If you register a deer on opening day of this year”s gun deer season, you might notice people going over deer carcasses with a fine-toothed comb. They”ll be collecting deer ticks, but please don”t question their sanity. Survey results will help researchers map the deer tick”s range in Wisconsin, according to Susan Paskewitz, an entomologist at the College of Agricultural and Life Sciences.
The data will help us understand the ecological determinants of deer tick distribution, says the University of Wisconsin-Madison researcher. This information will give us a better idea of how fast and far the ticks are going to spread through Wisconsin. Knowing where the ticks are, and what diseases they are carrying, will help physicians diagnose and treat victims of Lyme disease and other tick-borne ailments.
Volunteers will collect ticks and draw blood samples from deer at 11 registration stations around the state. They will focus on southern Wisconsin “edge” areas, such as Dane County, where past surveys have shown many ticks around Mazomanie and Arena but few ticks east of Madison. Surveys next year will target eastern Wisconsin, which has been fairly tick-free. The Wisconsin River valley, so far, seems to be limiting the ticks” eastward march.
Paskewitz has also trapped and sampled other tick hosts (deer mice and chipmunks) around Wisconsin. Samples from these surveys are screened for Lyme disease, human granulocytic ehrlichiosis (HGE) and human babesiosis. (see sidebar)
Deer ticks are equal-opportunity disease carriers, and can transmit multiple pathogens with a single bite. Infection with two diseases – Lyme disease and HGE, for example – may affect the course of Lyme disease, making it more severe and slower to respond to antibiotic treatment. Paskewitz is looking for overlaps in pathogen populations in ticks, and how those overlaps correlate with more severe cases of Lyme disease. This information should allow physicians in high Lyme/HGE areas to watch for both diseases in their patients.
Using computerized databases, Paskewitz can generate a map of points indicating where and how many cases of Lyme disease occurred, for example. She can take maps of Lyme disease occurrences, tick distribution, land use patterns and other factors, lay them over one another, and do an analysis that shows any associations. The system allows researchers to crunch a wide variety of variables, such as soils, climate, plants, vertebrate hosts, and infection levels in hosts.
“If we know all the parameters are there for ticks, then we can warn doctors to look for Lyme and other tick-borne diseases in patients, even if no ticks have been reported in the area,” Paskewitz points out.
“In Wisconsin, we find a different picture from survey to survey,” she says. “Each new survey shows more ticks in new places.” Unfortunately, she has no reports of tick declines in infested areas.
In Paskewitz”s 1994 survey, volunteers sampled 1,200 deer at 26 registration stations on opening day of Wisconsin”s gun deer season. Compared with earlier surveys, the results showed that deer ticks made steady progress in spreading through the western two-thirds of Wisconsin.
“This gave us baseline data, showed a few pockets of ticks in northeastern Wisconsin, and defined the eastern edge of prevalence in the state,” Paskewitz says. “We”re using these data to determine the ”patchiness” of tick distribution. One area may show severe infestation, while another sampling spot a few miles away turns up few or no ticks.
“We know there are areas where human cases have been reported but we don”t know anything about the tick populations – we”ve sampled but haven”t found ticks. The ticks may be very clustered, and we need a more intensive sampling to pick them up,” Paskewitz says.
“We”re interested in two things: how do ticks become established in new areas, and what is the risk for transmission of Lyme disease in areas where nobody has looked before,” she says. “We look at vertebrate host populations, the level of pathogens in those populations, and the characteristics of the habitat.” Ideal tick habitat includes well-drained, sandy soils with forested cover, she notes.
Some areas in eastern Wisconsin seem to have good tick habitat, but no ticks – yet. “Many studies have shown you can find a single tick on a deer, which suggests that ticks are being introduced into the range of that deer – but when you go back and check again later, you find no ticks, meaning the tick population isn”t getting established.
“This may be happening in eastern Wisconsin. Ticks are being introduced, but they don”t take off. We”re trying to figure out why,” Paskewitz says. She mentions two possible reasons for this – one is good news for eastern Wisconsin; the other is not so good:
There is something fundamentally different in the ecology of eastern Wisconsin, and the ticks just can”t survive there.
The Wisconsin River is a barrier, but a porous one. It slows tick movement, but ultimately, birds and deer will move the ticks into eastern Wisconsin, where they will establish themselves.
The Wisconsin project is of a joint effort to understand tick distribution in the Midwest. Other scientists involved in the project include Uriel Kitron and Carl Jones at the University of Illinois, Ned Walker at Michigan State University, and Louisa Beck and Byron Wood of NASA-Ames.