Small parcels of soft, fluffy cotton are providing a simple and effective way to interrupt the spread of Lyme disease, the dangerous ailment transmitted by ticks, scientists report.

Scattering cotton balls laced with a potent pesticide, researchers are using the nest-building instincts of wild mice to deliver poison to the ticks. As they build treated cotton into their nests, the mice create an environment that is lethal for ticks.

Field tests suggest that the system works well and that the pesticide, permethrin, is harmless to the mice, although it kills ticks attached to mice, as well as ticks that drop off into the nest.

In areas where white-footed mice are scarce, however, the scheme may not be as effective, since the cotton must be taken into nests for contact with many ticks.

The new tick-control scheme was devised by Dr. Andrew Spielman and colleagues at the Harvard School of Public Health. They demonstrated just a year ago that wild mice are an important link -- an animal reservoir -- in the chain that transmits Lyme disease to humans.

The better-known reservoir for the Lyme disease organism -- a bacterium called Borrelia burgdorferi -- is wild deer. Indeed, the tick that transmits the disease to humans and domestic animals is commonly known as the deer tick.

Lyme disease has been known only since 1975, but it has become an important public health problem, especially in the northeastern states. It has been reported in at least 33 states. It is now the most frequently diagnosed ailment spread by ticks.

Until now, no workable means of stopping the disease, or the ticks that transmit it, had been devised. One approach, which many people consider unacceptable, would require wild deer to be virtually eradicated in Lyme disease areas.

According to Christine Dively, a pesticide-registration officer for the Environmental Protection Agency, use of the new system will not end the Lyme disease problem. But, she said, it "will certainly reduce the population of infected ticks."

The release of small cardboard tubes packed with treated cotton was approved earlier this summer by the Environmental Protection Agency and the states of New York, Connecticut and Massachusetts. The pesticide was approved for use by the states because it is not harmful to mammals. The tubes are being produced commercially -- priced at $3 each -- by a small Boston firm, EcoHealth Inc.

Spielman explained: "We reasoned that if mice are the main reservoir host for the disease agent, and we know that mice take cotton to their nests, and scan an area about 20 yards across each night, that if we put out cotton at 10-yard intervals, there's an excellent chance the mouse will encounter the cotton."

Their assumptions were correct.

After tests of the tick-control scheme were run for three seasons on private land -- Naushon Island, near Falmouth on Cape Cod -- Spielman and his colleagues reported that 72 percent of the mice captured at treated sites were free of ticks. By contrast, almost all the mice trapped at untreated sites were burdened with ticks.

Demand for the tick-control tubes is already brisk. "The issue now is how fast we can produce and market them," said company president Alexander Kovel. The Lyme disease organism is a spirochete, a corkscrew-shaped bacterium similar to the agent that causes syphilis. It apparently causes no symptoms of disease in either the mice or the deer, although in humans it is quite dangerous if not treated quickly with antibiotics.

In humans, the first sign of infection is usually a crimson rash shaped somewhat like a bull's-eye. Symptoms can progress to chills, fatigue and backaches even 30 days later.

If left untreated, a second set of symptoms can develop, including severe headaches, shortness of breath, irregular heartbeat and dizziness. Third-stage symptoms -- occurring as much as two years after infection -- can include arthritis-like swelling of the joints, usually painful and sometimes disabling.