Speaker: Prof. Andrea Pugliese,
University of Trento

Title: Within-host immune dynamics, epidemic models and evolutionary dynamics of virulence

Abstract: Several recent papers have introduced explicit modelling of hosts' immune response to study epidemic dynamics and the evolution of pathogens. Following this idea, I extend the submodel for within-host dynamics by Gilchrist and Sasaki (2002) in which functional response in pathogen-immune system interaction, and decay of immune response are considered: different qualitative behaviours (pathogen clearance, pathogen uncontrolled growth, equilibrium or periodic coexistence of pathogen and immune response) are possible, according to parameter values and initial conditions. Passing to the population level, one obtains an epidemic model in which the infective class is structured through individual pathogen level and immune response. If, as assumed by Gilchrist and Sasaki (2002), the pathogen load at infection is fixed, the model can reduce to an age-of-infection structure, and its qualitative behaviour follows the usual properties of epidemic models. If, on the other hand, initial infection load depends on the pathogen load of the individual from whom the infection is acquired, more complex behaviours appear to be possible. Moving to evolutionary dynamics, these models indicate that an intermediate level of pathogen replication is selected for. However, for most parameter values, this `intermediate' level of pathogen virulence corresponds to rather high lethality rates compared to normal infections. If immune response rates in the host population, instead of being a constant, follow some probability distribution, computer simulations show that a much smaller replication rate is selected among pathogen; moreover, the overall lethality rate is smaller, and concentrated in the `weakest' individuals. This underlines the relevant role of non-genetical variability in populations.