Lyme Disease -- United States, 1991-1992

Surveillance for Lyme disease (LD) was initiated by CDC in 1982 (1), and in 1990, the Council of State and Territorial Epidemiologists (CSTE) approved a resolution making LD nationally reportable. During 1982-1991, states reported 40,195 cases of LD. In 1992, LD accounted for more than 90% of all reported vectorborne illnesses in the United States (CDC, unpublished, 1993). This report summarizes surveillance for LD in the United States during 1991-1992.

Forty-nine states and the District of Columbia require reporting of LD. The CSTE/CDC surveillance case definition requires the presence of an erythema migrans rash or at least one objective sign of musculoskeletal, neurologic, or cardiovascular disease and laboratory confirmation of infection (2).

During 1991, 47 states reported 9465 cases of LD to CDC (3); during 1992, 45 states reported a provisional total of 9677 cases, representing a 19-fold increase over the 497 cases reported by 11 states in 1982 (1) (Figure 1). Most cases were reported from the northeastern, mid-Atlantic, north central, and Pacific coastal regions (Figure 2). Established enzootic cycles of Borrelia burgdorferi, the causative agent of LD, have been identified in 19 states; these states accounted for 94% of cases reported during 1991-1992.

The overall incidence rate of reported LD during 1992 was 3.9 per 100,000 population. During 1992, Connecticut (53.6 cases per 100,000), Wisconsin (10.7), and California (0.8) reported the highest rates in the northeast, north central, and Pacific coastal regions, respectively. Rates in some counties in California, Connecticut, Massachusetts, New York, and Wisconsin exceeded 200 cases per 100,000; the incidence was highest in Nantucket County, Massachusetts (449.1). The number of reported cases in Connecticut and Rhode Island increased 48% and 93%, respectively, over 1991. New York reported a provisional total of 3370 confirmed cases during 1992, a decrease of 574 cases from 1991. From 1991 through 1992, decreases were greatest in Westchester (1762, compared with 1154) and Suffolk (860, compared with 654) counties. In 1992, these two counties accounted for 19% of the national total, compared with 28% in 1991.

Among 7507 cases analyzed for which patient age was given, the largest numbers were reported for persons aged 0-9 years (1087 {14.5%}), 30-39 years (1272 {16.9%}), and 40-49 years (1271 {16.9%}). Of 7642 cases, 3770 (49.3%) occurred among males.

Reported by: State health depts. Bacterial Zoonoses Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The distribution of LD in the United States is highly correlated with the distribution of the principal tick vectors Ixodes dammini (reported to be the same species as I. scapularis, the black-legged tick {4}) in the northeastern and north central regions and I. pacificus (i.e., the western black-legged tick) in the Pacific coastal states (5). The occurrence of sporadic cases in states without established enzootic transmission of B. burgdorferi may be due to infectious exposures in limited, unrecognized foci, exposures during visits to areas with endemic LD outside the state of residence, misclassification, or misdiagnosis. Enzootic foci are highly localized and are dependent on environmental factors favorable to vector ticks and their maintenance hosts (especially deer) and to rodent reservoirs of B. burgdorferi. Therefore, subtle ecologic differences may account for substantial differences in incidence between states, counties within states, and adjacent townships (6,7).

The 19-fold increase in reported LD cases since 1982 may reflect a combination of at least four factors: heightened awareness of LD by patients and physicians; increased use of laboratory testing in LD diagnosis; increased surveillance and health department requirements for reporting; and a true increase in the number of cases. Surveillance practices in particular have had an important impact on the reported occurrence of LD. For example, active physician-based surveillance conducted in 1992 by state health departments in collaboration with CDC in Connecticut and Rhode Island resulted in substantial increases in reported cases over 1991. By contrast, the decrease in reported cases in Suffolk and Westchester counties, New York, probably reflects reductions in state and county surveillance personnel necessary to maintain previous levels of case detection and validation.

LD is considered an emerging infectious disease because of the impact of changing environmental and socioeconomic factors, such as the transformation of farmland into suburban woodlots that are favorable for deer and deer ticks (8,9). Demographic profiles of persons with LD reflect mostly suburban and rural risk. Evidence suggests both continuing geographic spread and increasing incidence over time in established endemic foci (6,7).

The diagnosis of LD is based principally on clinical findings, and results of serologic testing are supportive. Serologic tests for LD are not standardized, and problems in the reliability and accuracy of serologic test results have limited their usefulness for surveillance purposes. CDC, in collaboration with the Association of State and Territorial Public Health Laboratory Directors, held a workshop on standardized serologic testing for LD in March 1993, and an evaluation of a standardized testing protocol by selected public health laboratories will be conducted during May-August 1993.

Although the numbers of LD cases reported by some states have fluctuated by year, the annual number of reported cases in the United States has remained relatively constant during 1989-1992, possibly reflecting the implementation of the uniform case definition and standardized reporting. However, the true incidence of LD in the United States is unknown, and estimates are subject to the influences of underreporting, misclassification, and overdiagnosis. The development of standardized, sensitive and specific serologic tests and better surveillance should result in improved estimates of LD.

References

1. Schmid GP, Horsley R, Steere AC, et al. Surveillance of Lyme disease in the United States, 1982. J Infect Dis 1985;151:1144-9.

2. CDC. Case definitions for public health surveillance. MMWR 1990;39(no. RR-13):19-21.

3. CDC. Summary of notifiable diseases -- United States, 1991. MMWR 1992;40(no. 53).

4. Oliver JH, Owsley MR, Hutcheson HJ, et al. Conspecificity of the ticks Ixodes scapularis and I. dammini (Acari: Ixodidae). J Med Entomol 1993;30:54-63.

5. Dennis DT. Epidemiology. In: Coyle PK, ed. Lyme disease. St. Louis: Mosby-Year Book, 1993:27-37.

6. Cartter ML, Mshar P, Hadler JL. The epidemiology of Lyme disease in Connecticut. Conn Med 1989;53:320-3.

7. White DJ, Chang H-G, Benach JL, et al. The geographic spread and temporal increase of the Lyme disease epidemic. JAMA 1991;266:1230-

9. Institute of Medicine. Factors in emergence. In: Lederberg J, Shope RE, Oaks SC Jr, eds. Emerging infections: microbial threats to health in the United States. Washington, DC: National Academy Press, 1992:34-112.

10. Dennis DT. Lyme disease: tracking an epidemic {Editorial}. JAMA 1991;266:1269-70.

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