Effects of host heterogeneity on parasite transmission are mediated by the dynamics of infectiousness determination

It is well established that heterogeneities in host susceptibility and infectiousness affect transmission, and are typically assumed to be pre-determined traits. However, they may arise dynamically during the transmission process. Specifically, while infectiousness may be an inherent trait of the recipient (‘recipient-dependent’), it may instead be determined by the donor host that infected them (‘donor-dependent’). We investigated how the effects of heterogeneities on transmission are affected by these contrasting scenarios by analysing two ‘Susceptible-Infected’ models for three metrics: the basic reproduction number ( R 0 ), changes in heterogeneity, and equilibrium host abundance. We show that the primary driver of R 0 differs between the two scenarios: covariance between susceptibility and infectiousness for recipient-dependent, versus maximum infectiousness for donor-dependent. Consequences for equilibrium host abundance also differed, but changes in heterogeneity did not. Our results show that these scenarios change epidemiological dynamics and should be considered when exploring the consequences of host heterogeneity on transmission.