Special Report: Population movement signals new threats

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."

Mosquitoes

Human and pet migration to the South and West, with accompanying building and landscape changes, is a major factor in spreading mosquito-borne animal and human illnesses, other experts say.

"You'll see the number of pet heartworm cases continue to grow in those parts of the country," says heartworm authority Thomas Nelson, DVM, surgical director and co-medical director of the Animal Medical Centers of Northeast Alabama at Anniston, Ala., and past president of the American Heartworm Society.

"As people move west, they're transforming the desert, putting in trees and lawns, irrigating, building ponds — lots of prime habitat for mosquitoes. The knothole mosquito (Aedes Sierrensis) is the prime vector in the West. It lays eggs in the knotholes of trees, and all it needs is a little rain or water from a sprinkler system," Nelson explains. Commercial real estate development is a factor, he says, in that parking lots and large buildings retain heat and push up the already-high temperatures in desert cities and towns, and that extra heat compounds the mosquito problem.

"A lot of people moving to these areas take heartworm-positive dogs with them, and they're a reservoir for infection. Literally thousands of infected dogs moved from the hurricane areas of the Gulf Coast."

Heartworm has long been a problem in the East, especially in high-density areas along the Atlantic Coast, the Gulf and the Mississippi, Nelson says. "There are about 70 different incriminating mosquitoes, with 22 of them significant factors, and now there's a new one, Aedes albopictus or 'tiger mosquito' that came in from southeast Asia in the 1980s and has proliferated in warm climates all over the country. It's voracious," Nelson says.

"What's concerning is that we have a tremendous arsenal of products that are 100 percent effective against heartworm, yet only about 55 percent to 60 percent of dog owners use them," Nelson adds. Until recent years, heartworm was not seen as a serious threat to cats, "but that's going to change. There's going to be a lot more emphasis going forward on feline testing and treatment," he says.

What effect will drought conditions, such in the Southeast and mid-South, have on mosquito-borne illnesses like West Nile Virus and eastern and western equine encephalitis this year?

Research entomologists in California and Florida say it's too early to say precisely, but do provide some insight.

"Even with reduced rainfall in parts of California, people still water grass; underground reservoirs in urban areas stagnate and provide a breeding place for mosquitoes that cause West Nile," says research entomologist Dr. William Reisen at the University of California-Davis' Center for Vectorborne Diseases.

"A lot of people walking away from foreclosed houses don't pump out their pools, but just leave them to stagnate. The chemicals evaporate, the water turns green and these pools crank out the mosquitoes. In Bakersfield alone last summer, 3,000 families abandoned houses. There were not that many with pools, but those that did have them can produce a lot of mosquitoes and become a big problem for cities."

Wherever in the nation the upcoming summer is hot and dry, West Nile and other viruses "will do better," Reisen says. "The Midwest has been having warmer summers, and it's seen some of the highest incidence of West Nile in the country."

West Nile is cyclical, Reisen explains, which puts California on the watch to see what will happen this summer. "The Los Angeles area had a West Nile epidemic in 2004. In the two succeeding years, bird hosts died back and the disease subsided but there is herd immunity in the survivors. This year, the crows are back and finch population is up. We've already had some 100-degree days, so we'll be looking intensively at what may happen."

"It's hard to say if this will be an epidemic year in Los Angeles," agrees Aaron C. Brault, PhD, associate professor also at UC-D's Center for Vectorborne Diseases, who also noted the cyclical nature of West Nile.

"The ambient air temperature is directly proportional to mosquito activity. When it's consistently warmer, usually about late August, the virus gets to their salivary glands faster and that's when you would see the most virus transmission."

Western equine encephalitis hasn't been a problem in California for several years, largely because of vaccination coverage, Brault says, but the eastern variety has been an issue recently for the Northeast.

Officials in Massachusetts and New Hampshire already are preparing to go on the attack against the mosquitoes that cause both eastern equine encephalitis (EEE) and West Nile Virus after EEE affected eight persons in New Hampshire and 13 in Massachusetts over the last three years.

Both viruses affect horses and birds, but usually not pets.

New Hampshire earmarked $180,000, a third of it for mosquito surveillance and two-thirds to reimburse southeastern New Hampshire towns that adopt mosquito-control programs aimed at killing larvae.

Last year's severe drought in the Southeast eased somewhat over the winter and with recent spring rains. "Typically drought tends to shut down mosquito-borne illness like West Nile and EEE, but our drought situation turned around to some degree. Still, it's hard to predict what it (mosquito activity) will be like before about mid-June," says Jonathan F. Day, University of Florida professor of medical entomology at the Florida Medical Entomology Laboratory at Vero Beach.

"In urban areas, like Atlanta, even though the drought shuts things down, mosquitoes still are in storm drains. They just need a way to get out. If the drought persists, they don't do well, but if there's some wetting they can get out and that would favor tranmission of virus."

In Kentucky, meanwhile, University of Kentucky entomologists say they're seeing no indication of higher than normal numbers of the Eastern tent caterpillar, linked to mare reproductive loss syndrome (MRLS) in horses.

With the egg-hatch stage just completed, horse farms were advised to watch for the insect's white tents in cherry and crabapple trees and keep mares away from suspected caterpillar locations, or apply foliar sprays to the trees if the horses can't be moved from areas of potential exposure.

MRLS hit the state's Thoroughbred horse country especially hard in 2001.