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). 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. Final annual 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 the fall of the following year. A more detailed presentation of the data, is published in the annual MMWR Summary of Notifiable Diseases.
2. Selected Lyme disease statistics, tables and charts are available on the CDC Lyme disease website.
3. To facilitate the public health and research community’s access to NNDSS data on Lyme disease, CDC has developed a public use dataset. This dataset provides the number of reported cases by county for the years 2000-2015. The 2000-2007 data reflect the surveillance case definition in place during that time. Data from 2008 to the present reflect the case definition which allowed for reporting of “confirmed” and “probable” cases. County codes of “999” represent “unknown” county of residence within each state. More recent case counts are not publicly available at this time. County-level Lyme disease data from 2000-2015 [XLS – 202 KB] ––Right–click the link and select “save”.