228.1 🎓 醫孞生版

228.1.0.1 📌 䞀頁重點

  • Pathogens:
    • Babesia microti — USA NE/Midwest predominant
    • Babesia divergens — Europe (especially asplenic; very severe)
    • Babesia duncani — USA West Coast (rare)
    • Babesia venatorum — Asia + Europe (rare)
  • Vector: Ixodes scapularis (same as Lyme, USA); I. ricinus (Europe)
  • Reservoir: White-footed mouse + deer
  • Transmission:
    • Tick bite (most common)
    • Blood transfusion — transfusion-transmitted babesiosis (major USA concern in NE; some states screen)
    • Vertical
    • Organ transplant
  • Risk Groups for Severe:
    • Asplenic / functional asplenia (sickle cell)
    • Immunocompromise (HIV, B-cell depletion like rituximab, malignancy, post-transplant)
    • Elderly
    • Pregnancy
  • Clinical:
    • Most asymptomatic / mild in immunocompetent
    • Symptomatic: fever + malaise + chills + myalgia + headache + sweats + thrombocytopenia + hemolytic anemia
    • Severe: hemolysis + AKI + multi-organ failure + DIC + ARDS + sepsis-like
    • Asplenic mortality 20-50% without treatment
  • Diagnosis:
    • Thick + thin blood smears (Giemsa) — intra-erythrocytic ring forms; “Maltese cross” tetrads pathognomonic (rare)
    • PCR of blood (sensitive)
    • Serology (IgG) — confirmatory; cross-reacts with malaria
    • Don’t confuse with malaria — clinical + geographical context essential
  • Treatment:
    • Mild-Moderate: Atovaquone 750 mg PO bid + Azithromycin 500 mg PO day 1 → 250 qd × 7-10 days
    • Severe: Clindamycin 600 mg IV q6h + Quinine 650 mg PO q8h × 7-10 days + supportive
    • Refractory severe: + exchange transfusion (especially asplenic, parasitemia > 10%)
    • Immunocompromise: longer course (6+ wk) + monitor relapse
  • Co-Infection (Ixodes tick): Lyme + Anaplasma + Babesia + B. miyamotoi common combinations

228.1.0.2 1⃣ Babesia Microbiology + Life Cycle

228.1.0.2.1 Taxonomy
  • Protozoan parasite
  • Apicomplexa phylum (like Plasmodium)
  • Babesia genus; many species
  • Babesia microti human-relevant USA NE/Midwest
  • B. divergens human-relevant Europe (also cattle)
  • B. duncani USA W Coast (rare; “Washington-1”)
  • B. venatorum Asia + Europe (rare)
  • Cattle / rodent reservoirs
228.1.0.2.2 Life Cycle
  1. Tick takes blood meal from infected rodent
  2. Tick attaches to human → sporozoites injected
  3. Sporozoites invade RBCs directly (no liver stage, unlike Plasmodium)
  4. Asexual reproduction in RBCs → merozoites
  5. RBC rupture + new RBC infection cycles
  6. Some become gametocytes → ingested by next tick
228.1.0.2.3 Differs from Plasmodium
  • No exoerythrocytic (liver) stage — direct RBC entry
  • No hypnozoites
  • “Maltese cross” tetrads (4 merozoites in 1 RBC) pathognomonic but rare
  • Single rings can mimic Plasmodium

228.1.0.3 2⃣ Epidemiology

228.1.0.3.1 USA
  • NE + Midwest (especially MA, CT, RI, NY, NJ, PA, WI, MN)
  • 2,500+ reported cases/yr (likely underreported)
  • Ixodes scapularis vector (same as Lyme)
  • Co-circulation with Borrelia burgdorferi, Anaplasma, Powassan virus
228.1.0.3.2 Europe
  • B. divergens — primarily Britain + France + Scandinavia
  • B. microti reported (less common)
  • Cattle reservoir
  • Severe in asplenic
228.1.0.3.3 Asia
  • B. venatorum (Japan, China, Russia)
  • B. crassa (Eastern Europe + Asia)
  • Sporadic cases
228.1.0.3.4 Transfusion-Transmitted Babesiosis
  • Major USA concern in NE blood supply
  • Asymptomatic Babesia-infected donors
  • 2024: FDA approved screening of blood donations in 14 NE states + several MidAtlantic
  • Transmission can occur to recipients in non-endemic areas
228.1.0.3.5 Seasonal
  • Spring + summer + fall (tick season)

