Lyme disease surveillance and available data

Background

Lyme disease has been a nationally notifiable condition in the United States since 1991. Reports of Lyme disease are collected and verified by state and local health departments in accordance with their legal mandate and surveillance practices. After removal of personal identifiers, selected information on cases is shared with CDC through the National Notifiable Diseases Surveillance System (NNDSS). For more information on NNDSS, see http://www.cdc.gov/osels/ph_surveillance/nndss/nndsshis.htm. Policies regarding case definitions, reporting, confidentiality, and data release are determined by states and territories under the auspices of the Council of State and Territorial Epidemiologists (CSTE). Surveillance data have a number of limitations that need to be considered in the analysis, interpretation, and reporting of results. Additionally, answers to frequently asked questions related to surveillance are available.

Limitations of surveillance data

1. Under-reporting and misclassification are features common to all surveillance systems. Not every case of Lyme disease is reported to CDC, and some cases that are reported may be due to another cause. Under-reporting is more likely to occur in highly endemic areas, whereas over-reporting is more likely to occur in non-endemic areas.
2. Surveillance data are subject to each state's abilities to capture and classify cases, which are dependent upon budget and personnel and varies not only between states, but also from year to year within a given state. Consequently, a sudden or marked change in reported cases does not necessarily represent a true change in disease incidence, and should not be construed as such without knowledge of that state’s historical surveillance practices.
3. Surveillance data are captured by county of residence, not county of exposure.
4. States may close their annual surveillance dataset at a different time than CDC. Thus, the final case counts published by CDC may not exactly match numbers published by each state agency for a given year.
5. Following its implementation in 1991, the national surveillance case definition for Lyme disease was modified in 1996 and again in 2008. Changes were generally minor but may have had some impact on surveillance and must be considered when attempting to interpret trends. Case definitions for each period are available.

Publicly available surveillance data

1. Selected Lyme disease statistics, tables and charts are available on the CDC Lyme disease website.
2. Provisional case counts for states and territories are published weekly in CDC’s Morbidity and Mortality Weekly Report (MMWR), Notifiable Diseases and Mortality Table II (available at: http://www.cdc.gov/mmwr/mmwr_wk/wk_cvol.html). Provisional data are subject to change. Cumulative totals for the current and previous year are presented in the table; however, due to variable reporting delays these numbers cannot be compared directly to determine whether cases are higher or lower than the previous year.
3. Final case counts are published after the year is over and all states and territories have verified their data. A table with the final numbers is published in MMWR Weekly Report in early August of the following year. A more detailed presentation of the data, complete with historical tables, in published in the annual MMWR Summary of Notifiable Diseases (available at http://www.cdc.gov/mmwr/mmwr_nd/).
4. To facilitate the public health and research community’s access to NNDSS data on Lyme disease, CDC has developed a public use dataset. Based on reports submitted to CDC, this dataset provides the number of confirmed cases by county for the years 1992-2011, in four 5-year intervals.  County tabulation is by American National Standard Institute (ANSI) [formerly Federal Information Processing Standard (FIPS)] codes. County codes of "0" represent “unknown” county of residence within each state. More recent county-level case counts are not publicly available at this time. County-level Lyme disease data from 1992-2011 [Excel CSV - 121KB] ––Right–click the link and select "save".