Alizon and Lion consider the role of cooperation between coinfecting parasites in the form of public goods (PGs) in the evolution of virulence. In general it is expected that co-infections should select for increasing virulence in parasites. Previous studies have suggested that co-infections generally select for less virulent strains when effective exploitation of the host requires cooperation by parasites. They use a nested model of within-host and between host (epidemiological) dynamics which allows them to describe the relationship between parasite densities and epidemiological parameters. Within host dynamics of a single parasite are determined by a differential equation with three terms: logistic growth, cost of PG production, and benefit of PG exploitation (based on the uptake rate of PG). This model assumes that all strains are equally competitive to obtain other non-PG resources within the host. They pair this within host model with two alternative epidemiological models (1) a transmission-virulence tradeoff model in which both transmission and virulence increase with within host densities transmission can either be a linear or saturating function of virulence and (2) a model in which virulence depends on parasite density and total PG density. Both models incorporate a new term, sigma, which represents the susceptibility of an already infected host to additional infection affecting the average multiplicity of infection (MOI). MOI is also determined by the epidemiological density of susceptible hosts, when this density is low the MOI is expected to be close to its maximum (2 in the case of a wild-type and a mutant strain). As a result varying demographic parameters is likely to change MOI and optimal investment in PG.
When the trade-off between transmission and virulence is linear and there is no co-infection selection leads to maximization of virulence. With increasing susceptibility to co-infection the investment in PGs will decrease as the number of singly infected hosts decreases. The decrease in singly infected hosts decreases the advantage of cooperating and favors cheaters who produce fewer PGs. When there is a saturating transmission-virulence tradeoff and high levels of susceptibility to secondary infection PG production is never selected for. When there is low suscpetibility to secondary infection, however, there is bistability depending on initial conditions: with low PG investment PG is not selected for, with high initial PG investment high PG production is selected for. In the second model in which virulence depends on densities and PGs there is a non-monotonic relationship between evolutionarily optimal PG production and susceptibility to secondary infection. As a result PG production and virulence are maximized at intermediate MOI because PG’s separate contribution to virulence increases the cost of parasite survival. The authors also explore the effect of variation of host background mortality on MOI and PG production. MOI is generally a decreasing function of mortality. Generally the relationship between optimal virulence and MOI is similar across a range of background mortality rates when the trade-off relationship is linear. For other trade-off functions, such as the saturating relationship the patterns of MOI, PG production, and virulence are not consistent across a range of demographic parameters.
Alizon, Samuel, and Sébastien Lion. “Within-host parasite cooperation and the evolution of virulence.” Proceedings of the Royal Society of London B: Biological Sciences (2011): rspb20110471.