228.1.0.4 3⃣ Clinical

228.1.0.4.1 Asymptomatic / Mild
  • Most immunocompetent
  • Mild flu-like
  • Self-limited 1-2 wk
228.1.0.4.2 Symptomatic (Moderate)
  • Onset 1-4 wk post-tick bite
  • Fever (intermittent or sustained)
  • Chills, sweats
  • Malaise + fatigue
  • Myalgia
  • Headache
  • Sometimes mild jaundice (hemolysis)
  • Splenomegaly (40%)
  • Hepatomegaly
  • Lab: thrombocytopenia + mild anemia + elevated LDH + ↑ AST/ALT + ↓ haptoglobin
228.1.0.4.3 Severe Disease
228.1.0.4.3.1 Risk Groups
  • Asplenic / functional asplenia (sickle cell disease, splenectomy)
  • Immunocompromise (HIV CD4 < 200, hematologic malignancy, anti-CD20 like rituximab, transplant, chemo, anti-TNF)
  • Elderly (≥ 65)
  • Pregnancy
228.1.0.4.3.2 Clinical
  • High fever
  • Severe hemolytic anemia (severe Hb drop, hemoglobinuria)
  • Acute kidney injury (ATN from hemoglobinuria + sepsis)
  • ARDS / pulmonary edema
  • DIC
  • Splenic rupture (rare)
  • Cardiac (HF, arrhythmia)
  • CNS (rarely encephalopathy)
  • Multi-organ failure
228.1.0.4.3.3 Mortality
  • Asplenic: 20-50% if untreated
  • Immunocompromise: variable
  • General hospitalization mortality ~ 5%
228.1.0.4.4 Special: Asplenic Patients
  • Loss of spleen → no clearance of intra-erythrocytic parasites
  • Parasitemia rises rapidly
  • Massive hemolysis + multi-organ failure
  • Exchange transfusion may be lifesaving

228.1.0.5 4⃣ Diagnosis

228.1.0.5.1 Blood Smears (Giemsa)
  • Thick + thin smears
  • Look for intra-erythrocytic ring forms
  • “Maltese cross” tetrads (4 merozoites in 1 RBC) — pathognomonic but rare
  • Differential from malaria:
    • Babesia rings may have multiple in single RBC
    • No pigment (vs Plasmodium hemozoin)
    • No gametocytes
    • Geographic + clinical context
228.1.0.5.2 PCR
  • Most sensitive method
  • Species-specific (B. microti vs B. divergens vs B. duncani)
  • Useful for screening + low parasitemia
228.1.0.5.3 Serology (IgG)
  • Confirmatory
  • Cross-reacts with malaria + other species
  • Indirect Fluorescent Antibody (IFA)
  • Available specialty labs
228.1.0.5.4 Lab
  • Thrombocytopenia
  • Hemolytic anemia (↑ LDH, ↑ unconjugated bilirubin, ↓ haptoglobin)
  • Mild ↑ AST/ALT
  • ↑ Creatinine in AKI
  • DIC parameters in severe
228.1.0.5.5 Co-Infection Testing
  • Lyme + Anaplasma + Babesia + B. miyamotoi panel (same Ixodes tick)
  • Multiplex PCR

228.1.0.6 5⃣ Treatment

228.1.0.6.1 Mild-Moderate Disease
  • Atovaquone 750 mg PO bid + Azithromycin 500 mg PO day 1 → 250 mg PO daily × 7-10 days
  • Well-tolerated
  • Standard for outpatient + immunocompetent
228.1.0.6.2 Severe Disease
  • Clindamycin 600 mg IV q6h + Quinine 650 mg PO q8h × 7-10 days
  • More potent but higher side effect profile
  • ICU
  • Supportive care
228.1.0.6.3 Refractory Severe / Asplenic / Parasitemia > 10%
  • Add exchange transfusion
  • Replace patient blood with donor blood
  • Removes parasitized RBCs + reduces parasitemia rapidly
  • Especially helpful in asplenic + high parasitemia
228.1.0.6.4 Immunocompromise
  • Longer course (6+ weeks) — relapse common
  • Combination therapy
  • Monitor PCR for recovery
  • ID consultation
228.1.0.6.5 Pregnancy
  • Atovaquone + azithromycin
  • Quinine + clindamycin alternative
  • Avoid in 3rd trimester due quinine side effects in fetus
228.1.0.6.6 Supportive Care
  • Fluid management
  • Blood transfusion for severe anemia
  • Dialysis for AKI
  • Mechanical ventilation
  • Vasopressors

228.1.0.7 6⃣ Prevention

228.1.0.7.1 Tick Avoidance
  • Same as Lyme + other tick-borne diseases (Ch 185)
  • Permethrin-treated clothing
  • DEET
  • Tick checks (multiple times daily during outdoor activities)
  • Remove ticks promptly (within 24 hr)
228.1.0.7.2 Blood Donor Screening
  • USA NE + MidAtlantic: routine donor screening for Babesia
  • 2024 FDA approved expansion
  • Reduces transfusion-transmitted babesiosis
228.1.0.7.3 Asplenic + Immunocompromise
  • Avoid endemic areas during tick season if possible
  • Strict tick avoidance
  • Awareness of post-tick illness
228.1.0.7.4 No Vaccine
  • Babesia vaccine in early research (animal model)
  • Cattle vaccines exist (for veterinary use)