Special Report: Population movement signals new threats

May 01, 2008

The weather always can be counted on to influence parasite populations around the country, but it plays a lesser role these days compared to what humans are doing environmentally to affect the number and spread of disease vectors, entomology experts say.

Human migration patterns, especially toward the South and West, that are spurring rapid new commercial and residential building, the accompanying movement of pets, plus reforestation and other changes to wildlife habitat — even the current mortgage-foreclosure crisis that results in thousands of abandoned homes — have a huge effect on disease-parasite numbers, the experts say.

"Suburbanization, reforestation, encroachment on and changes to wildlife habitat, and the exploding wildlife population — especially whitetail deer — these are huge issues," says flea and tick expert Michael W. Dryden, a veterinary parasitologist who is a professor of diagnostic medicine and pathobiology at Kansas State University.

"Global warming — yes, that's a player, but a minor player compared to this," Dryden says. "The winter we just had in most parts of the country was snowy but not bitterly cold. My guess is that it didn't reduce tick populations, but might delay them for a month to six weeks. That's important, but it's the wildlife population and these other factors that are of much greater importance than climate right now."

Backing up his point on the deer population, Dryden cites a 2007 University of Georgia study titled "Ecological havoc, the rise of whitetail deer and the emergence of Amblyomma americanum-associated zoonoses in the United States."

A portion of the study summary reads, "Because whitetails serve as a keystone host for all stages of lone star ticks, and an important reservoir host for Ehrlichia chaffeensis, E. ewingii and Borrelia lonestari, the near-exponential growth of whitetail deer populations that occurred in the eastern United States during the twentieth century is likely to have dramatically affected the frequency and distribution of A. americanum-associated zoonoses."

A question-and-answer interview (p. 14) with Edward Breitschwerdt, professor of medicine and infectious diseases at North Carolina State University's College of Veterinary Medicine, supports Dryden's point.

"We probably have more of a problem with ticks than with any other vectors in the United States," Breitschwerdt tells DVM Newsmagazine. "Tick movement takes place with the movement of wildlife populations, especially deer. Most authorities concur that the major factor contributing to the spread of ticks is the movement of deer populations throughout the United States."

The growth of Ohio's deer population is a good example, Dryden says. "Back in 1900, there were virtually no whitetail deer in Ohio. From 1901 through 1942, the state's deer (hunting) season was closed. But in 2004, Ohio issued 520,000 deer-hunting permits and 216,000 deer were killed. Now the state has an estimated 700,000 deer. And in the whole eastern half of the country, there may be 25 million to 30 million of them."

Other important tick hosts, including coyote and wild turkey, are also on the rise, Dryden says. "Reforestation brings in wildlife and concentrates them, and where ticks are concerned that's not a good thing."