Definition and epidemiology
Lyme disease is an infectious disease caused by spirochetes of the Borrelia burgdorferi sensu lato complex, transmitted to humans through the bite of infected Ixodes species ticks. In North America, the primary vector is Ixodes scapularis (black-legged or deer tick) in the eastern and central regions and Ixodes pacificus on the Pacific coast. In Europe, Ixodes ricinus is the main vector.
Lyme disease is the most common vector-borne illness in the United States, with approximately 30,000-40,000 confirmed and probable cases reported annually to the CDC, though actual incidence may be 10 times higher. Peak incidence occurs in children aged 5-14 years and adults aged 45-54 years. Geographic distribution is highly focal, with over 95% of cases concentrated in 14 states in the Northeast, Mid-Atlantic, and upper Midwest regions. In Europe, highest incidence is reported in central and eastern countries. Transmission is seasonal, with most cases occurring between May and September when nymphal ticks are most active.
- Erythema migrans in an endemic area is sufficient for diagnosis; serologic testing not needed and may be falsely negative in early disease
- Doxycycline is first-line for early Lyme disease; avoid in children <8 years and pregnant women (use amoxicillin)
- Bilateral facial nerve palsy is highly suggestive of Lyme disease neuroborreliosis
- Lyme carditis classically causes AV nodal blocks; high-grade blocks require hospitalization and IV antibiotics
- Two-tier serologic testing (EIA followed by Western blot) is standard; IgM alone after 30 days is not reliable
- Single dose doxycycline prophylaxis may prevent Lyme disease if Ixodes tick attached ≥36 hours in endemic area
- Lyme arthritis is typically a late manifestation affecting large joints, especially the knee, with large effusions
- Post-treatment Lyme disease syndrome causes persistent symptoms but does not respond to additional antibiotics
Etiology and risk factors
Causative organism: In North America, Borrelia burgdorferi sensu stricto is the sole pathogenic species. In Europe and Asia, Borrelia afzelii and Borrelia garinii also cause disease, with slightly different clinical manifestations.
Transmission: The spirochete is transmitted through the bite of an infected Ixodes tick. The tick must typically remain attached for 36-48 hours for effective transmission, as the bacteria reside in the tick's midgut and require time to migrate to salivary glands. Nymphal ticks (1-2 mm in size) are responsible for most human infections because they are small and often go unnoticed.
Risk factors:
- Residence in or travel to endemic areas
- Outdoor activities in wooded or grassy areas (hiking, camping, gardening)
- Spring and summer months (May-September)
- Failure to use tick repellents or protective clothing
- Lack of daily tick checks after outdoor exposure
- Occupational exposure (forestry workers, landscapers, wildlife biologists)
- Presence of deer and rodent populations that support tick life cycles
Person-to-person transmission does not occur. There is no evidence of transmission through blood transfusion or sexual contact, though rare cases of transplacental transmission have been reported.
Pathophysiology
After inoculation through a tick bite, Borrelia spirochetes proliferate locally in the skin, producing the characteristic erythema migrans lesion. The rash results from both direct bacterial invasion and local inflammatory response. Spirochetes can evade the immune system through antigenic variation and downregulation of surface proteins.
From the skin, bacteria disseminate hematogenously to multiple organ systems within days to weeks. Tropism exists for skin, nervous system, joints, and heart. The organism's outer surface proteins (particularly OspA, OspB, and OspC) facilitate tissue binding and immune evasion.
Stage-specific pathophysiology:
- Early localized (days 3-30): Local infection produces expanding erythema migrans through dermal infiltration with lymphocytes and plasma cells
- Early disseminated (days to weeks): Hematogenous and lymphatic spread causes secondary skin lesions, cranial neuropathies (particularly facial nerve), meningitis, and carditis with conduction abnormalities
- Late disease (months to years): Persistent infection in joints triggers chronic inflammatory arthritis, particularly in large joints. Neuroborreliosis causes encephalomyelitis and peripheral neuropathy through direct invasion and immune-mediated damage
Chronic inflammation in untreated cases may result from persistent viable organisms, retained bacterial antigens, or autoimmune mechanisms triggered by molecular mimicry between Borrelia proteins and host tissues.
Clinical features
Lyme disease classically progresses through three stages, though not all patients experience all stages.
