Babesiosis¶

Chapter 232 | Part 5: Infectious Diseases

KEY CLINICAL POINTS¶

Babesiosis is an emerging infectious disease caused by Babesia species, primarily transmitted via Ixodes ticks or blood transfusions.
Babesia microti is the most common causative agent in the U.S., with seasonal peaks in summer and transmission via Ixodes scapularis ticks.
Severe disease is more common in immunocompromised patients, asplenia, and splenic rupture, requiring atovaquone + azithromycin or exchange transfusion.
Diagnosis involves Giemsa-stained blood smears, PCR, and serology, with differential consideration for malaria and other tick-borne pathogens.
Prophylaxis for travelers includes atovaquone-proguanil, mefloquine, or doxycycline, with caution for G6PD deficiency and QT prolongation.

1. DEFINITION & OVERVIEW¶

Babesiosis is an emerging infectious disease caused by protozoan parasites of the genus Babesia that invade and lyse red blood cells (RBCs). Most cases in the U.S. are caused by Babesia microti, transmitted by Ixodes scapularis ticks. It presents as a flu-like illness, with severe complications in immunocompromised patients or asplenia.

Table 232-1: Treatment of Human Babesiosis¶

ADULTS	CHILDREN
Mild to Moderate B. microti Infectiona	Atovaquone (750 mg q12h PO) plus Azithromycin (500 mg/d PO on day 1 followed by 250 mg/d PO on subsequent days)
Severe B. microti Infectionb,c	Preferredd: Atovaquone (750 mg q12h PO) plus Azithromycin (500 mg qd IV followed by 250–500 mg qd PO). Alternativee,f: Clindamycin (600 mg q6h IV followed by 600 mg q8h PO) plus Quinine (650 mg q6–8h PO)
B. divergens Infectiong	Immediate complete exchange transfusion plus Clindamycin (600 mg q6–8h IV) plus Quinine (650 mg q8h PO)

1.1 Pathogenesis¶

Babesia parasites invade RBCs, leading to hemolysis, anemia, and systemic inflammation. Splenic complications (infarction, rupture) occur due to RBC lysis and oxidative stress. Immune response involves CD4+ T cells and B cells, with antibody roles uncertain.

1.2 Clinical Spectrum¶

Mild to moderate illness (fever, fatigue, hemolytic anemia) vs. severe disease (acute renal failure, respiratory failure, hemophagocytic lymphohistiocytosis). Splenic rupture and sepsis are life-threatening complications.

2. EPIDEMIOLOGY¶

Babesiosis is increasingly reported in the U.S. and globally. Incidence rose due to tick expansion and increased human exposure. Most cases (95%) are caused by B. microti in the Northeast and Midwest. TTB (transfusion-transmitted) cases are common in blood donations, with B. microti detected in RBC units for up to 42 days.

2.1 Risk Factors¶

Age ≥ 50 years, asplenia (congenital, functional, or acquired), immunosuppression (HIV, organ transplant, autoimmune disorders), and tick exposure. Splenectomy increases risk of severe disease.

2.2 Geographic Distribution¶

B. microti endemic in the U.S. Northeast and Midwest; B. divergens in Europe (France, Ireland), B. duncani in the U.S. West, and B. venatorum in Europe and China. Global spread is anticipated.

3. ETIOLOGY & PATHOPHYSIOLOGY¶

Babesia species (B. microti, B. divergens, B. duncani, B. venatorum) are transmitted via ticks or blood transfusions. Pathogenesis involves RBC invasion, hemolysis, oxidative stress, and immune-mediated complications. Splenic infarction and rupture occur due to RBC lysis and vascular damage.

3.1 Transmission¶

Tick bites (Ixodes scapularis, I. ricinus, D. albipictus) or blood transfusions. B. microti persists in RBC units for up to 42 days, causing transfusion-transmitted babesiosis.

3.2 Molecular Mechanisms¶

Parasites target RBC mitochondria (atovaquone), inhibit heme synthesis (quinine), and disrupt protein synthesis (clindamycin). Resistance linked to mutations in RPL4 and 23S rRNA gene.

