Lyme disease trends: Delaware, 2000 - 2004

Lyme disease is the most common vector-borne disease in the United States, with 19,804 cases reported in 2004. During 1992-1998, Delaware was among the top 10 states with the highest incidence rates in the United States. We analyzed Delaware Lyme disease surveillance data for the reporting period of 2000-2004 to characterize trends, identify populations at high risk, examine the frequency of the major clinical features of disease, and report antibiotic usage patterns among reporting clinicians.

A Lyme disease case was defined as a person with physician-diagnosed erythema migrans (EM) or at least one objective late manifestation of the disease (e.g., musculoskeletal, cardiovascular, or neurologic) with laboratory confirmation. We merged the information contained in the Delaware Electronic Reporting Surveillance System (DERSS) with an existing database containing case reports of Lyme disease collected at the Delaware Division of Public Health (DPH) for 2000-2004. The 2000 United States Census Bureau data relevant to Delaware was used to calculate incidence rates. Annual incidence rates by age, sex, county, race, and ZIP Code Tabulation Area (ZCTA) were calculated. Frequency of EM and other late manifestations of Lyme disease as well as antibiotic and median duration of use were calculated.

During 2000-2004, a total of 962 confirmed cases of Lyme disease were reported to DPH, for a crude mean incidence rate of 24.6 cases/100,000 persons/year (95% confidence interval [CI], 22.8-26.3). The annual incidence rate remained approximately constant during 2000-2004, with no statistically significant rate difference among the reporting years. Highest incidence rates were observed in Kent County, at 27.8 cases/ 100,000 persons/year (95% CI, 23.0-32.3). Age-related incidence followed a bimodal distribution, with the initial peak among children aged 5-14 years, followed by a second peak among adults aged 60-64 years. Males were disproportionately represented among the age ranges of 5-19 years, 60-69 years, 75-79 years, and >85 years. Lyme disease was most commonly diagnosed during June, July, and August. Fifty-one percent of reported cases had physician-diagnosed EM.

Lyme disease prevention programs should specifically target populations at high risk, as defined in this report. A substantial percentage of reported cases lacked physician-diagnosed EM. Physicians should be aware of the spectrum of Lyme disease clinical manifestations and the local Lyme disease epidemiology to best serve their patient population.