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 assess the conditions under which a novel strain, perhaps a mutant or migrant, may invade a population that already carries an endemic strain. Multiple strain dynamics can be modeled through coinfection 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 coinfection 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 understood to generate diversity, it is not always advantageous.