Morphology: Spirochete — long, slender, motile (axial flagella in periplasmic space). Microaerophilic. Does not gram-stain well; visualized with darkfield or silver stain. Cultured in Barbour-Stoenner-Kelly (BSK) medium but rarely done clinically.
Typical drugs
- #1Doxycycline— **100 mg PO BID × 10 d** (early local) or × 14 d (early disseminated). Single-dose 200 mg may prevent Lyme after a high-risk Ixodes scapularis bite (deer-tick exposure ≥36 h in endemic region).
- #2Amoxicillin— **500 mg PO TID** alternative — preferred in pregnancy + children <8 y. Equivalent efficacy to doxy for early Lyme.
- #3Ceftriaxone— **2 g IV daily × 14–28 d** for neuroborreliosis (meningitis, encephalitis, radiculopathy), high-degree AV block, or severe carditis.
Empiric therapy when resistant
No clinically meaningful resistance to first-line agents. Persistent symptoms after appropriate antibiotic course ("post-treatment Lyme disease syndrome") are NOT due to persistent infection in most cases — repeat antibiotic courses do not help (IDSA 2020 strong recommendation against).
Resistance notes
No documented resistance to doxycycline, amoxicillin, or ceftriaxone. Macrolides (azithro) less effective and reserved for true PCN + tetracycline allergy.
Common syndromes
Pearls
Erythema migrans (EM) — pathognomonic when present; >5 cm expanding annular rash 3–30 days after tick bite. NO labs needed; treat clinically. Carditis = AV block (often high-degree but transient); doesn't require permanent pacing. Lyme arthritis is late manifestation (months later); responds to PO doxy/amox × 28 d. Post-treatment Lyme disease syndrome (PTLDS): chronic subjective symptoms after adequate tx; NOT chronic infection; supportive care only. Don't co-infect-screen reflexively unless severe / refractory disease — Babesia + Anaplasma + Ehrlichia + Powassan all share Ixodes vector. Prophylaxis for high-risk Ixodes scapularis bite (>36 h attached, endemic region): doxycycline 200 mg PO × 1.