The Centers for Disease Control announced on Thursday that reported cases of Lyme disease have more than doubled since 1991.

People can get Lyme disease if they're bitten by an infected blacklegged tick (also known as deer tick). Spring and summer are prime time for those ticks.

The first sign of Lyme disease infection is usually a circular skin rash. Early symptoms of Lyme disease may also include fever, headache, and fatigue.

Untreated Lyme disease may spread to other parts of the body, including the muscles, joints, heart, and nervous system.

There were fewer than 10,000 reported cases of Lyme disease in 1991, when the CDC began tracking Lyme disease nationally.

The CDC's latest statistics, gathered from 2003 to 2005, show about 20,000 reported cases annually.

"This increase in cases is most likely the result of both a true increase in the frequency of the disease as well as better recognition and reporting due to enhanced detection of cases," says Paul Mead, MD, MPH, in a CDC news release.

Mead is a medical epidemiologist with the CDC's Division of Vector-Borne Infectious Diseases.

Lyme Disease in the U.S.

From 2003 to 2005, the CDC got reports of 64,382 people with Lyme disease in 46 states and Washington, D.C.

Most cases occurred in the following 10 northeastern, mid-Atlantic, and north-central states: Connecticut, Delaware, Maryland, Massachusetts, Minnesota, New Jersey, New York, Pennsylvania, Rhode Island, and Wisconsin.

Lyme disease was most common among children aged 5 to 14 and adults aged 45 to 54. Cases peaked during summertime.

Preventing Lyme Disease: 14 Tips

Preventing Lyme disease means avoiding tick bites. The CDC's tips include:

• Avoid areas with lots of ticks.Ticks like wooded, bushy areas with high grass and lots of leaf litter.

• Ask your local health department and park or extension service about tick-infested areas to avoid.

• Use insect repellent with 20 percent to 30 percent DEET on adult skin and clothing to prevent tick bites.

• Wear long pants, long sleeves, and long socks to keep ticks off your skin.

• Check your skin and clothes for ticks every day.Remove ticks before going indoors.

• In areas where ticks are found, walk in the center of trails to avoid contact with overgrown grass, brush, and leaf litter.

• Remove leaf litter and clear tall grasses and brush around homes and at the edges of lawns.

• Place wood chips or gravel between lawns and wooded areas to restrict tick migration to recreational areas.

• Mow the lawn and clear brush and leaf litter frequently.

• Keep the ground under bird feeders clean.

• Stack wood neatly and in dry areas.

• Keep playground equipment, decks, and patios away from yard edges and trees.

• Since deer can carry ticks that transmit Lyme disease, discourage deer from entering your yard.

• Take extra precautions in May, June, and July, when ticks that transmit Lyme disease are most active.

The American Academy of Pediatrics recommends not using repellents containing DEET on infants less than 2 months old. Always follow the product's recommended uses.

A vaccine against Lyme disease used to be available, but it was taken off the market in 2002, "reportedly because of poor sales," says the CDC.

The vaccine's protection against Lyme disease faded with time. People who got the Lyme disease vaccine before 2002 "are probably no longer protected against Lyme disease," says the CDC.

This article was reviewed by Louise Chang, MD.

SOURCES: CDC, Morbidity and Mortality Weekly Report, June 15, 2007; vol 56: pp 573-576. News release, CDC. CDC: "Lyme Disease Prevention and Control" and "Lyme Disease Symptoms."