Assessing the impact of host density on vector abundance and transmission scaling of Culicoides-transmitted pathogens

The spread of any pathogen depends on the dynamics of its hosts, with transmission rates typically assumed to either scale linearly with host population density (density-dependent transmission) or to be independent of it (frequency-dependent transmission). For vector-transmitted pathogens, a key determinant of transmission scaling is whether vector abundance is constant or sensitive to host abundance, with the latter being consistent with density-dependent transmission. Here, we assess whether Culicoides vector abundance increases in a manner that indicates density-dependent transmission of Culicoides-transmitted pathogens. To test this, we conducted trapping of Culicoides midges on eight livestock operations across a wide range of cattle abundances, placing traps at varying distances from the host aggregation. We used hierarchical Bayesian models to estimate the effect of host abundance on the vector-to-host ratio while accounting for differences in trapping efficacy between locations. Our results indicate a positive linear effect of host abundance on the ratio of vectors to hosts, with a posterior probability of 0.83. Median posterior values of the effect of host abundance on vector density predict a 2.3% increase (95% credible interval: -0.8, 23.2%) in the vector-to-host ratio with every 1,000 additional hosts. The weight of evidence from our study suggests that Culicoides-transmitted pathogens are likely subject to density-dependent transmission, and that transmission may be amplified in high-concentration livestock environments.