Environmental Public Health Tracking: Lyme Disease

Lyme disease is an infection caused by the bacterium Borrelia burgdorferi. The infection is spread by the bite of an infected black-legged tick (Ixodes scapularis), which are commonly called a deer tick. Symptoms can vary depending on the stage of infection and may include a characteristic bullseye rash, fever, arthritis, headache, fatigue, and facial paralysis. Lyme disease can be treated with antibiotics.¹ Lyme Disease Trends in Wisconsin, P-01295 (PDF) is a surveillance brief that offers more details on Lyme disease and its spread in Wisconsin.

The video below offers tips for preventing Lyme disease:

Tracking Lyme disease gives public health professionals a better understanding of how often Lyme disease happens in their county.

With Environmental Public Health Tracking, we can monitor how many cases of Lyme disease occur in a county over time and can use that information to educate our communities and plan prevention efforts.

The source of the data is the Wisconsin Vectorborne Disease Program. Healthcare providers are required to report cases of Lyme disease.

The Vectorborne Disease Program aggregates the cases and sends this information to Wisconsin Environmental Public Health Tracking Program.

• Cases of Lyme disease – The total number of Lyme disease cases in Wisconsin in a given year.
• Incidence of Lyme disease – The crude rate of new confirmed cases of Lyme disease in a given year, per 100,000 people. See our glossary entry on incidence rate for more information.

• Cases are based on the county of residence; some infections may have been acquired during travel to other areas.
• These data are obtained from the Wisconsin Electronic Disease Surveillance System (WEDSS). WEDSS is a secure, web-based system used by public health staff, infection control practitioners, clinical laboratories, clinics, and other disease reporters to report communicable diseases. "Month" and “Year" indicates the month or year of illness onset or specimen collection date, whichever is earlier. County-level data are based on the county of residence of the case; some infections may have been acquired during travel to other areas. Lyme disease is endemic in all Wisconsin counties; thus, any Wisconsin resident is considered to be "exposed."
• Case counts are determined by applying the Lyme disease surveillance case definition to reports of suspected Lyme disease. A case definition is a set of uniform criteria used to define a disease for the purpose of public health surveillance and enables public health to classify and count cases consistently across Wisconsin and the United States.
• Important changes in how surveillance was conducted, and thus how cases are counted, occurred in 2022, 2012, and 2008. Caution should be used when interpreting trends in Lyme disease case numbers and disease incidence over time, especially between years when surveillance methods changed. Information below provides context for these changes.

• Wisconsin Department of Health Services - Tickborne Infections
• Wisconsin Department of Health Services - Lyme Disease
• University of Wisconsin-Madison Department of Entomology - Tick Surveillance
• Lyme Disease Trends in Wisconsin, P-01295 (PDF)
• Wisconsin Climate and Health Program - Wisconsin Vectorborne Disease Toolkit, P-01109 (PDF)
• Centers for Disease Control and Prevention - Lyme Disease

Click the link below to download the data you're looking for:

County Data (CSV)

State Data (CSV)

Counts

Case counts, 2022-current

The current Lyme disease case definition was implemented January 1, 2022. This definition relies largely on laboratory data alone for case classification, whereas the previous case definition relied heavily on a combination of both laboratory and clinical data. One consequence of this case definition change was an overall increase in the number of reported cases statewide and in most counties for the years 2022 and later compared to previous years. This increase is a result of fewer criteria required for a report of suspected Lyme disease to be counted as a case (Probable or Confirmed). Therefore, case counts from 2022 or later are not directly comparable with counts from 2021 or earlier.
"Confirmed cases" of Lyme disease (2022 and after) include:
Those with an erythema migrans (EM) rash that is greater or equal to 5 cm in diameter and diagnosed by a medical professional.
"Probable cases" of Lyme disease include:
Those with laboratory evidence of infection that meets criteria, regardless of clinical presentation.
See the current Lyme disease Case Reporting and Investigation Protocol (EpiNet) for complete case definition details.

