Lyme Disease

Transmission and Establishment of Infection

• Ixodes tick nymphs (approximately 2mm in size, often unnoticed by patients) acquire B. burgdorferi spirochetes by feeding on infected rodents (particularly white-footed mice, which are the primary natural reservoir)
• Transmission requires approximately 36-48 hours of tick attachment; early removal within this window significantly reduces infection risk
• Spirochetes migrate from the tick midgut to salivary glands and inoculate into human dermis during blood meal
• The organism demonstrates remarkable phenotypic and antigenic variation, which facilitates immune evasion and persistence

Bacterial Mechanisms of Immune Evasion and Persistence

• B. burgdorferi produces outer surface proteins (OspA, OspB, OspC) that undergo stage-specific antigenic variation; OspC is particularly important for initial infection while OspA predominates in tick midgut
• The spirochete possesses a linear plasmid genome with genes encoding ~100 lipoproteins, many of which are immunogenic and trigger both adaptive and innate immune responses; paradoxically, these responses may contribute to tissue damage rather than pathogen clearance
• B. burgdorferi secretes proteases and collagenase that facilitate tissue invasion and dissemination through dermis and connective tissues
• The organism binds to host extracellular matrix proteins (fibronectin, decorin) via surface proteins, promoting dissemination and persistence in collagen-rich tissues (particularly joints and cardiac tissues)
• Spirochetes upregulate expression of complement-binding protein factor H, creating a "molecular camouflage" that prevents complement deposition and opsonization
• B. burgdorferi induces production of immunosuppressive cytokines (IL-10, TGF-β) that dampens Th1-mediated immune responses necessary for spirochete clearance

Tissue-Specific Pathology and Dissemination

• Early Local Infection (Erythema Migrans): After inoculation, spirochetes replicate in dermis and subdermis, triggering robust local inflammatory response with infiltration of CD4+ T cells, macrophages, and neutrophils; this produces characteristic expanding erythematous macule; the centrifugal expansion occurs as organisms spread centrally while immune response concentrates at margins
• Hematogenous Dissemination (Days 3-30): Spirochetes breach vascular endothelium via production of matrix metalloproteinases and direct endothelial cell engagement; bloodstream dissemination seeds multiple organs including skin (secondary lesions), joints, nervous system (CNS), and myocardium
• Neuroinvasion Mechanism: B. burgdorferi crosses blood-brain barrier through both transcytosis and paracellular routes; spirochetes possess neurotropic properties and preferentially replicate in perineurial and subarachnoid spaces; organisms trigger astrocyte activation and microglial response, producing pro-inflammatory cytokines (TNF-α, IL-6) that cause meningeal inflammation and cranial nerve dysfunction
• Arthritotropism: Spirochetes demonstrate particular tropism for synovial tissue; B. burgdorferi produces hyaluronate on its surface that mimics host hyaluronate, partially disguising organism from immune recognition; synovial inflammation occurs via both direct bacterial presence and immune-mediated mechanisms (molecular mimicry with host glycoproteins may perpetuate inflammation); this explains why some patients develop chronic Lyme arthritis despite appropriate antibiotic therapy

Molecular Mimicry and Autoimmunity

• B. burgdorferi outer surface proteins share epitopes with human proteins including decorin, tropomyosin, and keratin; this molecular mimicry may perpetuate autoimmune responses even after spirochete clearance
• In genetically susceptible individuals (those expressing HLA-DR4 alleles), persistent immune activation against spirochetal antigens cross-reacts with host tissue antigens
• This mechanism likely explains why some patients with documented past Lyme infection develop chronic symptoms in the absence of viable spirochetes

Chronic vs. Persistent Infection

• Most untreated B. burgdorferi infections are self-limited, with immune response eventually clearing organisms over months to years
• However, in articular sites, particularly the knee, organisms can persist despite robust immune response, leading to chronic Lyme arthritis (occurring in ~10% of untreated infections in North America)
• Persistent infection is not attributable to antibiotic resistance but rather to immune evasion mechanisms and sequestration in privileged immune sites