Early localized disease (3-30 days post-bite)
Erythema migrans is the hallmark manifestation, occurring in 70-80% of infected patients. The rash begins as a small red papule at the bite site, then expands over days to weeks to form a large (typically >5 cm diameter) annular lesion. Classic "bull's-eye" appearance with central clearing occurs in only 20-30% of cases; most lesions are uniformly erythematous. The rash is typically warm but not painful or pruritic. Multiple erythema migrans lesions indicate hematogenous dissemination.
Systemic symptoms include fever, fatigue, myalgia, headache, and arthralgia. These flu-like symptoms occur in approximately 50% of patients.
Early disseminated disease (days to weeks)
Neurologic manifestations (10-15% of untreated patients):
- Cranial neuropathies, most commonly unilateral or bilateral facial nerve palsy
- Lymphocytic meningitis with headache, neck stiffness
- Radiculoneuritis with radiating pain and paresthesias
Cardiac manifestations (1-5% of cases):
- Atrioventricular conduction blocks (first-, second-, or third-degree)
- Myopericarditis
- Syncope may occur with high-grade heart block
Secondary skin lesions: Multiple erythema migrans lesions, smaller than the primary lesion
Late disease (months to years)
Lyme arthritis (60% of untreated patients):
- Intermittent or chronic monoarthritis or oligoarthritis
- Knee is the most commonly affected joint
- Large joint effusions with inflammatory synovial fluid
Late neuroborreliosis:
- Encephalomyelitis with cognitive dysfunction, sleep disturbance, personality changes
- Peripheral neuropathy with distal paresthesias
- In Europe, acrodermatitis chronica atrophicans (progressive skin atrophy)
Only a minority of untreated patients develop late manifestations.
Diagnosis
Diagnosis of Lyme disease requires integration of clinical presentation, epidemiologic factors, and laboratory testing.
Clinical diagnosis
Erythema migrans in a patient with appropriate exposure history is sufficient for diagnosis and treatment initiation. The rash is pathognomonic enough that serologic testing is not required and may be falsely negative in early disease.
Laboratory diagnosis
Serology is the primary diagnostic modality for non-cutaneous manifestations. A two-tier testing approach is standard:
- Initial screening: Enzyme immunoassay (EIA) or immunofluorescence assay (IFA) detecting IgM and IgG antibodies
- Confirmatory testing: If screening is positive or equivocal, perform Western blot for IgM (if <30 days of symptoms) and IgG
Interpretation:
- IgM antibodies appear 2-4 weeks after infection; positive IgM requires 2 of 3 specific bands on Western blot
- IgG antibodies appear after 4-6 weeks; positive IgG requires 5 of 10 specific bands
- False-positive IgM results are common; do not test IgM after 30 days of illness
- Antibodies persist for years after successful treatment and do not indicate active infection
- Seronegative early disease is common; do not test patients with erythema migrans
Alternative testing: Modified two-tier testing using two different EIAs has been validated and may improve sensitivity.
Specialized testing
- CSF analysis for suspected neuroborreliosis: lymphocytic pleocytosis, elevated protein, normal glucose. Intrathecal antibody production confirms CNS infection
- Synovial fluid analysis: inflammatory fluid (WBC >10,000) with neutrophil predominance
- PCR: Low sensitivity; may be used on synovial fluid in patients with Lyme arthritis
- Culture: Rarely performed; low yield except from erythema migrans skin biopsy
Tests to avoid:
- Urine antigen tests
- Blood PCR
- CD57 lymphocyte counts
- C6 peptide testing as standalone
These lack validation and are not recommended by guidelines.
Differential diagnosis
Erythema migrans mimics
- Erythema multiforme: Target lesions with dusky centers, acute onset
- Tinea corporis: Scaly border, central clearing, KOH positive
- Granuloma annulare: Firm papules, no scale, chronic
- Southern tick-associated rash illness (STARI): Associated with Amblyomma americanum bites, self-limited
- Cellulitis: More acute, painful, warm, purulent
- Hypersensitivity reaction to tick bite: Smaller (<5 cm), resolves within 24-48 hours
Neurologic presentations
- Bell's palsy from other causes: HSV, HIV, sarcoidosis, idiopathic
- Viral meningitis: CSF findings similar but exposure history absent
- Multiple sclerosis: Chronic relapsing course, MRI findings
- Guillain-Barré syndrome: Ascending paralysis, albuminocytologic dissociation
Cardiac presentations
- Viral myocarditis: Acute presentation without preceding tick exposure
- Conduction system disease: Elderly patients, chronic progressive
- Drug-induced heart block: Medication history
Arthritis presentations
- Septic arthritis: Acute, febrile, positive Gram stain/culture
- Reactive arthritis: Post-infectious, often follows GI/GU infection
- Rheumatoid arthritis: Symmetric small joint involvement, positive RF/anti-CCP
- Juvenile idiopathic arthritis: Negative Lyme serology, chronic course
- Crystalline arthropathy: Gout or pseudogout, crystals in synovial fluid
Management
Treatment of Lyme disease depends on the stage and organ system involvement. Early treatment prevents progression to disseminated disease.
