infected with each pathogen at equilibrium. Virulent, directly transmitted pathogens prevailed solely when their infectivity was transient. When duration of infectivity exceeded that of host survival, the less virulent pathogen invariably prevailed. Although remotely transmitted virulent pathogens persisted somewhat longer than did virulent pathogens that were transmitted directly, they never perpetuated themselves. We conclude that populations of vector-borne pathogens may retain pathogenicity somewhat longer than do those that are directly transmitted, but that both kinds of pathogens tend to become nonvirulent.