Infectious Disease · Borrelia burgdorferi
The facts most likely to be tested
The hallmark clinical manifestation of early localized Lyme disease is erythema migrans, a bullseye or target-shaped rash that expands over several days.
Diagnosis of Lyme disease is confirmed via a two-tiered testing approach using ELISA followed by Western blot for confirmation.
Early disseminated Lyme disease frequently presents with bilateral facial nerve (CN VII) palsy, AV heart block, or meningitis.
Late-stage Lyme disease is characterized by recurrent, migratory monoarticular arthritis, most commonly affecting the large joints such as the knee.
The primary vector for *Borrelia burgdorferi* transmission is the Ixodes scapularis tick, which typically requires >48 hours of attachment for transmission.
First-line treatment for early localized Lyme disease in adults and children is doxycycline.
Treatment for Lyme disease with carditis or neurologic involvement (excluding isolated facial palsy) requires intravenous ceftriaxone.
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A 34-year-old hiker presents to the clinic with a 3-day history of fatigue and a rash on his left thigh. He reports spending significant time in the woods of Connecticut two weeks ago. Physical examination reveals a 12-cm erythematous annular patch with central clearing and a surrounding red border. He has no fever, joint pain, or neurological deficits. The patient is otherwise healthy and has no known drug allergies.
What is the most appropriate first-line treatment for this patient?
Oral doxycycline
The patient presents with the classic erythema migrans rash of early localized Lyme disease, which is treated with oral doxycycline as per the first-line recommendation.
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Etiology / Epidemiology
Caused by Borrelia burgdorferi, transmitted by Ixodes scapularis tick. High risk in Northeast/Mid-Atlantic US.
Clinical Manifestations
Early localized stage presents with erythema migrans. Bilateral facial nerve palsy is a classic finding in disseminated disease.
Diagnosis
Two-tiered testing: ELISA followed by Western Blot for confirmation. Clinical diagnosis for early stage.
Treatment
Doxycycline is the first-line treatment. Avoid in pregnancy/children (use Amoxicillin).
Prognosis
Most recover fully with prompt treatment. Lyme carditis and late-stage arthritis are rare complications.
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Epidemiology & Etiology
Transmitted by the Ixodes tick, which requires 36-48 hours of attachment for transmission. Endemic in the Northeast, Mid-Atlantic, and North-Central United States. Peak incidence occurs in summer months due to outdoor activity.
Pertinent Anatomy
The spirochete disseminates hematogenously to the skin, heart, nervous system, and joints. Involvement of the cranial nerves, particularly the facial nerve (CN VII), is a hallmark of early disseminated disease.
Pathophysiology
The spirochete Borrelia burgdorferi invades tissues, triggering a robust inflammatory response. The immune system produces antibodies that are the basis for serologic testing. Untreated infection leads to immune-mediated damage in the synovium and myocardium.
Clinical Manifestations
Early localized disease features erythema migrans, a bulls-eye rash. Early disseminated disease presents with bilateral facial nerve palsy, heart block (AV block), and meningitis. Late disease manifests as monoarticular arthritis, typically in the knee.
Diagnosis
Diagnosis is clinical in early stages with erythema migrans. For later stages, use two-tiered testing: initial ELISA followed by Western Blot (IgM/IgG). A positive Western Blot confirms the diagnosis.
Treatment
Doxycycline is the treatment of choice for adults. Contraindicated in pregnancy and children <8 years, where Amoxicillin is the preferred alternative. For severe cardiac or neurologic involvement, IV Ceftriaxone is required.
Prognosis
Early treatment prevents late-stage arthritis and chronic neurologic sequelae. Lyme carditis usually resolves with appropriate antibiotic therapy, though temporary pacemaker placement may be required for high-grade blocks.
Differential Diagnosis
Southern Tick-Associated Rash Illness (STARI): Similar rash but milder course
Cellulitis: Lacks the central clearing of erythema migrans
Bell's Palsy: Usually unilateral; Lyme should be suspected if bilateral
Rheumatoid Arthritis: Typically symmetric and polyarticular
Fibromyalgia: Lacks objective inflammatory markers or rash