Early localized disease (erythema migrans)
Oral antibiotics for 10-14 days:
- Doxycycline (first-line): Adults and children ≥8 years
- Amoxicillin: Children <8 years, pregnant/lactating women
- Cefuroxime axetil: Alternative for doxycycline-intolerant patients
- Azithromycin: Less effective; use only if above contraindicated
Early disseminated disease
Multiple erythema migrans or isolated facial palsy: Oral therapy as above for 14 days
Carditis:
- High-degree AV block or symptomatic: Admit for telemetry, consider temporary pacing, IV ceftriaxone for 14 days initially, then transition to oral therapy
- First-degree AV block: Oral therapy for 14-21 days
Meningitis or radiculoneuritis: IV ceftriaxone for 14-28 days. Alternatives include IV cefotaxime or IV penicillin G. Oral doxycycline may be equally effective for early neuroborreliosis.
Late disease
Lyme arthritis:
- Oral doxycycline or amoxicillin for 28 days
- If persistent or recurrent after one course: Repeat 28-day course of oral or IV therapy
- NSAIDs for symptomatic relief
- Arthroscopic synovectomy for antibiotic-refractory arthritis
Late neuroborreliosis: IV ceftriaxone for 14-28 days
Special considerations
Pregnancy: Amoxicillin or cefuroxime; IV ceftriaxone for disseminated disease
Jarisch-Herxheimer reaction: Transient fever and worsening symptoms within 24 hours of starting antibiotics; manage supportively
Post-treatment Lyme disease syndrome (PTLDS): Persistent fatigue, pain, or cognitive complaints after appropriate treatment in approximately 10-20% of patients. Pathophysiology unclear. Prolonged or repeated antibiotic therapy is not beneficial and may be harmful. Manage symptomatically with supportive care.
Prevention
- Avoid tick-infested areas when possible
- Wear protective clothing and apply DEET-containing repellents
- Perform daily tick checks and prompt removal (within 24-36 hours)
- Remove attached ticks with fine-tipped tweezers by grasping near the skin
- Prophylaxis: Single dose of doxycycline within 72 hours of tick removal may be considered if: (1) tick identified as Ixodes species, (2) estimated attachment ≥36 hours, (3) endemic area with ≥20% infection rate, (4) doxycycline not contraindicated
No vaccine is currently available; a previous vaccine was withdrawn from the market.
Complications and prognosis
Complications
Early treatment complications:
- Jarisch-Herxheimer reaction (fever, worsening symptoms within 24 hours)
- Antibiotic-associated adverse effects
Untreated or late-treated disease:
- Chronic Lyme arthritis with joint damage
- Persistent neurocognitive dysfunction
- Peripheral neuropathy with pain and paresthesias
- Complete heart block requiring pacemaker (rare)
- Acrodermatitis chronica atrophicans (Europe)
Antibiotic-refractory Lyme arthritis: Persistent synovitis despite appropriate antibiotic therapy occurs in approximately 10% of Lyme arthritis cases, particularly in patients with HLA-DR4 alleles. Thought to result from autoimmune mechanisms triggered by molecular mimicry.
Post-treatment Lyme disease syndrome (PTLDS): Subjective symptoms (fatigue, arthralgia, cognitive complaints) persist in 10-20% of treated patients despite microbiologic cure. Symptoms typically improve gradually over 6-12 months. Mechanism unclear but not due to persistent infection.
Prognosis
Prognosis is excellent with early diagnosis and appropriate antibiotic therapy. Erythema migrans resolves within weeks, and early treatment prevents progression to disseminated disease in the vast majority of cases.
