Vermont in 2013 led the nation in Lyme disease infection rates, and Maine and New Hampshire ranked second and third. Yet in 2002, Vermont was not even mentioned in a U.S. Centers for Disease Control report ranking the top 12 states in the incidence of Lyme disease.
What has changed? It is the significant northward expansion of blacklegged “deer” ticks carrying Lyme disease into communities throughout northern New England.
In March, the Small Woodland Owners Association of Maine reported deer ticks “were coming in” to Houlton, a North Woods community where the January temperature is about 0 degrees and snowfall averages 100 inches. The idea that colder temperatures would inhibit the spread of ticks and Lyme disease has proved to be false.
Scientists and health professionals tell us today that the entire New England region supports a potent combination of weather and humidity, mice and other small rodent populations, dense thickets of invasive barberry and, most importantly, overabundant deer populations to serve as the blood meal for the nymphs of deer ticks to emerge as egg-laying adults.
* * *
Three numbers link the presence of barberry in the landscape to abundant populations of infected deer ticks.
According to a study by the University of Connecticut’s Department of Natural Resources and the Environment and the University’s Agriculture Experiment Station, the number of deer ticks per acre is 120 where no effort is made to control barberry. Making this control effort reduces ticks per acre to 40. Where barberry is absent, the number declines to 10 ticks per acre.
Ticks require about 80 percent humidity to actively feed and reproduce. In the shade of a barberry plant, this level of humidity is sustained for 23 hours a day. In open, sunny areas with no barberry, 80 percent humidity is maintained for only one hour each day.
Studies in Maine and elsewhere confirm tick population findings in studies of barberry as well as invasive honeysuckle. Both shrubs also provide similar habitat for the white-footed mouse and other small rodents, which provide the blood meal required by tick larvae to become nymphs.
Charles Lubelczyk, a vector biologist at the Maine Medical Center Research Institute notes that a warming climate and shorter winters help to expand tick habitat as well as increase the number of days ticks are active during the year. Lubelczyk said spring migrant birds import deer tick larvae and nymphs into new environments.
The tiny nymph form of the tick is responsible for an estimated 98 percent of the cases of human infection with Lyme. The bite of the much larger adult deer tick attached to human skin, in comparison, is much easier to detect and remove.
* * *
While normally, the numbers reported for Lyme disease fluctuate annually, in northern New England, there is a dramatic upward trend.
Vermont reported 12 cases in 2002. The number of confirmed and probable cases of infection increased to 893 in 2013. Underreporting is widespread, with some experts suggesting as few as 10 percent of actual cases are recorded.
It is hoped that increased public awareness might halt the increase in public infection rates. Certainly, it is not uncommon to learn of infections from neighbors and individuals, such as hikers and foresters, who spend a lot of time in the woods.
We can take steps to reduce or remove barberry and honeysuckle. But this growing problem is massive and ongoing. There is little possibility that control efforts on a regional level are capable of reducing the threat of a bite by an infected blacklegged tick. However, local efforts by landowners are useful and well advised.
Thus far, we have recounted a number of factors in Lyme disease expansion: a warmer climate, migrant birds as carriers of deer tick larvae and nymphs, and invasive plants as ideal habitat for both ticks and white-footed mice.
What about the relationship to deer populations? To complete the life cycle into an egg-laying adult, the deer tick nymph requires a final blood meal. Most often, the host for this meal is a white-tailed deer.
On Monhegan Island in Maine, when Lyme disease infection rates reached 13 percent of the small year-round population, residents voted to eliminate all deer. Local hunters completed this task in about two years, reducing deer density from about 60 per square mile.
As a result, new Lyme disease cases also dropped to zero. Bird migrations flew in additional tick larvae and nymphs. But without the deer to provide a blood meal, there was no tick reproduction.
The link between deer and Lyme disease cannot be argued. Which brings us to the question of deer density, measured as the number of deer per square mile.
The white-tailed deer population of Vermont, about 120,000, equally spread out over the state’s 9,200 square mile land area, would create an average of 13 deer per square mile. Some areas experience populations well above the average.
* * *
Forty-eight percent of Vermont respondents to a public-opinion survey on deer numbers said deer numbers should remain the same, and 32 percent said they should be increased.
As deer browse (the leaves, twigs, and buds of woody plants) is often observed on woods walks, a good question is: How many deer can a woodland support without serious negative impact on vegetation and timber regeneration?
This number, reports a U.S. Forest Service publication, White-tailed Deer in Northeastern Forests, is 20 deer per square mile. The same number of 20 per square mile is noted in Deer, Ticks and Lyme Disease, a fact sheet authored by the Connecticut State Entomologist and a wildlife biologist at the state’s Agricultural Experiment Station.
The fact sheet noted: “Observational studies and computer models suggest that a reduction of deer density to less than twenty per square mile may significantly reduce a tick bite risk, while lower levels (less than 8 deer per square mile) would interrupt the enzootic cycle of Lyme disease and transmission [...] to wildlife and humans.”
Another perspective is shared by Susan Elias, clinical research associate at the Maine Medical Research Center’s “tick lab.” Elias says, “Reducing deer density to 12 per square mile is necessary to control tick populations.”
In future years, new data on Lyme disease trends in Vermont will give a more complete picture of the scale of the epidemic based on human experience. The cumulative impact of new cases of Lyme disease may lead to more discussion of the issue of deer density in some regions of the state. Is deer overabundance a preventable public health risk?
It is essential to increase public awareness of the risks of Lyme and several other tick borne diseases carried by blacklegged ticks. Careful tick checks at the end of each day in the field are a well-established habit for many of us, along with wearing clothing protected by Deet or Permethrin.