Current Project 2

Effects of Sustained Reduction of White-tailed Deer on the Abundance of Ixodes scapularis (Acari: Ixodidae) in an Endemic Lyme Disease Area of New Jersey

Background

Lyme disease has become the most common vector-borne disease in the United States. The importance of white-tailed deer (Odocoileus virginianus) as hosts for the blacklegged tick (Ixodes scapularis Say), the principal vector of Lyme borreliosis in the eastern and mid-western United States, is well documented, and the incidence of Lyme disease appears to be directly related to deer density. Because white-tailed deer have become overabundant in many suburban areas, local governments have conducted or are considering deer reduction programs with greater frequency. Justifications for such programs have included increasing concern for highway and road safety (reduction of deer-vehicle collisions); observed ecological effects due to over-browsing (replacement of native plants by invasive exotic plants, shifts in mammal and avian diversity); and reducing tick-borne disease risk.

Objective

We monitored the abundance of black-legged ticks and the Lyme disease incidence rate during a municipally sponsored deer reduction program within a suburban residential area to determine if there was a measurable decrease in the abundance of ticks due to deer removal, and if the reduction in ticks resulted in a reduction in the incidence rate within the human population.

Study Design & Methods

The study was conducted in Bernards Township, located in northern Somerset County, New Jersey. In Fall 2001, the Township began a five-year program of incremental deer removal and annual aerial census of the local deer population. We established tick sampling plots throughout the Township and in surrounding areas with no deer removal program to monitor the numbers of host-seeking ticks. We also conducted active surveillance for Lyme disease in the Township. Bernards Township is among 18 municipalities in the DHSS Active Surveillance.

Study Results

After three seasons, the estimated deer population was reduced by 46.7 %, from the 2002 post-fawning estimate of 2,899 deer (45.6 deer/km2) to a 2005 estimate of 1,540 deer (24.3 deer/km2). There was no apparent effect of the deer culling program on numbers of questing nymphal ticks in the culling areas and the overall numbers of host-seeking ticks in the culling areas appeared to increase in the second year of the program. The Lyme disease incidence rate generated by both passive and active surveillance systems showed no clear trend among years and did not appear to vary with declining deer density.

Conclusions

Given the resources required to mount and maintain a community-based program of sufficient magnitude to effectively reduce vector tick density in ecologically open situations where there are few impediments to deer movement, it may be that deer reduction, while serving other community goals, is unlikely to be a primary means of tick control by itself. However, in concert with other tick control interventions, such programs may provide one aspect of a successful community effort to reduce the abundance of vector ticks.

These results will be published in the Journal of Medical Entomology later this year.