National case definition: Lyme disease

Date of last revision: November 2024

On this page

• National notification
• Type of surveillance
• Case classification
• Laboratory criteria
• Clinical manifestations
• ICD code
• Comments
• Footnotes
• Appendix: Description of the change to the national case definition
• References

National notification

Confirmed and probable cases of disease should be notified.

Type of surveillance

Routine case-by-case notification to the federal level.

Case classification

Confirmed case

Clinical evidence of illness with laboratory confirmation by one of the following methods:

• isolation of Borrelia burgdorferi (B. burgdorferi) from a clinical specimen as specified by current guidelines^{Footnote 1Footnote 2Footnote 3}.
• detection of B. burgdorferi DNA by PCR testing on skin biopsies, synovial fluid or cerebrospinal fluid and by methods specified by current guidelines^{Footnote 1Footnote 2Footnote 3}.

OR

Clinical evidence of illness with a history of residence in, or visit to, a Lyme disease risk area^{Footnote *}, and with laboratory evidence of infection in the form of a positive serologic test using the standard two-tiered testing (STTT) or modified two-tiered testing (MTTT) approach^{Footnote 3Footnote 4} (see laboratory comments).

Probable case

A case that meets laboratory evidence of infection (using one of the methods as defined above under confirmed case, also see Laboratory comments), with history of residence in, or visit to, a Lyme disease risk area^{Footnote *} and without clinical evidence^{Footnote **}.

OR

Clinical evidence of illness without a history of residence in, or visit to, a Lyme disease risk area^{Footnote *}; and with laboratory evidence of infection in the form of a positive serologic test as defined above under confirmed cases (see Laboratory comments).

OR

Clinician-observed erythema migrans without laboratory evidence but with history of residence in, or visit to, a Lyme disease risk area^{Footnote *}.

Laboratory criteria

Criteria for serologic testing are described by the guidelines of the Association of Public Health Laboratories^{Footnote 3Footnote 4}. Serologic evidence is confirmatory only in patients with objective clinical evidence of disseminated Lyme disease, and a history of residence in, or visit to, a Lyme disease risk area. Serologic testing is not recommended in patients with early localized Lyme disease with exposure from a Lyme disease risk area.

The STTT approach consists of a screening ELISA followed by an immunoblot assay. The MTTT approach consists of a screening ELISA, followed by a different, sequential ELISA, replacing an immunoblot. Immunoblots include traditional Western blots^{Footnote 3} or newer line blots, and both formats target B. burgdorferi immunoreactive proteins^{Footnote 5}.

Laboratory testing should be done through a licensed and accredited public health laboratory. It's not recommended to:

• perform tests via non-licensed laboratories or private laboratories that don't use U.S. Food and Drug Administration or Health Canada approved tests, nor
• use alternative interpretive criteria.

Clinical manifestations

The clinical information presented below is not intended to describe the complete range of signs and symptoms that may be used in a clinical diagnosis of Lyme disease. Symptoms of early or late disseminated Lyme disease are described in scientific literature^{Footnote 1Footnote 2}. Other symptoms that are associated with Lyme disease (such as post Lyme disease syndrome) are considered too non-specific to define cases for surveillance purposes, even though they are caused by B. burgdorferi infection.

Objective evidence of Lyme disease includes the following when an alternative explanation is not found:

• In simple terms, Lyme disease has three stages if left untreated:
  1. early Lyme disease characterised by a red rash (greater than 5 cm; called erythema migrans) that spreads from the site of the tick rash (as described below);
  2. early disseminated Lyme disease characterised by multiple erythema migrans rashes and/or neurological (facial paralysis or meningitis-like) manifestations and/or heart problems (palpitations caused by heart block) which may last several weeks to months; and
  3. late disseminated Lyme disease which is most commonly intermittent arthritis and may last months to over a year.
• The following signs and symptoms constitute objective clinical evidence of illness for surveillance purposes for Lyme disease. In detail the manifestations are:
• Erythema migrans: a round or oval expanding erythematous area of the skin greater than 5 cm in diameter and enlarging slowly over a period of several days to weeks. It appears one to two weeks (range 3 to 30 days) after infection and persists for up to eight weeks. Some lesions are homogeneously erythematous, whereas others have a prominent central clearing or a distinctive target-like appearance. On the lower extremities, the lesion may be partially purpuric. Signs of acute or chronic inflammation are not prominent. There is usually little pain, itching, swelling, scaling, exudation or crusting, erosion or ulceration, except that some inflammation associated with the tick bite itself may be present at the very centre of the lesion. Note: An erythematous skin lesion present while a tick vector is still attached or that has developed within 48 hours of detachment is most likely a tick bite hypersensitivity reaction (i.e. a non-infectious process), rather than erythema migrans. Tick bite hypersensitivity reactions are usually less than 5 cm in largest diameter, sometimes have an urticarial appearance and typically begin to disappear within 24 to 48 hours. Diagnosis of erythema migrans requires careful examination by a physician to eliminate alternative types of skin rash. Note that it is recommended that physicians would normally treat patients with erythema migrans without recourse to serological testing as specific antibodies are often not detectable in early Lyme disease^{Footnote 1Footnote 5}.