Causative Agent: Borrelia burgdorferi

• B. burgdorferi is a slow-growing, microaerophilic Gram-negative spirochete with a 16S rRNA gene sequence distinct from other Borrelia species
• Three major genospecies cause human disease: B. burgdorferi sensu stricto (North America), B. garinii (Eurasia), and B. afzelii (Eurasia); phenotypic differences result in variable clinical presentations between continents
• The organism's unique circular and linear plasmids encode genes essential for tick transmission and mammalian infection

Geographic Risk Factors

• Endemic regions in North America: Northeastern states (Massachusetts, Connecticut, New York, New Jersey, Pennsylvania, Maryland, Delaware), Upper Midwest (Wisconsin, Minnesota, Illinois), and Pacific coastal areas (Northern California, Oregon); risk gradually expanding westward and northward with climate change
• Endemic regions outside North America: Central and Northern Europe, particularly Germany, Austria, Switzerland, and Scandinavia; portions of Russia and Eastern Europe; Japan and temperate areas of China
• Risk of infection increases substantially in residential or recreational exposure to wooded areas, grasslands, and brush where Ixodes ticks are established; travel to endemic regions during tick season increases risk

Seasonal Risk Factors

• Peak incidence occurs June-August when nymphal Ixodes scapularis ticks are most active; nymphs are responsible for ~90% of human infections due to small size allowing unnoticed feeding
• Secondary peak in fall (September-October) corresponds to adult tick activity
• Winter months show markedly reduced transmission risk in most regions, though infections can occur year-round in milder climates

Occupational and Recreational Exposure

• Outdoor workers (forestry workers, park rangers, wildlife biologists) have elevated occupational risk
• Hikers, campers, and individuals with extensive woodland property exposure have substantially increased risk
• Dog owners in endemic areas have increased risk, as infected ticks can attach to dogs and then transfer to humans; however, dogs themselves do not appear to be competent reservoirs for human-to-human transmission

Host Factors Influencing Severity

• Age: Early Lyme arthritis and neurologic disease occur more commonly in adults aged 40-60 years; acrodermatitis chronica atrophicans (chronic skin manifestation seen primarily in Europe) is more common in elderly populations
• Immunocompromised status: Patients with HIV/AIDS or those receiving immunosuppressive therapy may develop more severe and prolonged infections, including unusually aggressive Lyme arthritis or CNS involvement
• Genetic susceptibility: HLA-DR4 and HLA-DRB1*0401 alleles are associated with increased risk of chronic Lyme arthritis, suggesting genetic predisposition to persistent infection or autoimmune sequelae
• Delayed treatment: Progression from early to late manifestations is significantly increased when early infection is not recognized and treated promptly

Vector Biology

• Ixodes scapularis ticks in North America have multiple-year life cycles; nymphs acquire infection by feeding on infected reservoir hosts (particularly white-footed mice, which are infected at rates of 20-40% in endemic areas)
• Tick infection rates vary geographically from <1% to >50% depending on local reservoir prevalence and ecological factors
• Co-infection with other *Ixodes*-transmitted pathogens (Anaplasma phagocytophilum, Babesia microti, Powassan virus) occurs in 5-10% of patients with Lyme disease, potentially worsening clinical outcomes

Early Localized Disease (Stage 1: Days 3-30)

Erythema Migrans (EM)

• Characteristic lesion appears at site of tick bite, typically 3-30 days after exposure (median 7-14 days); represents the initial local inflammatory response to spirochetal infection
• Classic teaching describes "bull's-eye" lesion with central erythema, clear zone, and peripheral erythema; however, most lesions (~70%) are uniformly erythematous macules or papules without the classic central clearing, making them easily confused with simple cellulitis
• Size typically expands centrifugally at rate of 5-10 cm per day, eventually reaching 5-80 cm in diameter if untreated; expansion reflects centrifugal spread of organisms coupled with immune response concentration at periphery
• Lesion is typically warm and may be pruritic or painful, but is usually painless and non-tender (important distinguishing feature from cellulitis, which is typically tender)
• Approximately 80% of early Lyme disease patients develop EM; in patients with disseminated disease, multiple secondary lesions develop days to weeks later in 50% of cases
• Associated systemic symptoms (flulike illness): fever (present in only 10-15% of EM patients, contrary to common belief), malaise, arthralgia, myalgia, fatigue, and headache; these may be mistakenly attributed to concurrent viral illness

