What Is Chronic Lyme Disease? ILADS Definition Explained

ILADS proposed an evidence-based definition of chronic Lyme disease
The definition describes persistent multisystem symptoms and clinical features
It aims to improve research and clinical recognition

What is chronic Lyme disease? The term chronic Lyme disease is used differently by different medical organizations. In 2019, an International Lyme and Associated Diseases Society (ILADS) working group published an evidence-based definition intended to describe patients with persistent or relapsing multisystem illness attributed to Borrelia burgdorferi sensu lato infection. Other organizations use different terminology, such as post-treatment Lyme disease syndrome (PTLDS), reflecting ongoing scientific debate regarding diagnosis, mechanisms, and patient classification.

In their evidence review, Shor and colleagues identified a broad range of symptoms, signs, and conditions reported in patients with acute and chronic Lyme disease, regardless of whether they had previously received treatment. The cases evaluated by the ILADS working group had laboratory evidence of infection with Borrelia burgdorferi sensu lato (Bbsl), the group of bacteria that causes Lyme disease.

The findings illustrate the multisystem nature of Lyme disease and the wide spectrum of clinical manifestations that may occur in some patients.

Neurologic and neuropsychiatric manifestations described in the review included:

• Memory difficulties
• Decreased concentration
• Dizziness and vertigo
• Abnormal taste
• Decreased sensation
• Paresthesias or tingling
• Generalized and radicular pain
• Decreased dexterity
• Abnormal gait and balance
• Limb paralysis
• Spastic paraparesis
• Positive Babinski sign
• Areflexia or hyperreflexia
• Fasciculations
• Urinary incontinence

READ MORE: How do Lyme disease symptoms persist long term?

Additionally, the authors described a broad range of clinical characteristics reported in patients with chronic Lyme disease, including fatigue, weight gain, orthostatic intolerance, palpitations, dyspnea, cardiac arrhythmias, mitral regurgitation, and numerous musculoskeletal and head, ear, eye, nose, and throat manifestations.

The review also identified numerous conditions reported in the reviewed literature as occurring in patients with Lyme disease, including encephalomyelitis, encephalopathy, endocarditis, epilepsy or seizures, facial palsy, meningitis, Alzheimer’s disease, carpal tunnel syndrome, panuveitis, polyarthritis, radiculoneuropathy, sensory-motor polyneuropathy, sensory neuropathy, and synovitis.

How does the ILADS definition differ from other guidelines?

The ILADS working group defines chronic Lyme disease as a persistent multisystem illness that may continue or recur for at least six months and may be associated with ongoing infection by members of the Borrelia burgdorferi sensu lato complex. In contrast, the 2020 guidelines from the Infectious Diseases Society of America (IDSA), American Academy of Neurology (AAN), and American College of Rheumatology (ACR) generally use the term post-treatment Lyme disease syndrome (PTLDS) for patients with persistent symptoms following recommended treatment rather than using chronic Lyme disease as a diagnostic term.

These differing definitions reflect ongoing scientific and clinical debate regarding terminology, mechanisms, and patient classification. Understanding the distinctions can help patients interpret recommendations from different medical organizations and discuss their symptoms with their healthcare professional.

ILADS defines chronic Lyme disease as:

“A multisystem illness with a wide range of symptoms and/or signs that are either continuously or intermittently present for a minimum of six months.

The illness is the result of an active and ongoing infection by any of several pathogenic members of the Borrelia burgdorferi sensu lato complex (Bbsl).

The infection has variable latency periods and signs and symptoms may wax, wane and migrate.

CLD has two subcategories: CLD untreated (CLD-U) and CLD previously treated (CLD-PT). The latter requires that manifestations persist or recur following treatment and are present continuously or in a relapsing-remitting pattern for six months or longer.”

Functional impairment in chronic Lyme disease

The authors noted that persistent manifestations of Lyme disease may interfere with daily activities, employment, education, and social functioning. However, not every patient who meets the proposed ILADS definition will experience measurable functional impairment.

