Tick-borne intraerythrocytic protozoal infection (Babesia microti in US); causes hemolytic anemia, fever, and severe disease in asplenic or immunocompromised hosts.
Also known as: Babesia microti, Babesia divergens, babesiosis
Overview
Tick-borne zoonotic infection caused by intraerythrocytic protozoa of the genus Babesia, most commonly Babesia microti in the United States and Babesia divergens in Europe. Clinical spectrum ranges from asymptomatic parasitemia to severe hemolytic anemia and multiorgan failure.
Epidemiology
Endemic to the northeastern United States (especially southern New England, New York, New Jersey) and upper Midwest (Wisconsin, Minnesota). Transmitted by the Ixodes scapularis (black-legged) tick, the same vector as Lyme disease and anaplasmosis. Co-infections with B. burgdorferi and Anaplasma phagocytophilum are common. Also transmitted by blood transfusion and rarely by congenital transmission.
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Question 1Infectious DiseaseMedium
A 68-year-old man from coastal Massachusetts presents in July with a week of fevers, drenching sweats, fatigue, and dark urine. He spends time gardening but recalls no rash. Labs show hemoglobin 9.0 g/dL, elevated LDH, low haptoglobin, and thrombocytopenia. A Giemsa-stained thin blood smear shows intraerythrocytic ring forms and rare tetrads arranged in a 'Maltese cross,' without pigment. Which of the following is the most likely diagnosis?
ARocky Mountain spotted fever
BHuman granulocytic anaplasmosis
CPlasmodium falciparum malaria
DBorrelia burgdorferi infection
EBabesia microti infection
Reveal answer & full explanation
Correct answer: E — Babesia microti infection
ARocky Mountain spotted fever
BHuman granulocytic anaplasmosis
CPlasmodium falciparum malaria
DBorrelia burgdorferi infection
EBabesia microti infection✓
Why Babesia microti infection is correct
Babesiosis is a tick-borne infection by the intraerythrocytic protozoan Babesia microti, transmitted by the same Ixodes scapularis tick that spreads Lyme disease in the coastal Northeast and upper Midwest.
It causes a febrile hemolytic anemia (elevated LDH, low haptoglobin, hemoglobinuria) with thrombocytopenia.
The smear shows intraerythrocytic ring forms and the pathognomonic tetrad ('Maltese cross') and — unlike malaria — produces NO hemozoin pigment.
Why the others are wrong
Plasmodium falciparum malaria — requires travel to an endemic region, shows banana-shaped gametocytes and pigment, and lacks the Maltese-cross tetrad.
Human granulocytic anaplasmosis — shares the Ixodes vector but shows morulae within neutrophils, not intraerythrocytic parasites.
Rocky Mountain spotted fever — presents with a petechial rash spreading from the wrists and ankles, not intraerythrocytic ring forms.
Borrelia burgdorferi infection — the Lyme spirochete shares the Ixodes vector and coastal Northeast summer exposure, and erythema migrans can go unnoticed, but it does not infect red cells or cause hemolytic anemia with intraerythrocytic ring forms and tetrads; coinfection with Babesia does occur.
Question 2Infectious DiseaseMedium
An infectious-disease consultant is asked to identify which patient is at greatest risk of severe, fulminant babesiosis with high-grade parasitemia. Which of the following host factors is the strongest risk factor for severe disease?
AHereditary hemochromatosis
BPrior elective cholecystectomy
CPrior surgical splenectomy
DLong-standing hypertension
EHistory of iron-deficiency anemia
Reveal answer & full explanation
Correct answer: C — Prior surgical splenectomy
AHereditary hemochromatosis
BPrior elective cholecystectomy
CPrior surgical splenectomy✓
DLong-standing hypertension
EHistory of iron-deficiency anemia
Why Prior surgical splenectomy is correct
The spleen is the principal organ that clears Babesia-infected erythrocytes from the circulation, so asplenia — most often from prior splenectomy — is the strongest risk factor for severe, high-parasitemia babesiosis, along with older age and immunosuppression.
Asplenic patients can develop overwhelming hemolysis, ARDS, and disseminated intravascular coagulation.
Why the others are wrong
Long-standing hypertension — is a chronic vascular comorbidity with no effect on clearance of parasitized erythrocytes, and it is not among the recognized predictors of severe babesiosis (asplenia, age 50 or older, immunocompromise).
Prior elective cholecystectomy — removes the gallbladder while leaving splenic function fully intact, so it carries none of the clearance defect that drives fulminant parasitemia.
History of iron-deficiency anemia — is unrelated to the splenic clearance of parasitized red cells and does not predispose to fulminant disease.
Hereditary hemochromatosis — iron overload is a classic risk factor for severe infection with siderophilic bacteria such as Vibrio vulnificus and Yersinia, which makes it tempting, but hemochromatosis leaves splenic clearance of parasitized erythrocytes intact and is not among the recognized predictors of severe babesiosis.
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Receipt of red blood cell transfusion from an infected donor (FDA-recommended Babesia NAT donor screening in endemic states since 2019 has reduced but not eliminated risk)
Co-infection with Lyme disease or anaplasmosis
Pathophysiology
Babesia sporozoites injected by tick saliva invade erythrocytes, where they replicate and lyse cells in a manner analogous to malaria. Hemolysis, hemoglobinuria, and microvascular sludging produce anemia and end-organ ischemia. Splenic clearance is critical; asplenic patients are at greatest risk for fulminant disease.
