Cases of Lyme disease rose nearly 66 percent in Maryland last year to reach their highest level since the state health department started tracking the tick-borne disease in 1979.
Doctors reported 343 cases last year, compared with 207 the previous year. There were no deaths.
The disease, which produces flu-like symptoms, can affect the heart and cause lifelong joint pain if not treated promptly.
Following a typical pattern, the highest rates occurred on the Eastern Shore where low-lying areas of high grasses provide ideal habitat for the infected deer ticks.
But Dr. Jack R. Grigor, Maryland's public health veterinarian, said significant numbers of cases were reported in suburban and rural areas throughout the state. In Baltimore County, for instance, the rate was 12.9 cases per 100,000 people -- compared with the statewide average of 6.9 cases per 100,000.
Only in the mostly paved areas of the inner city and the higher-altitude areas of Western Maryland is the disease considered rare.
"To give people the impression that there are some safe places and more risky places is not the answer," Dr. Grigor said. "The key is not to leave the impression that certain places are safe havens."
Maryland mirrored a national trend described this week by the federal Centers for Disease Control and Prevention. The agency said 13,083 cases of the disease were reported last year -- up 58 percent over the previous year. The previous record was 9,902 in 1992.
Six states -- New York, Maryland, New Jersey, Connecticut, Rhode Island and Pennsylvania -- accounted for 95 percent of the increase last year, the CDC said.
Reasons for the trend are not completely clear. Dr. Roy Campbell, an epidemiologist at the CDC's National Center for Infectious Diseases, said the increase along the East Coast is at least partly a result of larger deer-tick populations.
As human cases have risen, scientists tracking ticks in Rhode Island and Westchester County, N.Y., also have reported higher tick densities. Such studies have not been done in Maryland.
But Dr. Campbell said, "It makes sense to me that what we probably saw was a regional increase in tick population, not just locally. . . . That tick occurs all along the eastern seaboard from Virginia to southern Maine."
The disease is caused by corkscrew-shaped bacteria transmitted to humans by infected deer ticks. It is most commonly spread in May, June and July, when the tick reaches its nymph stage. At that point, it is no larger than the head of a pin.
Although the tick feeds on deer and other large mammals, the disease usually travels to people by a different route: ticks biting infected field mice, then feasting on humans.
"They are gotten by walking through tall grass and low brush," Dr. Grigor said. "They hang on the edge of the grass trying to catch any creature."
Doctors have become more attuned to the symptoms of Lyme disease and are more likely to diagnose and report cases, according to Dr. Grigor and Dr. Campbell.
Within three to 32 days of exposure, a circular red rash resembling a bull's eye will appear in about three-quarters of infected people. It can be three-quarters of an inch to 26 inches in diameter.
Even when the rash doesn't occur, some people will soon develop the characteristic fever, aching joints and fatigue. Antibiotics usually prevent further symptoms, which can include memory loss, facial paralysis and heart rhythm disturbances.
About half of all people who are not treated will develop arthritic pain that can progress to a chronic lifelong condition.
Tips to avoid Lyme disease:
* Avoid exposure to deer ticks by using insect repellent and wearing long, tucked-in clothing while walking through areas of high grasses.
* Carefully inspect twice a day for the presence of ticks, especially on children who have been outdoors.
* To remove a tick, grasp it with tweezers and tug gently but firmly until it releases its hold on the skin.
* Do not mangle, burn or squash the tick to avoid spreading the bacteria.
* Disinfect the site and wash your hands.
* Save the tick in a closed container. Identification can help in diagnosis if the person bitten develops symptoms.