Early Disseminated Disease (Stage 2: Weeks to Months)

Neurologic Manifestations (Lyme Neuroborreliosis)

• Occurs in 15-20% of untreated infections, typically weeks to months after EM
• Aseptic meningitis: Headache (often severe), neck stiffness, photophobia, and CSF pleocytosis (lymphocytic predominance); B. burgdorferi produces meningeal inflammation via direct invasion and cytokine production; distinguishable from viral meningitis by higher protein levels and lower glucose levels in CSF
• Cranial nerve palsies (Bannwarth syndrome in Europe, or Lyme neuroborreliosis in North America): Facial nerve (CN VII) involvement is most common (bilateral in ~50% of cases), causing Bell's palsy; CN VIII involvement causes sensorineural hearing loss; CN III, IV, V, and VI involvement occurs but less commonly; mechanism involves direct spirochetal inflammation of nerve roots and meninges
• Peripheral neuropathy: Radiculoneuropathy with sensory symptoms in dermatomal distribution; presents as burning pain, paresthesias, or sensory loss; reflects inflammation of dorsal root ganglia and peripheral nerves
• Encephalopathy: Cognitive impairment, subtle personality changes, difficulty concentrating, and sleep disturbance; mechanism less clearly defined, possibly representing direct CNS infection versus autoimmune process

Cardiac Manifestations (Lyme Carditis)

• Occurs in <1% of early disseminated Lyme disease (much less common than arthritis or neuroborreliosis)
• Atrioventricular (AV) block: Most common cardiac manifestation; caused by spirochetal infiltration and inflammation of conduction tissue, particularly around AV node; presents as first-degree through third-degree AV block on electrocardiography; patients may present with syncope or presyncope
• Myopericarditis: Myocardial inflammation and pericardial involvement causing chest pain, dyspnea, and heart failure; myocarditis reflects direct spirochetal invasion and inflammatory cell infiltration
• Acute heart failure: Dilated cardiomyopathy and ventricular dysfunction can develop acutely, though complete recovery typically occurs with antibiotic therapy

Arthrologic Manifestations (Lyme Arthritis)

• Lyme arthritis develops in ~60% of untreated North American infections (much higher percentage than in European strains)
• Pattern of involvement: Migratory or additive arthralgia preceding frank arthritis; knees are affected in >90% of cases (usually oligoarticular); other common sites include ankles, elbows, shoulders, and wrists
• Clinical characteristics: Typically acute in onset; brief episodes of joint pain and effusion lasting days to months; can alternate between joints; mild to moderate joint swelling without warmth (distinguishing from septic arthritis); patients typically do not appear toxic
• Chronic Lyme arthritis: Develops in ~10% of untreated North American infections; characterized by persistent monoarticular or oligoarticular arthritis, particularly of the knee; may continue for years despite treatment; this represents either persistent spirochetal infection in synovial tissue or immune-mediated perpetuation
• Pathophysiology: Direct bacterial invasion of synovium combined with host immune response; B. burgdorferi produces hyaluronate on surface, which mimics host tissue and escapes immune recognition; immune complexes deposit in synovium, triggering inflammation; molecular mimicry with host antigens may perpetuate inflammation after spirochete clearance

Secondary Skin Manifestations

• Secondary EM lesions: Occur in 50% of disseminated disease; resemble primary lesion but typically smaller
• Acrodermatitis chronica atrophicans (rare in North America, more common in Europe): Chronic skin atrophy and inflammation, typically on extremities; associated with B. afzelii infection; may appear as bluish-red nodules progressing to atrophic skin changes; thought to represent chronic stage manifestation

