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Definition: Epidemiology is the study of the distribution and causes of disease in populations. Tracking the number of cases of disease by person, place, and time allows public health authorities to better identify who is at risk, trends of occurrence, and development of strategies for disease prevention and control.
Surveillance of reported cases: Lyme disease is a rapidly emerging vector-borne infectious disease in the United States. In 2002, 23,763 cases of Lyme disease were reported to the Centers for Disease Control and Prevention (CDC) (MMWR 52(31):741-750). More than 157,000 cases have been reported to health authorities in the U.S. since 1982, when a systematic national surveillance was initiated. The overall incidence rate of reported cases in the U.S. is approximately 7 per 100,000 population, but there is considerable underreporting. The disease occurs in distinct and geographically limited areas. The incidence in a few of the most highly endemic communities may reach 1 to 3% per year. Persons of all ages and both genders are equally susceptible, although the highest attack rates (see chart) are in children aged 0-14 years, and in persons 30 years of age and older. Although cases of Lyme disease have been reported from 49 states and the District of Columbia, significant risk of infection with the agent of Lyme disease, Borrelia burgdorferi, is found in only about 100 counties in 12 states located along the northeastern and mid-Atlantic seaboard and in the upper north-central region, and in a few counties in northern California. See the table for data by year, state, and region for the ten year period 1990-2001.
United States map of reported cases: Lyme disease cases have been reported by 49 states and the District of Columbia; there is, however, a distinctive geographic pattern in which cases remain concentrated in the northeastern, north-central, and Pacific coastal regions. The trend of increasing incidence in some established endemic areas continues, as well as geographic spread of B. burgdorferi to new areas. The distribution of the principal vectors of B. burgdorferi in the United States mirrors the areas of high reported incidence of Lyme disease and identifies areas of potential disease emergence. Lyme disease surveillance case definition: State and local public health officials rely on health-care providers, laboratories, and other public health personnel to report the occurrence of notifiable diseases to state and local health departments. Without such data, trends cannot be accurately monitored, unusual occurrences of diseases might not be detected, and the effectiveness of intervention activities cannot be easily evaluated. In the United States, requirements for reporting diseases are mandated by state laws or regulations, and the list of reportable diseases in each state differs. CDC and the Council of State and Territorial Epidemiologists (CSTE) have established a policy (see MMWR May 2, 1997;46(RR-10):1-55) that requires state health departments to report cases of selected diseases to CDC's National Notifiable Diseases Surveillance System (NNDSS). Based on a surveillance case definition, Lyme disease is a reportable disease in all 50 states and the District of Columbia.
Map of tick vectors: The map at right is based on a comprehensive review of the literature in the United States from 1907 through 1996 of established and reported distribution of the Lyme disease vectors Ixodes scapularis and Ixodes pacificus. Data compiled from the literature review are presented by county. "Established" means at least 6 ticks or 2 life stages (larvae, nymphs, adults) were identified in the county. "Reported" indicates at least 1 tick was identified in the county. Note that the principal vectors are Ixodes scapularis in the northeast and upper-midwest states and Ixodes pacificus along the West Coast of the United States. Although I. scapularis is widely distributed in the southern U.S., it is not an established vector of Lyme disease to humans in that area. References: Centers for Disease
Control and Prevention. Notice
to Readers: Final 2002 Reports of Notifiable Diseases. MMWR. 8 August
2003; 5(31):741-750. Centers for Disease
Control and Prevention. Case
Definitions for Infectious Conditions Under Public Health Surveillance.
MMWR May 2, 1997;46(RR-10):1-55. Centers for Disease
Control and Prevention. Recommendations
for Test Performance and Interpretation from the Second National Conference
on Serologic Diagnosis of Lyme Disease. MMWR Aug 11, 1995;44:590-591. Centers for Disease
Control and Prevention. Summary
of Notifiable Diseases, United States, 1995. MMWR Oct 25, 1996;44:1-87. Dennis DT. Epidemiology, ecology, and prevention of Lyme disease. in Rahn DW, Evans J eds. Lyme disease. Philadelphia, PA: American College of Physicians, 1998;7-34. Dennis DT; Nekomoto TS; Victor JC; Paul WS, and Piesman J. Reported distribution of Ixodes scapularis and Ixodes pacificus (Acari: Ixodidae) in the United States. J Med Entomol. 1998; 35(5):629-638. Koo D, Wetterhall SF. History and current status of the National Notifiable Diseases Surveillance System. Journal of Public Health Management Practice 1996;2:4-10. Orloski KA, Hayes
EB, Campbell GL, Dennis DT. Surveillance
for Lyme Disease—United States, 1992-1998. In: CDC Surveillance
Summaries. MMWR Apr 28, 2000;49(SS-3):1-11. | ||||||||||||||||||||||||||||||||||||||||
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