Case counts during 2008-2021

"Confirmed cases" of Lyme disease included:
Those with an erythema migrans (EM) rash that is greater or equal to 5 cm in diameter and diagnosed by a medical professional or those with at least one non-EM confirmatory sign or symptom indicating late manifestation of disease (arthritis, Bell's palsy or other cranial neuritis, encephalomyelitis, lymphocytic meningitis, radiculoneuropathy, or 2nd or 3rd degree atrioventricular block) that also has laboratory evidence of infection that meets criteria.

“Probable cases" of Lyme disease included:
Any other physician-diagnosed Lyme disease cases with laboratory evidence of infection that meets criteria and at least one non-confirmatory sign or symptom. Non-confirmatory signs and symptoms include fever, sweats, chills, fatigue, neck pain, arthralgias, myalgias, fibromyalgia syndromes, cognitive impairment, headache, paresthesias, visual/auditory impairment, peripheral neuropathy, encephalopathy, palpitations, bradycardia, bundle branch block, myocarditis, or other (non-EM) rash.
See the CDC Lyme Disease case definition page for access to current and previous national case definitions.

Case estimation during 2012-2021

From 2012-2021, the Division of Public Health and local and tribal health departments modified the way they conducted Lyme disease surveillance compared with pre-2012 surveillance. Complete public health follow-up during this time involved a case investigation with each suspected case report or positive laboratory report to collect information on clinical signs and symptoms, possible exposures, all laboratory results, and treatment (Routine Surveillance). To address the increasing number of Lyme disease cases and the significant burden of conducting Routine Surveillance for all suspected cases of Lyme disease, a Partial Surveillance approach was used by many local and tribal agencies during 2012-2021. This means that in many counties, some but not all reported suspect cases were investigated (Partial Surveillance). During this time-period when Partial Surveillance was being conducted, case estimation was also completed to account for reports of possible Lyme disease that were unable to be investigated by public health. A statistical method was developed and then implemented to estimate statewide cases based on the number of total individuals with at least one laboratory report received for each year during 2012-2021. The total statewide case count measured during 2012-2021 was the sum of confirmed, probable, and estimated cases. Estimates were not available by county, only for statewide numbers. This means for counties using Partial Surveillance methods, county level case counts likely underreport the actual number of cases during this time period.

Case counts during 1991-2007

Total state and county level cases were calculated using only confirmed cases.
“Confirmed cases” of Lyme disease included:
Those with an erythema migrans (EM) rash that is greater or equal to 5 cm in diameter and diagnosed by a medical professional, or those with at least one non-EM confirmatory sign or symptom indicating late manifestation of disease (arthritis, Bell's palsy or other cranial neuritis, encephalomyelitis, lymphocytic meningitis, radiculoneuropathy, or 2nd or 3rd degree atrioventricular block) that also has laboratory evidence of infection that meets criteria.

See the CDC Lyme Disease case definition page for access to current and previous national case definitions.

Incidence rates

Statewide incidence rates have been calculated using confirmed only (1991-2007), confirmed and probable (2008-2011 and 2022-present), and confirmed, probable, and estimated cases (2012-2021). County-specific incidence rates have use confirmed only, and confirmed and probable cases and did not use estimated cases at any time. The incidence, or crude rate, is calculated by dividing the number of cases by the total number of people in the population of interest (for example, a county). This is expressed as a number per unit population such as "per 100,000 population." Population estimates were derived from the Wisconsin Interactive Statistics on Health (WISH).

As of November 2023, the most current population estimates available in WISH is for the year 2020. For calculating rates for the years 2021 and 2022, population estimates for 2020 were used.

Note: a crude rate does not account for the differences in age distributions across counties and is therefore subject to bias. For example, as Lyme disease is less common among working age individuals, areas of the state with more working age individuals could appear, artificially, to have fewer cases generally.