Late Disseminated Disease (Stage 3: Months to Years)

Lyme Arthritis (Chronic)

• As noted above, persistent synovitis in knees particularly; some cases show remarkable resistance to antibiotics
• Can present acutely even years after initial infection in some patients

Tertiary Neuroborreliosis (Chronic Encephalomyelitis)

• Rare in North America but well-described in Europe
• Progressive cognitive decline, leukoencephalopathy on MRI, and chronic demyelinating disease
• Associated with B. garinii and B. afzelii more than B. burgdorferi sensu stricto

Physical Examination Findings

• Erythema Migrans: Flat to slightly raised erythematous macule/papule, warm, usually nontender, expanding centrifugally; may be present at multiple sites in disseminated disease
• Systemic findings in early disseminated disease: Lymphadenitis (regional or widespread lymphadenopathy), hepatosplenomegaly (uncommon but reported)
• Neurologic examination: CN VII palsy (facial asymmetry, inability to close eye, loss of nasolabial fold on affected side); other CN palsies manifest with specific motor or sensory deficits depending on nerve involved; meningeal signs (neck stiffness, Kernig sign, Brudzinski sign); cognitive testing may reveal subtle deficits
• Joint examination: Effusion (detected by ballottement or bulge sign), limited range of motion, mild erythema or warmth (typically less pronounced than in septic arthritis)
• Cardiac examination: Irregular pulse or bradycardia (in AV block); new murmur (pericardial friction rub in pericarditis); signs of heart failure (peripheral edema, hepatic congestion, pulmonary rales)

Important Clinical Variants

• Single erythema migrans only: Some patients present with and never progress beyond localized disease
• Arthritis as sole manifestation: Rare patients develop Lyme arthritis without prior documented EM or systemic symptoms
• Asymptomatic seroconversion: Serologic testing may reveal evidence of past infection in asymptomatic individuals from endemic areas; clinical significance unclear

Clinical Assessment and History

• Key historical elements: Recent travel to or residence in endemic area, specific exposure history (hiking, camping, outdoor work, living near wooded areas), timing of symptom onset

Stabilise first (Lyme carditis)

• Telemetry admission: the 2020 IDSA/AAN/ACR Lyme disease guideline advises hospitalisation with continuous monitoring for symptomatic carditis, PR prolongation beyond roughly 300 ms, or any high-grade AV block, because block can migrate from first-degree to complete block within hours.
• Temporary pacing: transcutaneous then transvenous pacing for symptomatic high-grade block. Conduction disease is inflammatory and reversible — a permanent pacemaker should not be implanted during acute Lyme carditis.
• Parenteral therapy: third-generation cephalosporin, ceftriaxone IV, transitioning to oral therapy once block resolves.

First-line therapy for early localised disease (erythema migrans)

• Tetracyclines: doxycycline orally is preferred (IDSA/AAN/ACR 2020) because it also treats Anaplasma phagocytophilum co-infection; a 10-day course suffices for isolated EM.
• Aminopenicillins / second-generation cephalosporins: amoxicillin or cefuroxime axetil for 14 days when doxycycline is unsuitable — the preferred agents in pregnancy.
• Macrolides (azithromycin) are second-line only, reserved for intolerance of both above, as efficacy is lower.

Stage-specific escalation

• Neuroborreliosis: oral doxycycline is adequate for cranial neuropathy and meningitis in adults; IV ceftriaxone for parenchymal CNS or spinal cord involvement.
• Lyme arthritis: an initial 28-day oral course; persistent synovitis warrants a second oral course or IV ceftriaxone.
• Antibiotic-refractory arthritis (HLA-DR4-associated, immune-mediated): stop antibiotics and treat with NSAIDs, intra-articular glucocorticoids, DMARDs such as hydroxychloroquine or methotrexate, and arthroscopic synovectomy as definitive salvage — per ACR/IDSA.