Stage-specific outcomes:
- Early localized disease: Complete resolution with 10-14 days of antibiotics
- Facial nerve palsy: Recovery in >90% within 2-3 months, regardless of treatment
- Carditis: Conduction abnormalities typically resolve within 1-6 weeks; permanent pacemaker rarely needed
- Lyme arthritis: Resolves in approximately 90% within months after antibiotic treatment
- Neuroborreliosis: Most patients improve, though recovery may be slow and incomplete in late-treated cases
Mortality is extremely rare and typically related to cardiac complications. Long-term disability is uncommon with appropriate treatment. Even without treatment, most manifestations eventually resolve spontaneously, though this may take months to years.
Chronic Lyme disease: The term "chronic Lyme disease" is controversial and not a recognized clinical entity. Many patients with this self-diagnosis do not have evidence of prior or current Borrelia infection. Prolonged antibiotic therapy for such patients is not supported by evidence and carries significant risk of harm.
FACE for Lyme neuroborreliosis: Facial nerve palsy (most common), Asymptomatic meningitis, Cranial neuropathy, Encephalitis
Practice questions
1. A 32-year-old woman presents in July with a 5-day history of an expanding rash on her thigh. She returned from hiking in Connecticut 10 days ago and recalls removing a small tick from the area. She has mild fatigue but no fever. Examination shows a 9 cm erythematous annular lesion without central clearing, warm to touch and non-tender. What is the most appropriate next step in management?
2. A 45-year-old man with a history of Lyme disease treated 3 years ago presents with 4 months of progressive right knee swelling and pain. He denies fever, rash, or recent tick bites. Examination shows a large knee effusion without erythema or warmth. Arthrocentesis yields 18,000 WBC/μL with 70% neutrophils; Gram stain is negative. Lyme IgG by ELISA is positive with confirmatory Western blot showing 7 of 10 bands. What is the most appropriate treatment?
3. A 7-year-old boy is brought to the emergency department with sudden-onset right facial droop noticed this morning. His mother reports he had a "red rash on his back" 3 weeks ago after camping in upstate New York. He has no fever, headache, or other neurologic symptoms. Examination shows complete right peripheral facial nerve palsy with inability to close his right eye. Which of the following is the most appropriate initial management?
Frequently asked questions
What causes Lyme disease?
Lyme disease is caused by the spirochete bacterium Borrelia burgdorferi, transmitted through the bite of infected Ixodes ticks (black-legged or deer ticks). The tick must remain attached for at least 36-48 hours for effective transmission. Peak transmission occurs during late spring and summer months when nymphal ticks are most active in endemic areas of the northeastern, mid-Atlantic, and upper midwestern United States.
What are the first symptoms of Lyme disease?
The first symptom is typically erythema migrans, a characteristic expanding red rash that appears 3-30 days after a tick bite in 70-80% of infected people. The rash starts as a small red bump and expands to at least 5 cm, sometimes with central clearing creating a "bull's-eye" appearance. Accompanying symptoms often include fever, fatigue, headache, muscle aches, and joint pain resembling flu-like illness.
How is Lyme disease diagnosed?
Erythema migrans rash in someone with tick exposure in an endemic area is sufficient for clinical diagnosis without testing. For other manifestations, diagnosis uses two-tier serologic testing: an initial enzyme immunoassay (EIA) screening followed by confirmatory Western blot if positive. Testing is not reliable in the first few weeks of infection. CSF analysis, synovial fluid studies, or PCR may be used for specific organ involvement like neuroborreliosis or arthritis.
Is Lyme disease curable?
Yes, Lyme disease is curable with appropriate antibiotic treatment. Early localized disease responds well to 10-14 days of oral doxycycline, amoxicillin, or cefuroxime with complete resolution. Disseminated disease may require longer courses or intravenous antibiotics. Most patients recover fully, though 10-20% may have persistent nonspecific symptoms (post-treatment Lyme disease syndrome) that gradually improve over months. Early treatment prevents progression and improves outcomes.
Lyme disease vs Southern tick-associated rash illness (STARI) — what is the difference?
Lyme disease is caused by Borrelia burgdorferi transmitted by Ixodes ticks, while STARI follows bites from Amblyomma americanum (lone star tick) in the southeastern United States. Both produce expanding erythema migrans-like rashes, but STARI's causative agent is unknown and the rash is typically smaller. STARI does not cause the neurologic, cardiac, or arthritic complications seen in Lyme disease. STARI is self-limited and resolves without specific treatment, though antibiotics are often given empirically.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: A69.2
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.