The Ecology of Lyme-Disease Risk
Complex interactions between seemingly unconnected phenomena determine risk of exposure to this expanding disease
Controlling Ticks and their Hosts
Ecologists and entomologists have searched unsuccessfully for biological agents that might control populations of black-legged ticks. Natural enemies of ticks, such as parasitic wasps, wolf spiders and foraging guinea fowl, have been found to be either highly localized in their impacts on tick survival or ineffective. Although further studies of the potential effectiveness of parasites, pathogens and predators as control agents are warranted, the biological control of ticks appears elusive. In many animals, population control is imposed naturally by crowding, which reduces reproduction rates or increases mortality, but whether this mechanism of control operates for ticks and, if so, in which life stages, is largely unknown.
My former student Kirsten Hazler, currently at North Carolina State University, and I designed a study to determine whether crowding of larval ticks on Peromyscus mice reduces the tick's ability to survive, feed successfully and molt into viable nymphs. We placed varying numbers of ticks on captive mice and followed their feeding and molting success. We found that the same proportion of larval ticks fed successfully on mice and molted into nymphs when the mice were heavily infested at 100 ticks per mouse as when they were lightly infested with only 5 ticks per mouse. Thus, at the densities we used, tick populations do not appear to be regulated by crowding on their most important host. In fact, our experiments suggested that ticks fed more successfully when they were more crowded on mice, a result that would be expected if the anticoagulants and immunosuppressive agents in tick saliva are more effective at higher doses.
A recent effort in our laboratory to create computer models of the dynamics of Lyme disease revealed that the degree of species diversity in the community of hosts for ticks may have a strong impact on the risk of Lyme disease. This simulation model, devised by Josh Van Buskirk at the Zoologisches Institut, University of Zurich, created a habitat in which ticks encounter host species in proportion to the population density of the host. As in nature, the computer-created hosts varied in their likelihood of infecting feeding juvenile ticks with bacteria, but did not vary substantially in the probability that a tick feeding on that host species would gain a full blood meal and molt into the next stage.
The model suggested that reducing the density of deer, which serve as hosts for adult ticks, would have only a modest effect on tick density, and that nothing short of the near eradication of these hosts would substantially reduce tick numbers. This result is supported by empirical data collected by Mark Wilson at the University of Michigan and his coworkers indicating that deer hunting has little if any effect on tick numbers, whereas eliminating deer by fencing them or removing them from islands drastically reduces tick density. On the other hand, the model suggested that the number of ticks can be reduced steadily by gradually reducing the density of hosts for juvenile ticks (that is, rodents and lizards), a prediction that has not yet been tested rigorously.
Perhaps most interesting, the model suggested that infection rates of juvenile ticks, and hence the risk of Lyme disease, are lower when the host community is highly diverse than when only a few species of hosts are available. Because only one species of host, the white-footed mouse, is a highly competent reservoir for B. burgdorferi, any increase in the species diversity of hosts will dilute the effect of mice by offering ticks alternative, reservoir-incompetent hosts from which to feed. This result is consistent with empirical results obtained from the southeastern and western United States by Robert Lane of the University of California at Berkeley and his colleagues. In these areas the host community includes several species of reservoir-incompetent rodents and lizards, and the infection rate of Ixodes ticks is much lower than in the northeastern and north-central United States. A key implication of the model is that maintaining high species diversity in vertebrate communities, for instance by maintaining high habitat diversity or high predator density, may reduce the risk of Lyme disease.
Recently Josh Van Buskirk and I extended the simulation model to a landscape consisting of several different types of habitat patches among which both rodent and deer hosts were able to move. We arranged these habitat patches according to our understanding of the nature of semirural and suburban landscapes in Lyme-disease-endemic areas of the northeastern United States. In these areas, patches are relatively small compared to the dispersal capabilities of mammals, and some habitat types constitute "source patches," as they are net exporters of emigrants, whereas others constitute "sink patches" because they are net importers of immigrants.
One key result of the model was that considerable densities of highly infected ticks can be maintained in “sink” habitats even when the average density of rodent hosts is low. The density of infected ticks in any particular habitat type is a consequence of the proximity of other patch types and the dispersal patterns of rodent and deer hosts, which our empirical studies demonstrate are affected by acorn availability. Thus, our model and field data indicate that the local risk of Lyme disease cannot be predicted simply by focusing on conditions within the local habitat. Rather, land-use policy makers must also consider the dynamics of mice and ticks in the surrounding landscape.