The number of Loudoun County residents testing positive for Lyme disease more than tripled this year, baffling health officials and raising concerns, especially among parents of young children.

Of 55 reported Lyme disease cases so far this year, half were among children younger than 15, according to preliminary figures made available to The Washington Post. In 1998, there were 17 such cases in the county, and only nine in 1997.

The county Health Department said it still was studying the data and was not yet ready to issue an advisory or make a recommendation. A fourth-year University of Maryland medical student is conducting a two-month study of the problem in Loudoun, county officials said.

Depending on the outcome, the county could take a number of steps, including advising children to wear long-sleeve shirts and long pants and recommending a new vaccine for those who work outside or hike in wooded areas.

Lyme disease is a bacterial infection transmitted by ticks that feed on deer and other mammals, typically during warmer seasons.

"We've got more reported cases than in previous years, but whether that's because there is more of the disease or not we don't know yet," said Diana Helentjaris, the county's health director. "At this time, I'm not prepared to make a recommendation."

Several factors could be contributing to Loudoun's sharp increase in reported cases of Lyme disease, Helentjaris said, among them, rapid population growth, more accurate diagnosis and improved reporting procedures.

"When you get a larger population in what was once a rural area, there is a likelihood of increased contact with wildlife," she said.

Although Lyme disease is not considered life-threatening, it can be painful and serious. Common symptoms include headaches, fever, fatigue and muscle and joint aches. Severe cases can result in arthritis, heart abnormalities and neurological complications.

If treated promptly with antibiotics, more than 90 percent of cases resolve themselves without serious long-term complications. A vaccine came out earlier this year that can stop new cases, but it cannot cure a person already infected. What's more, the vaccine is not suitable for children younger than 15.

Lyme disease was first identified in 1975 after several children in Lyme, Conn., began experiencing swollen joints, misdiagnosed at first as juvenile arthritis.

In the Washington area, Loudoun posted the sharpest rise in reported cases of Lyme disease this year. Fairfax health officials said their county is notching an increase in reported incidences; there have been 22 cases since July 1 in Fairfax, compared with 21 in the previous 12 months. But officials noted that only nine of the new Fairfax cases involved residents who contracted the disease in the county and that the rise might stem in part from Fairfax's new reporting rules.

In Montgomery County, reported cases of Lyme disease fell from 52 in 1998 to 35 so far this year, continuing a three-year drop.

Michelle Dunne, who lives in Loudoun's fast-growing Cascades neighborhood, is convinced that the county's growing deer population contributed to her 8-year-old daughter contracting the disease last month. Seven years ago, when the family first moved into its town house near Algonkian Regional Park, members saw seven or eight deer walking in the woods.

Lately, Dunne said, she has seen 30 to 40 deer roaming nearby, some within yards of a children's playground between the town house complex and the park. She is concerned that some neighbors might be contributing to the problem by putting food out in their yards for the deer.

Dunne said she thinks it was at the playground that her daughter, Bridgit, was bitten by a tick a month ago. Bridgit, whose illness included severe headaches and loss of muscle control in her face and legs, is on antibiotics and is expected to make a full recovery.

"It was frightening," Dunne said. "For weeks, we couldn't figure out what it was because she never had the classic bull's-eye rash that everybody talks about."

Doctors say the disease is difficult to diagnose because symptoms mimic those of other illnesses such as influenza, mononucleosis and arthritis. A specific blood test is required to detect Lyme disease.