Borrelia

Borrelia are gram-negative, microaerophilic spirochetes with a unique segmented genome (linear chromosome plus linear/circular plasmids) that drive extensive antigenic variation of surface proteins (VlsE), allowing immune evasion and relapsing infections. Lyme disease (B. burgdorferi, B. mayonii, B. afzelii, B. garinii) is transmitted by Ixodes scapularis/pacificus ticks, with reservoir in white-footed mice and deer as the tick's host. Clinical stages: Stage 1 (early localized) - erythema migrans (target/bull's-eye rash) at bite site, often with flu-like symptoms; Stage 2 (early disseminated, weeks-months) - secondary annular skin lesions, migratory arthralgias, cardiac involvement (AV block), neurologic findings (facial nerve palsy, meningitis); Stage 3 (late, months-years) - chronic monoarthritis (especially knee) and encephalopathy. Diagnosis uses two-tier serology (ELISA screening followed by Western blot confirmation); clinical diagnosis is sufficient if classic erythema migrans is present. Treatment: doxycycline for early disease (drug of choice in most adults and children); amoxicillin or cefuroxime as alternatives (e.g., pregnancy, young children); IV ceftriaxone for neurologic or cardiac involvement. Prevention includes tick avoidance, prompt removal, and prophylactic doxycycline after high-risk tick bites. Relapsing fever Borrelia (B. recurrentis - epidemic, louse-borne; B. hermsii - endemic, soft tick-borne) cause recurring febrile episodes due to antigenic variation of surface lipoproteins, evading antibody response cycle after cycle. Jarisch-Herxheimer reaction (fever, chills, worsening symptoms) can occur after antibiotic treatment due to rapid spirochete lysis and cytokine release, seen in both Lyme and relapsing fever treatment.