March 15, 2001 -- Though the number of cases of Lyme disease in the U.S. decreased slightly in 1999, the number of cases of this tick-borne illness more than doubled during the 1990s, according to the latest government data.
Lyme disease cases decreased by 3% in 1999, compared with 1998, to nearly 16,300 reported cases. However, when the 1999 figures were compared with 1997, there was a 21% increase, according to the data published by the CDC in the March 16 issue of Morbidity and Mortality Weekly Report.
The 1999 data show that most cases were reported in the northeastern, mid-Atlantic and north central states. Nine states -- Connecticut, Rhode Island, New York, Pennsylvania, Delaware, New Jersey, Maryland, Massachusetts, and Wisconsin -- accounted for the lion's share of new cases.
"Although the cases reported in 1999 are fewer than the peak number of cases reported in 1998, the data from the 1990s -- and since the epidemic began -- are showing a trend toward higher numbers," says researcher Stacie Marshall, MPH, an epidemiologist at the CDC's division of vector-borne infectious diseases in Fort Collins, Colo. Vector-borne diseases are passed by ticks, insects, and rodents, and Lyme disease is the most common vector-borne disease.
The reasons for the apparent increase are twofold, she explains. There has been an increase in surveillance efforts, so more cases are being detected, and people are spending more time outdoors in tick-infected areas.
Lyme disease is usually caused by deer ticks that transmit the bacterium Borrelia burgdorferi to humans. A bull's-eye rash is typically the first sign of infection, but a few months after the bite, arthritis may appear as brief bouts of pain and swelling in one or more large joints, especially the knees. Additionally, numbness, pain, Bell's palsy (paralysis of the facial muscles, usually on one side), and meningitis (fever, stiff neck, and severe headache) may occur later in the infection.
But serious consequences can be staved off with early diagnosis and proper antibiotic treatment.
Most people who developed Lyme disease in 1999 were either younger than 15 or between 45 and 59, and cases were mainly reported during June and July, the data show.
The new data reinforce the importance of preventing Lyme disease by dressing properly and doing daily tick checks if you spend time outdoors in high-risk areas, according to Marshall and other experts.
"Lyme disease prevention starts with preventing tick bites," she says, and personal protective measures can help reduce the risk of getting bitten.
Individuals who are exposed to tick-infested areas should wear light-colored clothing so that ticks can be spotted and removed more easily. "Wearing long-sleeved shirts and tucking pants into socks or boot tops can help keep ticks from reaching the skin," she says.
Ticks are usually located close to the ground, so wearing high rubber boots may provide additional protection. The CDC recommends applying insect repellents containing DEET to exposed skin, and permethrin (which kills ticks on contact) to clothes.
There are ways to reduce the number of ticks in residential areas such as removing leaf litter, removing brush-and-wood piles around houses and at the edges of yards, and clearing trees and brush to admit more sunlight and reduce the amount of suitable habitat for deer, rodents, and ticks.
"Since reporting started in 1980, the numbers have gone up and down and up and down," points out Karen Forschner, chair of the board of directors of the Lyme Disease Foundation, based in Hartford, Conn.
People should not derive a false sense of security from the new data, she says. "The numbers for 2000 may be down again because of droughts in many states during that year," she says. "Ticks don't survive in dryness. They need moist, humid weather."
States such as Texas, Maryland, Nebraska, and West Virginia, which typically have small numbers of Lyme disease cases, are also showing increases, she says.
"People shouldn't interpret Lyme disease case-count numbers as indicating whether or not ticks in their area will spread disease," she says. "Other ticks spread other diseases in others states."
Her advice is similar to Marshall's. "Dress properly in the summer in tick-infested areas, which means wearing light clothing, shirt tucked into pants and pants tucked into socks, but when it gets really hot in the Northeast, that's not practical advice," she notes.
Forschner lets her daughter play outside in shorts and a T-shirt, but she is careful to do daily tick checks
If ticks are found, they must be removed properly with fine-point tweezers, she explains. "Place the tweezers next to the skin and around the mouth part of the tick and pull back," she says. Don't squeeze their bodies.
If the ticks are removed as soon as possible after they're found, and haven't been on the skin too long, then a person might not be infected. Remember, Marshall says, "a tick has to be attached for two days to cause the disease."
Once again, vigilance is key. "Rates are down very little and that means that the risk still remains largely what it has been in the past," says Aravinda M. de Silva, PhD, an assistant professor of microbiology and immunology at the University of North Carolina at Chapel Hill School of Medicine.
De Silva and colleagues recently published research on how the bacteria that causes Lyme disease moves from a tick to an animal host. What they found was there was great variability in the bacteria -- which will make it tough to develop a vaccine.
The Lyme vaccine currently available is not recommended for widespread use, he says. Only people who live in high-risk areas and spend a lot of time outdoors are advised to get vaccinated.