As the working group explained:

“Although many patients with persistent manifestations of Lyme disease following treatment are functionally impaired at some point in their illness, others will not meet the criteria for functional impairment.”

For this reason, functional status was not included as a required component of the ILADS definition. A patient may have persistent or recurring symptoms without meeting a specific threshold for disability.

Can chronic Lyme disease symptoms come and go?

Under the ILADS definition, symptoms and signs may be continuously present or may follow a relapsing and remitting pattern. Patients may experience periods of improvement followed by the recurrence or migration of symptoms.

Recurring symptoms do not by themselves establish the cause of an illness. Other infections, inflammatory conditions, neurologic disorders, medication effects, sleep disorders, endocrine problems, and autonomic dysfunction may produce overlapping symptoms. A careful clinical evaluation remains important.

Why does terminology matter?

The terms chronic Lyme disease, persistent Lyme disease, and post-treatment Lyme disease syndrome are not always used interchangeably. Each term may reflect different assumptions about prior treatment, the cause of continuing symptoms, and whether an active infection remains present.

The ILADS working group proposed its definition to create a more consistent framework for clinical discussion and research. The IDSA/AAN/ACR guidelines use different terminology and emphasize that persistent nonspecific symptoms following recommended treatment do not necessarily demonstrate ongoing infection.

These differences are clinically important because research findings may vary depending on how patients are selected and classified. A clearly stated definition can help readers understand which patients were included in a study and whether the findings apply to untreated illness, previously treated illness, or PTLDS.

Why is a standardized definition needed?

A standardized definition can help researchers identify comparable patient populations, design clinical trials, and examine possible differences between untreated and previously treated patients. It may also help clarify symptom duration, laboratory evidence, treatment history, and functional outcomes.

However, a proposed definition should not replace an individualized medical evaluation. Patients with persistent symptoms may require assessment for Lyme disease manifestations, tick-borne coinfections, treatment complications, and unrelated conditions that can produce similar symptoms.

Frequently Asked Questions

What is chronic Lyme disease?

According to the ILADS working group, chronic Lyme disease is a multisystem illness with persistent or relapsing symptoms lasting at least six months and attributed to infection with Borrelia burgdorferi sensu lato. Other medical organizations use different terminology, including post-treatment Lyme disease syndrome (PTLDS), reflecting ongoing scientific debate regarding diagnosis and underlying mechanisms.

Can Lyme disease symptoms come and go?

Yes. The ILADS definition recognizes that symptoms may wax and wane or migrate over time. However, recurring symptoms are not specific to Lyme disease, and other medical conditions should also be considered during evaluation.

Why do different medical organizations define chronic Lyme disease differently?

ILADS and the IDSA/AAN/ACR guidelines use different terminology and patient classifications. ILADS proposes a definition of chronic Lyme disease, while the IDSA/AAN/ACR guidelines generally use the term post-treatment Lyme disease syndrome (PTLDS) for patients with persistent symptoms following recommended treatment rather than using chronic Lyme disease as a diagnostic term.

Clinical Takeaway

The ILADS working group proposed an evidence-based definition of chronic Lyme disease to provide a standardized framework for describing patients with persistent or relapsing multisystem illness. The proposed definition recognizes the broad spectrum of neurologic, musculoskeletal, autonomic, and constitutional symptoms reported in these patients while acknowledging that functional impairment varies between individuals.

Different medical organizations use different terminology for persistent symptoms associated with Lyme disease, underscoring the need for continued research into diagnosis, pathophysiology, and optimal treatment strategies.

Understanding how different organizations define chronic Lyme disease can help patients and clinicians interpret research findings and make more informed clinical decisions.

Related Articles

Persistent Lyme disease overview
Post-treatment Lyme disease syndrome (PTLDS)
Lyme disease diagnosis
How do Lyme disease symptoms persist long term?

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Dr. Daniel Cameron, MD, MPH
Lyme disease clinician with over 30 years of experience and past president of ILADS.

Symptoms • Testing • Coinfections • Recovery • Pediatric • Prevention