Evaluating the risks of engineered viruses: Modeling pathogen competition

Recently there has been a great deal of interest in the potential use of ge netically engineered baculoviruses as environmentally benign insecticides. Because baculoviruses often have a significant impact on the population dyn amics of their hosts, any effort to assess the environmental impact of rele asing engineered viruses must confront the question: Will genetically engin eered baculoviruses outcompete wild-type strains, thereby altering the natu ral population dynamics of the host? To begin to answer this question, we d evelop a mathematical model of competitive interactions between genetically engineered and wild-type baculoviruses. We find that the interactions betw een these viruses are characterized mostly by dominance of one strain or th e other, and that the chance that an engineered strain will outcompete a wi ld-type strain depends on its particular combination of speed of kill and i nfectiousness. That is, baculoviruses must kill their host to become infect ious, so the faster speed of kill of most recombinant viruses confers a com petitive advantage. Most such strains, however, also produce fewer infectio us particles and so are less infectious. Our model shows that the extent of this decrease in infectiousness must be rather small for an engineered str ain to become dominant. Nevertheless, even engineered strains that are at a substantial competitive disadvantage relative to the wild type may take de cades to go extinct. An additional complicating factor is that the outcome of competition depends on the overwinter survival of these viruses, about w hich little is known even for wild-type viruses. Caution is therefore neces sary in predicting the outcome of competitive interactions involving introd uced baculoviruses, and further work is needed in understanding pathogen ov erwinter survival rates.