Prophylaxis and what to avoid

• Single-dose doxycycline 200 mg within 72 hours of removing an identified engorged Ixodes tick attached ≥36 hours in a highly endemic area.
• Contraindicated: prolonged, repeated, or IV antibiotic courses for post-treatment Lyme symptoms — IDSA explicitly recommends against them. Doxycycline is avoided in pregnancy; short courses are acceptable in children per AAP Red Book.

Emergencies

• Complete (third-degree) AV block: spirochetal and lymphocytic infiltration of the AV node interrupts conduction; signalled by syncope, presyncope, bradycardia, or AV dissociation on ECG. Requires monitoring and temporary pacing — rarely, fatal Lyme carditis occurs in young patients with sudden cardiac death.
• Myopericarditis with acute heart failure: direct myocardial invasion causes ventricular dysfunction; flagged by chest pain, dyspnoea, elevated troponin, and a falling ejection fraction.
• Jarisch–Herxheimer reaction: massive spirochete lysis releases lipoproteins, driving a cytokine surge in the first 24 hours of therapy; fever, rigors, and worsening rash. Supportive care only — it is not an antibiotic allergy and therapy should continue.
• Missed co-infection: Babesia microti (haemolysis, Maltese cross on smear) or Powassan virus (encephalitis) share the Ixodes vector; suspect when fever is high or the patient fails doxycycline.

Disease complications

• Antibiotic-refractory Lyme arthritis: molecular mimicry and persistent synovial immune activation in HLA-DR4 carriers; signalled by continued knee effusion with a negative synovial PCR after full therapy.
• Exposure keratopathy: incomplete lid closure from CN VII palsy; signalled by corneal punctate staining — treat with lubrication and taping.
• Post-treatment Lyme disease syndrome: persistent fatigue, arthralgia, and cognitive complaints for months after documented, adequately treated infection, with no evidence of ongoing infection.
• Acrodermatitis chronica atrophicans: chronic B. afzelii skin infection producing atrophic, cigarette-paper skin in Europe.

Treatment complications

• Doxycycline: photosensitivity and pill oesophagitis — take upright with water and use sun protection.
• Ceftriaxone: biliary sludging/pseudolithiasis, C. difficile colitis, and catheter-associated bloodstream infection with prolonged IV courses — a key reason IDSA opposes extended parenteral therapy.

• Erythema migrans is a clinical diagnosis: in a patient from an endemic area, the single best next step is to start doxycycline, not to order serology. Two-tier testing is frequently negative in the first weeks because IgM has not yet developed — a negative ELISA early does not exclude disease.
• Doxycycline is the answer for most stems: it is first-line for EM, cranial neuropathy, and arthritis, and it uniquely covers Anaplasma co-infection. Amoxicillin or cefuroxime axetil is the substitute in pregnancy.
• Bilateral facial nerve palsy is the classic association examiners test: think Lyme first in an endemic-area stem, with sarcoidosis (Heerfordt syndrome), Guillain–Barré, and HIV as the competing answers.
• Monoarticular knee effusion, large but not exquisitely tender, in a non-toxic patient is Lyme arthritis; septic arthritis is hot, agonising, and immobile. Arthrocentesis still comes first if infection cannot be excluded.
• New AV block plus a summer tick exposure = Lyme carditis. Do not implant a permanent pacemaker — the block resolves with antibiotics; use temporary pacing as a bridge (IDSA/AAN/ACR 2020).
• Transmission needs roughly 36–48 hours of attachment: a promptly removed, non-engorged tick needs no prophylaxis. Single-dose doxycycline 200 mg is reserved for high-risk bites within 72 hours.
• The commonest distractor: prolonged or repeat IV antibiotics for persistent fatigue and arthralgia after adequate therapy. IDSA recommends against this; post-treatment Lyme disease syndrome is managed symptomatically.
• Second distractor: treating a positive serology alone. IgG Western blot bands persist for years after cure, so serology cannot be used as a test of cure or to justify retreatment.