Deer Tick Populations Have Exploded
by Jeanne Siviski
Back in 1997, an article in the Maine Woodland Owners newsletter raised questions about the spread of Lyme disease in Maine. Would environmental constraints keep deer tick populations at bay? What role would host populations or forest type play? Forty one Lyme diseases cases had been reported in 1996.
Today, the numbers are staggering, with 1,334 confirmed cases for 2014 so far. That number will climb, since reporting lags by several months. Most cases are in York and Cumberland counties, but incidence rates, per 100,000 people, have trended higher in Knox, Waldo and Lincoln. There are confirmed Lyme cases statewide. In Houlton and Farmington, deer ticks “are really coming in,” said Charles Lubelczyk, vector ecologist at Maine Medical Center Research Institute. Nor is Lyme alone among diseases for which Ixodes scapularis, or deer ticks, are a vector. Maine recorded 42 cases of babesiosis, a disease of the red blood cells in 2014, and anaplasmosis, a disease of white blood cells, doubled in a year from 94 to 190. Lubelczyk said spread of these diseases is about 10 years behind Lyme.
Ecological and environmental influences on I. scapularis are complex and interwoven. Take the classic biology textbook case of the interplay between gypsy moths, oak trees, white-footed mice, and Lyme disease. Damage done by gypsy moths can lower acorn production, but higher acorn yields leads to larger populations of white-footed mice and the presence of deer – two major deer tick hosts. Thus, a bumper crop of acorns can be used as a predictor of rising Lyme disease cases.
Between July and September, deer ticks begin a two-year life span as hatched larvae, and start questing for a meal from a small mammal or bird, often a white-footed mouse, chipmunk or robin. At this point, deer ticks can become infected if their host is carrying the spirochetal pathogen, Borrelia burgdorferi, which causes Lyme disease. White-footed mice, the prime deer tick hosts at the larval stage, appear to be more tolerant to ticks than other species, which remove them by grooming. As the last to leave in low species diversity locations, white-footed mice are often abundant. An oral vaccine has been developed to treat mice. According to a 2013 study, the vaccine would both protect uninfected mice and reduce transmission of B. burdorferi, in already infected mice. Another commercially available solution, tick bait boxes, lure mice and contain an insecticide. Treating mice may have limitations, however. “Deer ticks are highly adaptable,” Lubelczyk said.
After their first meal during the larval phase, I. scapularis drop into the leaf litter, then emerge in the spring as nymphs and quest for their second meal. Adult ticks feed and mate on deer, their preferred host. In York County, deer densities can be as high as 75 per square mile. Deer populations below 15 reduce tick populations, Lubelczyk says.
Deer ticks depend on humidity for hydration and are vulnerable to drought, so higher humidity in coastal regions may increase survival. Overly wet conditions do not suit them; they need leaf litter with moderate moisture. A study by the Maine Medical Center Research Institute illustrates tick density in several southern Maine forest sites (see chart on page 1). Tick counts dropped significantly in Cape Elizabeth spruce/fir forests, despite proximity to areas with high deer tick density and deciduous forest cover. For long-term management, Lubelczyk suggests invigorating small stands of spruce/fir as a possible control measure.
A 2006 Maine study found that deer ticks thrive on browse-resistant invasive plants like Japanese barberry and Eurasian honeysuckles – deer tick populations doubled in plots with browse-resistant invasives. The dense thickets create an ideal “microclimate’ for all deer tick life stages: leaf litter below, birds and mice within reach, and a vantage point for questing deer. Over-browsing of native plants by deer has contributed to the spread of invasives and is another illustration of the complex interrelationships that affect deer tick populations.
To date, seven different pathogens are transmitted by deer ticks, including Powassan virus, an arbovirus that killed a Maine resident in 2013. This was the first case in nearly a decade. But deer ticks are not the only vectors of disease. Spotted fever ricketsiosis, a group of bacterial illnesses, is carried by infected dog ticks; lone star ticks can carry ehrlichiosis. Neither illness is common here, but a 2014 report said ehrlichiosis “may be a disease to watch as the [lone star] tick appears to be moving north.” Looking ahead, Lubelczyk said that “We will probably see more recognized diseases transmitted by ticks. Lyme will just be one among many.”