Increase in ticks, disease feared Snowy winter made conditions right for more pests

June 23, 1996|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

As if this year's snowy winter and wet spring haven't caused enough misery, scientists are now concerned that the weather may mean a surge in tick populations, and in the incidence of tick-borne Lyme disease.

"That does seem to be a possibility," said Dr. Dewey M. Caron, a professor of entomology and applied ecology at the University of Delaware.

Two years ago, heavy snow and a cool, wet spring in the Northeast were followed by a jump in tick populations and a 58 percent increase in the number of reported cases of Lyme disease.

Last year, there was little snow and a dry spring, followed by a drop in reported cases of Lyme disease.

This year, Caron said, "conditions have been fairly favorable" for ticks, and their numbers do seem to be up. And "the greater the exposure, the more people that end up with ticks on them, and the more probability of disease."

Lyme disease is caused by corkscrew-shaped bacteria transmitted from wild mice to humans by the bite of the tiny deer tick. It usually produces a distinctive bull's eye-shaped rash and flulike symptoms. Without prompt treatment with antibiotics, it can affect the heart and cause lifelong joint pain.

First identified in Lyme, Conn., in 1976, the disease is now found in 43 states but is most common in eight -- Connecticut, Rhode Island, New York, New Jersey, Pennsylvania, Maryland, Wisconsin and Minnesota. More than 13,000 cases were

reported nationwide in 1994, a record. In 1995 there were 11,600 cases.

Maryland counted 343 cases in 1994, also a record, and up nearly 66 percent from the prior year. Last year, 336 cases were reported.

Deer ticks also have been found to transmit human granulocytic ehrlichiosis or HGE, which causes severe headaches, chills, mental confusion and high fevers. It can be fatal if untreated, but it remains rare. Only a handful of cases have occurred in Maryland.

There is no proof that tick numbers are up this year in Maryland, because there have been no counts. But there is a growing perception that they are.

Caron said inquiries to his University of Delaware office, from Delaware, Maryland and Pennsylvania residents who have found ticks on themselves or in their yards, seem to be up.

Maryland agriculture officials reported last month that the number of people sending ticks in for identification was up 30 percent over last year. But that might reflect only a growing public awareness of Lyme disease.

The notion that snowy winters and damp springs bring more ticks is something that federal health officials have been watching for several years, according to Dr. David T. Dennis, coordinator of the Centers for Disease Control and Prevention's Lyme disease program at Fort Collins, Colo. He noted the decline in deer tick populations and Lyme disease in 1995, after a mild winter and a dry spring.

But scientific data to support the idea are scarce.

"We don't know enough about what natural forces keep their numbers in check," Caron said. "Perhaps it's a combination of environmental factors."

Scientists at the Louis Calder Center, Fordham University's biological field station in Armonk, N.Y., have been counting ticks for a decade and attempting to link environmental conditions with changes in tick populations.

"But it hasn't proven a very fruitful thing so far," said Thomas J. Daniels, an adjunct assistant professor at the center.

"Intuitively, you expect there is going to be a relationship," he said. But statistical analyses "haven't revealed a consistent pattern."

Scientists at the center are now counting deer tick nymphs, the immature stage in which they spend their first year. The count has not peaked, but already the totals appear headed for a level at least 30 percent to 40 percent higher than last year's.

"In the past five years we have been seeing a rather radical oscillation between very high and very low nymphal numbers," Daniels said.

At the 6th International Lyme Borreliosis Conference in San Francisco this week, he expected to be asked what environmental factors influence tick populations, and "at the moment," he said, "I don't have a very good idea what those things are." The ticks' ups and downs don't seem to correlate in any statistically significant way with any of the climatological and environmental variables the center has been monitoring.

That doesn't surprise Dr. Nancy L. Breisch, an entomologist and researcher at the University of Maryland. To prove such a link, she said, you would have to know how many ticks there were as each winter began.

Unfortunately, ticks burrow into the leaf litter as the weather cools and can't easily be counted.

Breisch suspects any relationship between the weather and tick numbers is more indirect.

For example, she said, "vegetation is a lot more lush because of the prolonged, rainy spring." The dense vegetation, in turn, would attract more of the mice, birds, raccoons and possums on which the ticks feed.

The increased wildlife activity would mean more ticks drop from wild hosts into areas where people are active, she said.

The Calder Center, meanwhile, has begun new long-term winter mortality studies -- watching caged ticks throughout the winter to learn more about what weather factors kill them.

By discovering how nature keeps tick numbers in check, and encouraging those natural agents or conditions, Daniels said, people might begin to have an impact on the incidence of Lyme disease.

To avoid tick bites and the risk of Lyme disease:

Use a tick repellent, applying it to clothing, not to the skin. Wear long sleeves and pants, tucked-in, while walking through areas of high grasses.

Carefully inspect twice a day for the presence of ticks, especially on children and pets that have been outdoors.

Pub Date: 6/23/96

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