Theoretical considerations of cross-immunity, recombination and the evolution of new parasitic strains

We explore the dynamics of multiple strains of a parasite in order to asses s the conditions under which a novel strain, perhaps a mutant or migrant, m ay invade a population that already carries an endemic strain. Multiple str ain dynamics can be modeled through coinfection, superinfection or complete cross-immunity. We examine these three modes to discuss the relationships among cross-immunity, the basic reproductive rates of each strain, and the invasion of the new strain. Superinfection is more restrictive than coinfec tion in the proportion of parameters that allows invasion. The coinfection model is extended to allow haploid strains to undergo recombination within the host. We investigate the effects of recombination and cross-immunity on the invasion of new strains. Interestingly, although recombination is unde rstood to generate diversity, it is not always advantageous. (C) 1999 Acade mic Press.