4. CLINICAL FEATURES¶

Symptoms include fever, fatigue, chills, myalgia, and hemolytic anemia. Severe cases present with acute renal failure, respiratory distress, splenic rupture, or hemophagocytic lymphohistiocytosis. Asymptomatic infections are common in immunocompetent individuals.

4.1 Mild vs. Severe Disease¶

Mild: fever, fatigue, hemolysis. Severe: renal failure, respiratory failure, splenic rupture, or hemophagocytic syndrome. Duration of symptoms correlates with parasitemia levels.

4.2 Complications¶

Acute renal failure (20%), respiratory failure (7%), splenic rupture, hemophagocytic lymphohistiocytosis, and relapsing infections in immunocompromised patients.

5. DIFFERENTIAL DIAGNOSIS¶

Differential includes malaria, Lyme disease, anaplasmosis, and other tick-borne infections. Key distinguishing features: absence of gametocytes/schizonts in babesiosis vs. malaria; co-occurrence with Lyme disease (erythema migrans rash).

5.1 Tick-Borne Co-Infections¶

Babesiosis may co-occur with Lyme disease (B. burgdorferi), anaplasmosis (A. phagocytophilum), or B. miyamotoi. Symptoms may overlap, requiring PCR or serology for differentiation.

6. INVESTIGATIONS & DIAGNOSIS¶

Diagnosis via Giemsa-stained blood smears, PCR for Babesia DNA, and serology. TTB is detected in blood donations using 18S rRNA amplification. Differential diagnosis includes malaria and other hemolytic anemias.

6.1 Diagnostic Criteria¶

Microscopic identification of Babesia parasites in RBCs, PCR confirmation, or serology (IgM ≥ 1:20, IgG ≥ 1:64 for B. microti). TTB screening uses Procleix Babesia assay.

6.2 Laboratory Findings¶

Anemia, elevated lactate dehydrogenase, low haptoglobin, elevated bilirubin, and renal dysfunction. PCR detects parasites at 1–10 parasites/ µ L of blood.

7. MANAGEMENT & TREATMENT¶

First-line therapy: atovaquone + azithromycin (7–10 days). Severe cases require exchange transfusion, clindamycin + quinine, or RBC exchange. Prophylaxis for travelers includes atovaquone-proguan, mefloquine, or doxycycline.

7.1 Antimicrobial Therapy¶

Atovaquone + azithromycin (preferred), clindamycin + quinine (alternative), or exchange transfusion for severe cases. Duration extended for immunocompromised patients.

7.2 Adjunctive Therapies¶

RBC exchange for high parasitemia (>10%), supportive care for renal failure, and monitoring for hemophagocytic syndrome. Tafenoquine may substitute for drugs targeting specific genes.

8. PROGNOSIS & COMPLICATIONS¶

Mortality <1% with prompt treatment, but 20% in immunocompromised patients. Complications include renal failure, respiratory failure, splenic rupture, and relapsing infections. Long-term anemia may occur due to autoantibodies.

9. SPECIAL CONSIDERATIONS¶

Pregnancy: Avoid primaquine; use atovaquone + azithromycin. Pediatric: Avoid doxycycline; use atovaquone-proguanil. Immunocompromised: Prolonged therapy and monitoring for relapse. TTB prevention via blood donation screening.

9.1 Pregnancy¶

Avoid primaquine due to G6PD deficiency risk. Use atovaquone + azithromycin for treatment. Monitor for HELLP syndrome in pregnant patients.

9.2 Blood Transfusion Safety¶

Screen donations for B. microti using 18S rRNA PCR. Asymptomatic carriers may transmit infection via RBC units.

10. KEY POINTS & CLINICAL PEARLS¶

Babesiosis is a tick-borne and transfusion-transmitted disease, most commonly caused by B. microti in the U.S.
Severe disease is more common in immunocompromised patients, asplenia, or splenic rupture.
Atovaquone + azithromycin is first-line therapy; exchange transfusion for high parasitemia.
Prophylaxis for travelers includes atovaquone-proguanil, mefloquine, or doxycycline.
TTB prevention relies on blood donation screening and avoiding tick exposure.