Tick-borne infectious agents in nature: Simulated effects of changes in host density on spatial-temporal prevalence of infected ticks

Abstract

Ticks (Ixodidae) are important vectors of infectious agents that affect human and animal health, and the spatial-temporal dynamics of tick-host-pathogen-landscape interactions are difficult to understand based on empirical observations alone. We used a spatially explicit simulation model to examine the effects of changes in host density on the prevalence of a hypothetical transstadially transmitted infectious agent in a population of a prototypical three-host tick under ecological conditions representative of the predominantly forested areas of the south-central United States. The model was parameterized such that baseline conditions yielded a landscape-level nymphal infection prevalence (NIP) fluctuating seasonally around a threshold of 0.1 (indicative of pathogen endemicity in some disease systems) roughly paralleling seasonal fluctuations in wildlife host densities, with seasonal highs in late summer and early fall and seasonal lows in winter and spring. In simulated scenarios of both small-sized and medium-sized host reduction, the densities of both uninfected and infected off-host nymphs decreased markedly from year-to-year. The number of habitat patches in which NIP>0.1, however, increased when small-sized hosts were removed, yet decreased when medium-sized hosts were removed. Simulation of the reduction in density of large-sized hosts resulted in trends similar to those produced by reducing density of small-sized hosts, but trends were less pronounced. Under the conditions simulated, both NIP and off-host nymph densities (DON) were particularly sensitive to changes in the proportion of larvae obtaining their blood meal from medium-sized hosts. Variation in simulated NIP values can be explained by the fact that larval, nymphal, and adult tick loads were distributed differently among the different-sized hosts, each with their distinct range of movement and degree of variation in population size. Simulation results of this hypothetical case study offer insight into the complex landscape-level interactions of a prototypical 3-host tick and suggest that medium-sized hosts could play a key role in sustaining and dispersing a tick-borne infectious agent in nature.