EVOLUTION OF VIRULENCE - A UNIFIED FRAMEWORK FOR COINFECTION AND SUPERINFECTION

Models of the evolution of parasite virulence have focused on computin g the evolutionarily stable level of virulence favored by tradeoffs wi thin a host and by competition for hosts, and deriving conditions unde r which strains with different virulence levels can coexist. The resul ts depend on the type of interaction between disease strains, such as single infection (immunity of infected individuals to other strains), coinfection (simultaneous infection by two strains), and superinfectio n (instantaneous takeover of hosts by the more virulent strain). We pr esent a coinfection model with two strains and derive the superinfecti on model as the limit where individuals are rapidly removed from the d oubly-infected class. When derived in this way, the superinfection mod el includes not only the takeover of hosts infected by the less virule nt strain, but new terms which take into account the possibility of in creased mortality of doubly-infected individuals. Coinfection tends to favor higher virulence and support more coexistence than the single i nfection model, but the detailed results depend sensitively on two fac tors: (1) whether and how the model is near the superinfection limit, and (2) the shape of the coinfection function (the function describing the rate at which a more virulent strain can infect a host). If the s uperinfection limit arises due to rapid mortality of doubly-infected h osts, there is a region of uninvadable virulence levels rather than co existence. When the coinfection function is discontinuous, as in many previous models, neither the coinfection model nor the superinfection limit can support an evolutionarily stable virulence level. Piecewise differentiable and differentiable coinfection functions produce qualit atively different results, and we propose that these more general case s should be used to study evolution of virulence when other mechanisms like space, population dynamics, and stochasticity interact. (C) 1998 Academic Press.