Eight. That’s the magic number. This weekend the Tick Index for Westchester County and the surrounding area is 8—so if you’re out in the woods, or even your yard, you have a moderately high risk of being bitten by a tick. To be specific, a blacklegged tick, also called the deer tick (and in some parts of the country, the bear tick). And if you’re bitten, and don’t remove the tick very quickly, you stand a pretty good chance of contracting Lyme Disease.
The blacklegged tick, Ixodes scapularis, has a complicated two-year life cycle that revolves around feeding on the blood of animals. It’s the whole bloodsucking thing that makes the tick a vector, or carrier, of diseases like Lyme (and babesiosis and anaplasmosis, among others). Ticks are born without the bacterium that causes Lyme Disease, Borrelia burgdorferi. When they feed on animals and birds that are “reservoir competent,” meaning they can carry and transmit the Lyme bacterium, the ticks acquire it. If you provide the tick’s next meal, it can return the favor by infecting you with the disease. (White footed mice, chipmunks and robins are reservoir competent; gray squirrels, deer and opossums are not.)
So what does the Tick Index number mean? Several times a week, Thomas J. Daniels, Ph.D., Fordham associate research scientist and co-director of the Calder Center’s Vector Ecology Laboratory, takes drag samples at Calder. He and colleague Rich Falco, Ph.D., a medical entomologist with the New York State Department of Health, use a one-meter square piece of while corduroy attached to a wooden bar to go hunting ticks.
“We drag the fabric over a known distance—usually ten or twenty meters—then turn it over and start counting ticks,” Daniels says. “The ticks are host seeking: they don’t care if it’s me or a square of fabric. We know the size of the drag and the distance, so it’s easy to come up with a mean number of ticks per square meter.”
What the scientists are especially looking for are ticks in the nymphal stage, between 25 and 30 percent of which carry the Lyme bacterium. Nymphs are active and abundant from late spring through summer and into the fall. Because of their great numbers, and because they are so small and often escape detection, nymphs cause 90 percent of Lyme Disease infections. Altogether, Daniels and Falco perform drag counts from late March through early December, weather permitting. Adult ticks, active in the late fall, carry the Lyme bacterium at an even higher rate than nymphs, but because there are so many fewer adults, and because they are easier to spot and pick off before they transmit the disease to humans, they cause many fewer cases.
The scale of the tick index measures the relative risk of being bitten in a particular season. “This season isn’t as bad as last one was,” Daniels says, “so you might not have as much risk with an index of eight this year as you would have with an index of five least season. It’s not an absolute measure of risk.”
Daniels said the infection rate of Lyme Disease in the northeast is fairly steady from year to year, but vastly underreported, which is why he doesn’t rely on epidemiological data on Lyme Disease cases to assess risk.
“I would risk my life on my tick data, because I know how to count ticks,” he says. “The epidemiological data are terrible. Lyme disease is often hard to recognize, and as it’s become more common, physicians are less interested in it, and don’t report it reliably to health agencies.”
Daniels estimates that the 25,000 reported cases of Lyme Disease in the United States each year represents only 10 percent of the real number, “and I’m being generous,” he says.
Daniels and Falco have been collecting tick data since 1987. The first incarnation of the Tick Index debuted about 10 years ago, when Falco was working with the American Lyme Disease Foundation. The project languished when funding for the foundation was short. In 2008, the Index was revived and began publishing each season on the Fordham website: www.fordham.edu/tick.
Daniels received his doctorate from the University of Colorado in 1987, for research on the behavioral ecology of feral dogs on the Navajo reservation. He studied how well dogs that were raised initially as pets adapted to living in a semi-wild state on the reservation’s garbage dumps. Not well, it turned out. By 1985 he had already completed that research and was studying ticks and Lyme Disease, which was just coming to prominence as a medical issue in the United States. He has been at the Calder Center since 1994, and has seen the emergence not only of Lyme disease, but also of West Nile virus, a mosquito-borne disease, in the region.
West Nile is a case study of how a disease agent, introduced into the right animal community under the right conditions, can quickly become part of the everyday landscape.
"In 1999, when West Nile first arrived, we had 60 cases in just one state: New York," Daniels says. "In 2000 it had spread to two neighboring states, New Jersey and Connecticut, and by 2005, the virus was reported in 43 states. Vector-borne diseases are dynamic and they will continue to pose a significant health challenge in the future."