Clinical presentation
Symptoms
Gradual onset of fever, chills, sweats, fatigue, myalgia, headache 1-4 weeks after tick exposure
Anorexia, nausea, abdominal pain, dark urine (hemolysis)
Dyspnea and chest discomfort
Asplenic or immunocompromised patients: rapidly progressive severe disease with high parasitemia and multiorgan failure
Asymptomatic parasitemia in many immunocompetent hosts
Signs / physical exam
Fever, tachycardia
Mild splenomegaly or hepatomegaly
Jaundice and scleral icterus in significant hemolysis
Petechiae uncommon (in contrast to viral hemorrhagic fevers)
Rash is NOT typical — distinguishing feature from erythema migrans of Lyme
Classic findings
Patient from coastal Massachusetts or eastern Long Island with summer tick exposure, fever, hemolytic anemia, and intraerythrocytic ring forms or pathognomonic Maltese cross tetrads on Giemsa-stained thin blood smear.
Differential diagnosis
Malaria — Travel to a malaria-endemic region; ring forms can mimic Babesia, but Plasmodium produces hemozoin pigment, gametocytes, and schizonts, whereas Babesia shows none of these and may show Maltese-cross tetrads and extracellular merozoites
Lyme disease — Erythema migrans rash, arthritis, neurologic features; serology; co-infection common with babesiosis
Anaplasmosis / ehrlichiosis — Fever, leukopenia, thrombocytopenia, elevated LFTs in same vector region; PCR or morulae in granulocytes
Sepsis with DIC — Source identification, blood cultures, coagulation profile
TTP/HUS — Microangiopathic hemolysis with schistocytes, neurologic or renal signs; ADAMTS13
Diagnostic workup
Diagnostic criteria
Definitive: visualization of Babesia parasites on blood smear or positive PCR. Serology supports diagnosis but does not establish active infection alone.
Labs
Giemsa- or Wright-stained thin blood smear — intraerythrocytic ring forms; Maltese cross (4 merozoites in tetrad) is pathognomonic but uncommon
Babesia microti PCR (more sensitive at low parasitemia)
Babesia serology (IgM and IgG; supports rather than establishes diagnosis)
BMP and LFTs (renal failure and transaminase elevation in severe disease)
Parasitemia level (% parasitized RBCs) is important for severity classification and treatment monitoring
Concurrent testing for Lyme and anaplasmosis given shared vector
Imaging
Generally not required for diagnosis
Chest imaging if dyspnea or hypoxia for ARDS or pulmonary edema
Abdominal imaging if splenic infarct or rupture suspected
Treatment
First-line
Mild to moderate disease: atovaquone 750 mg PO BID + azithromycin 500 mg PO on day 1 then 250 mg daily for 7-10 days
Severe disease (parasitemia >=4%, hemodynamic instability, end-organ dysfunction, asplenia, immunocompromise): atovaquone 750 mg PO BID + azithromycin 500 mg IV daily (preferred per IDSA 2020), switching to the oral regimen once improved to complete at least 7-10 days; clindamycin 600 mg IV every 6 hours + quinine 650 mg PO every 8 hours is the alternative for nonresponse or intolerance
Red blood cell exchange transfusion for parasitemia >10%, severe hemolytic anemia, or severe pulmonary, renal, or hepatic compromise (IDSA 2020: consider with expert consultation)
Supportive care: transfusion, hemodialysis if needed, ICU monitoring
Concurrent treatment for Lyme disease if co-infection identified
Second-line / adjunct
Highly immunocompromised hosts (especially post-rituximab B-cell depletion) may require prolonged combination therapy (at least 6 weeks, including 2 final weeks with no parasites on smear) and have higher relapse rates
Clindamycin + quinine is the alternative regimen for patients who fail or cannot take atovaquone + azithromycin; it is more toxic (quinine causes cinchonism: tinnitus, vertigo, GI upset)
Reporting to state and local health department; transfusion-transmitted cases must be reported
Complications
Severe hemolytic anemia and DIC
ARDS, congestive heart failure
Acute kidney injury
Splenic infarct or rupture
Persistent or relapsing infection in immunocompromised hosts
Coma and death in fulminant disease
Transfusion-transmitted infection in blood-bank settings
PANCE pearls
Maltese cross tetrads on a thin blood smear are pathognomonic for Babesia but seen in only a minority of cases; PCR is more sensitive.
No rash — helps distinguish from Lyme disease (erythema migrans) and from spotted-fever rickettsioses.
Asplenic patients tolerate babesiosis poorly and are candidates for early aggressive therapy and exchange transfusion.
Co-infection with Lyme disease and/or anaplasmosis is common — test for all three in patients with tick-borne febrile illness in endemic regions.
Atovaquone + azithromycin is preferred for both mild outpatient disease (all oral) and severe hospitalized disease (IV azithromycin); clindamycin + quinine is the poorly tolerated alternative for failure or intolerance.
References
IDSA 2020 — Krause PJ, Auwaerter PG, Bannuru RR, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America (IDSA): 2020 Guideline on Diagnosis and Management of Babesiosis. Clin Infect Dis. 2021;72(2):e49-e64. doi:10.1093/cid/ciaa1216
CDC — CDC — Babesiosis: epidemiology, diagnosis, and treatment
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