Latest figures indicate incidence of Lyme disease may be increasing

April 2001

ATLANTA — Could Lyme disease be on the rise?

According to a recent CDC report, Lyme disease cases increased from 1991-1999. Although the number of cases decreased from 16,801 reported in 1998 to 16,273 reported in 1999 (3% decrease), the number of cases was still higher than that reported during the early 1990s. Most cases continue to occur in the Northeast, mid-Atlantic and North Central states.

In 1999, 713 (22.7%) out of 3,143 U.S. counties reported at least 1 case and 90% of the cases were from 109 (15.3%) reporting counties. Incidence exceeded 100 cases per 100,000 population in 24 counties in Connecticut, Maryland, Massachusetts, Minnesota, New Jersey, New York, Pennsylvania, Rhode Island and Wisconsin. The highest county-specific incidence (950.7) occurred in Nantucket County, Mass.

Limitations and prevention

Most patients were younger than 15 years and between 45-59 years. This could be the result of many different scenarios, one being that they experienced greater exposure than other groups to infected ticks. Others included less use of personal protective measures, differential use of health care services or reporting bias. Also, the large number of reported cases during June and July reflects the seasonal peak of tick activity in endemic areas.

With these findings come at least three limitations. First, distribution of reported cases could be distorted by reporting bias. The second limitation could be that Lyme disease is underreported in areas where it is endemic and overreported where it is not endemic. Third, the Lyme disease case definition also is limited in sensitivity and specificity. Because all cases of Lyme disease do not present the same symptoms, other conditions may be misdiagnosed. Laboratory testing may also be inaccurate.

Avoiding tick-infested areas, using repellents, promptly removing ticks from clothing or skin and reducing tick populations can prevent Lyme disease. If the disease is caught early and treated, morbidity can be reduced. A vaccine (LYMErix, GlaxoSmithKline) for individuals aged 15-70 years, approved by the FDA in 1998, is 76% effective in preventing Lyme disease after 3 doses. New methods of reducing tick vectors are also being developed including baited devices that passively apply acaricides to deer and rodents.

The CDC is collaborating with health departments and organizations to increase prevention efforts. Throughout the year, community projects are being initiated to reduce incidence to 9.7 per 100,000 population by 2010 in states where Lyme disease is endemic.

For more information:

• CDC. Lyme disease – United States, 1999. MMWR. 2001;50(10):181-5.