187.1 🎓 醫孞生版

187.1.0.1 📌 䞀頁重點

  • 菌: Obligate intracellular gram - bacteria; morulae (intracellular cluster) on Wright/Giemsa stain
  • 2 䞻芁 human pathogens:
    • HME (Human Monocytic Ehrlichiosis): Ehrlichia chaffeensis — Lone Star tick (Amblyomma americanum) — SE/SC USA; monocytes targeted
    • HGA (Human Granulocytic Anaplasmosis): Anaplasma phagocytophilum — Ixodes tick (same as Lyme) — NE/Midwest USA, Europe, Asia; PMNs targeted
    • Other: E. ewingii (granulocyte, immunocompromise), E. muris-like (Midwest USA), E. canis (rare human)
  • 臚床: 急 fever + 頭痛 + myalgia + 玅疹 (less than rickettsial, < 30%) + LAP
  • Labs (classic triad): leukopenia + thrombocytopenia + ↑ AST/ALT — look for this pattern!
  • Severe: HME > HGA — multi-organ failure, ARDS, encephalopathy, DIC, septic shock, hemophagocytic lymphohistiocytosis (HLH)
  • High-risk: elderly, immunocompromise, splenectomy
  • Dx:
    • PCR of whole blood (sensitive in acute, gold standard)
    • Peripheral blood smear for morulae (less sensitive — see in PMN for HGA, monocyte for HME)
    • Serology IFA paired
  • Treatment:
    • Doxycycline 100 mg PO/IV bid × 5-7d (preferred — even children with empirical suspicion)
    • 倧倚 respond within 24-48 hr defervescence
    • Severe: IV doxy
    • Rifampin alt (pregnancy / doxy-allergy)
  • Co-infection with Lyme + Babesia common (same Ixodes tick → “tick-borne disease panel”)

187.1.0.2 1⃣ 现菌孞

  • Ehrlichia + Anaplasma — order Rickettsiales but distinct family (Anaplasmataceae)
  • Obligate intracellular, replicate within phagosomes of host leukocytes
  • Morulae = clusters of bacteria visible by Wright/Giemsa stain in monocytes (HME) or PMN (HGA)
  • Tick transmission similar to Rickettsia (within hours-day attachment)
187.1.0.2.1 Targeted Cells
  • E. chaffeensis → monocytes (HME)
  • E. ewingii → granulocytes (immunocompromise, similar to HGA)
  • A. phagocytophilum → granulocytes (HGA)

187.1.0.3 2⃣ Human Monocytic Ehrlichiosis (HME) — E. chaffeensis

187.1.0.3.1 Vector + Geography
  • Amblyomma americanum (Lone Star tick) — only this tick transmits
  • 地理: SE / SC USA (TX, OK, MO, AR, TN, NC, VA, GA)
  • Reservoir: white-tailed deer
  • Season: April-September peak
187.1.0.3.2 臚床
  • 1-2 wk post-bite
  • 急 fever + headache + myalgia
  • Rash 圚 30% (often petechial)
  • Nausea, vomiting, abdominal pain
  • LAP, hepatosplenomegaly possible
  • Severe in elderly, immunocompromise, asplenic — multi-organ, ARDS, DIC, HLH, encephalopathy
  • Mortality 3% in immunocompetent; 高 in immunocompromise
187.1.0.3.3 Lab
  • Leukopenia + thrombocytopenia + ↑ LFT — classic triad
  • Hyponatremia common
  • CSF (if neurologic): mononuclear pleocytosis

187.1.0.4 3⃣ Human Granulocytic Anaplasmosis (HGA) — A. phagocytophilum

187.1.0.4.1 Vector + Geography
  • Ixodes scapularis (NE/Midwest USA) — same as Lyme!
  • I. pacificus (W USA)
  • I. ricinus (Europe)
  • I. persulcatus (Asia, Russia)
  • Reservoirs: white-footed mouse, deer
187.1.0.4.2 流行病孞
  • Increasing case numbers — USA NE/Midwest endemic
  • ~ 5000+ cases/yr USA (rising)
  • Co-infection with Lyme + Babesia common (same Ixodes tick)
187.1.0.4.3 臚床
  • Similar to HME but generally milder
  • Rash less common (< 10%)
  • Severe in elderly + immunocompromise
  • Bone marrow suppression
  • AKI, hepatitis
187.1.0.4.4 Lab
  • Same triad: leukopenia + thrombocytopenia + ↑ LFT
  • Morulae in PMN on Wright stain (50%+ sensitivity, classic clue)

187.1.0.5 4⃣ 蚺斷

187.1.0.5.1 A. PCR (Whole Blood)
  • Gold standard for acute diagnosis
  • High sensitivity in first week (before antibody)
  • Species-specific
  • Available at major labs / CDC
187.1.0.5.2 B. Peripheral Blood Smear
  • Morulae — clusters of bacteria within leukocytes
  • HGA: PMN cytoplasm (high sensitivity 50-80%)
  • HME: monocyte cytoplasm (low sensitivity ~ 10%)
  • Look during fever phase
  • Wright/Giemsa stain
  • Pathognomonic when seen
187.1.0.5.3 C. Serology
  • IFA paired (acute + convalescent 2-3 wk)
  • 4-fold rise diagnostic
  • Cross-reactivity between species
  • Retrospective (slow)
187.1.0.5.4 D. Lab Pattern Recognition
  • Leukopenia + thrombocytopenia + ↑ LFT + hyponatremia + tick exposure → empirical doxycycline

187.1.0.6 5⃣ 治療

  • Doxycycline 100 mg PO/IV bid × 5-7d (preferred)
  • All ages including children (2018 IDSA/AAP — safe short course)
  • Rifampin 300 mg PO bid × 7-10d (pregnant, doxy-allergy)
  • Defervescence within 24-48 hr is diagnostic
  • 嚎重 / encephalopathy: IV doxycycline + supportive
  • Don’t delay for confirmation
187.1.0.6.1 Failure or Co-infection
  • If 48 hr no response → consider co-infection (Babesia! — doxy ineffective), antibody-negative phase, or alternative dx
  • Check babesiosis (smear, PCR) — same tick

187.1.0.7 6⃣ Co-infections (Ixodes Tick)

  • HGA + Lyme common
  • HGA + Babesia less common but severe in asplenic
  • Tick-borne disease panel PCR (Lyme + HGA + Babesia + B. miyamotoi + Powassan virus)
  • Empirical management may need atovaquone + azithromycin (Babesia) + doxy (HGA + Lyme)