Effects of host heterogeneity on epidemiological dynamics are mediated by how host infectiousness is determined

It is well established that heterogeneities in host susceptibility and infectiousness can affect population-level parasite transmission. However, the role played by the underlying ‘infectiousness determination scenario’, or how infectious a newly infected host (the ‘recipient’) becomes upon infection, in mediating the effects of host heterogeneity on parasite transmission is unknown.

Broadly, host infectiousness may be an inherent, pre-determined trait of the recipient (which we call ‘Recipient-Dependent’ (RD)), or it may be determined by the infectiousness of the ‘donor’ host that infected them (which we call ‘Donor-Dependent’ (DD)). To investigate how these alternative infectiousness determination scenarios mediate the epidemiological consequences of host heterogeneity we developed two Susceptible-Infected compartmental models which incorporate host heterogeneity in both susceptibility and infectiousness, under the contrasting RD and DD scenarios. We then quantified how changing initial population-level heterogeneity affected three epidemiological measures in each case: the basic reproduction number ( R ₀ ), temporal changes in heterogeneity, and equilibrium host abundance. We found that the primary driver of R ₀ differs between the two scenarios – covariance between host susceptibility and infectiousness drives R ₀ in the RD scenario, versus maximum infectiousness in the DD scenario. Heterogeneity stayed the same over time in both scenarios, though there were differences between the scenarios in the consequences of host heterogeneity on equilibrium host abundance. Overall, the findings we present demonstrate that the infectiousness determination scenario can change epidemiological outcomes considerably and should be accounted for when modelling host heterogeneities in parasite transmission.