Objective evidence of disseminated Lyme disease includes any of the following when an alternative explanation is not found:

• Multiple erythema migrans: erythema migrans lesions, similar to the single erythema migrans lesions described above, but in multiple locations on the body and may be smaller (less than 5 cm).
• Neurological: Early neurological Lyme disease: acute peripheral nervous system involvement, including radiculopathy, cranial neuropathy and mononeuropathy multiplex (multifocal involvement of anatomically unrelated nerves), and central nervous system involvement, including lymphocytic meningitis and, rarely, encephalomyelitis (parenchymal inflammation of brain and/ or spinal cord with focal abnormalities). Late neurologic Lyme disease may present as encephalomyelitis, peripheral neuropathy or encephalopathy.
• Musculoskeletal: Lyme arthritis is a monoarticular or oligoarticular form of arthritis most commonly involving the knee, but other large joints or the temporo-mandibular joint may be involved. Large effusions that are out of proportion to the pain are typical. Lyme arthritis is often intermittent if untreated, with episodes of joint inflammation spontaneously resolving after a few weeks to a few months. Persistent swelling of the same joint for 12 months or more is not a usual presentation.
• Cardiac: Cardiac involvement associated with Lyme disease includes intermittent atrioventricular heart block often involving the atrioventricular node (although heart block may occur at multiple levels) and sometimes associated with myopericarditis. Carditis can occur in the early stages of the disease.

ICD code(s)

ICD-10 code
A69.2 Lyme disease (Erythema chronicum migrans due to Borrelia burgdorferi)

Comments

These are definitions for surveillance and epidemiologic purposes only, they do not represent clinical case definitions used for diagnosis purposes.

Footnotes

Appendix: Description of the change to the national case definition

In 2024, the Tick-borne Diseases Task Group of the Public Health Network's Communicable and Infectious Disease Steering Committee (CIDSC) proposed several changes that have been incorporated to the case definition including:

1. introduction of the modified two-tier testing (MTTT) approach as an alternative for serologic testing;
2. addition of laboratory-based cases in high-incidence areas; and
3. update of the reference to the serologic testing guidelines.

These updates aim to enhance flexibility and accuracy for Lyme disease surveillance, taking into account the varying surveillance approaches for Lyme disease across jurisdictions.

Rationale for the change

• In 2007-2008, the national surveillance case definition for Lyme disease was developed by a Working Group of the Public Health Network. The case definition incorporated evidence of exposure to environmental risk ("residence in, or visit to, an endemic area") in categorizing reported cases into 'confirmed' or 'probable' classifications, to limit the number of reported cases that may be due to false-positive test results. Lyme disease became nationally notifiable in 2009.
• In 2016, the Lyme disease surveillance case definition was updated to replace the term "endemic area" with "risk area" and specify five methods for identifying Lyme disease risk areas.
• In 2024, the Lyme disease case definition has been revised to reflect changes to laboratory diagnostics and reflect the latest scientific advancements and emerging trends in Lyme disease epidemiology in Canada. These updates are part of our ongoing commitment to improving public health and ensuring that our diagnostic and surveillance methods remain current with scientific advancements.
• The MTTT approach has been added as an alternative to the standard two-tiered testing (STTT) method. The MTTT approach replaces immunoblots with a second round of enzyme immunoassays (EIA). Cases identified only on the basis of laboratory test results have been incorporated into the case definition to facilitate a transition to laboratory-based surveillance in regions with high Lyme disease incidence. This change aims to simplify reporting of cases by public health organizations in high incidence locations where the follow-up for each case, to obtain information on environmental exposure and clinical manifestations as required by the existing definition, is highly burdensome for the public health organizations. This change will facilitate reporting of more accurate case numbers from high incidence regions.

Expected impact on the number of cases reported

• The revised case definition will enable provinces/territories to detect and report Lyme disease cases in a less resource intensive, more timely and standardized manner, allowing more cases to be reported.
• It will also allow some provinces/territories to report laboratory-based cases as probable cases when previously they were not able to do.

Consultation process followed for the change

• In February 2024, a revision of the Lyme disease case definition was added to the work plan of Communicable and Infectious Disease Steering Committee (CIDSC).
• The Tick-Borne Diseases Task Group was tasked in February 2024 with a mandate to revise and achieve consensus on the Lyme disease surveillance case definition.
• The Tick-Borne Diseases Task Group was composed of 26 participants from 10 provinces (including epidemiologists, Lyme disease laboratory diagnostics and infectious disease specialists from the Canadian Public Health Laboratory Network).
• The revised case definition was approved by the Canadian Public Health Laboratory Network in October 2024.
• The revised case definition was approved by CIDSC in November 2024.

Provinces and territories implementing the change

All provinces and territories have approved the revised case definition.

Date of implementation

February 2025

Previous case definitions

Lyme disease case definition (2016)
Lyme disease case definition (2009)

References