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?
APlasmodium falciparum malaria
BBabesia microti infection
CHuman granulocytic anaplasmosis
DRocky Mountain spotted fever
Reveal answer & full explanation
Correct answer: B — Babesia microti infection
APlasmodium falciparum malaria
BBabesia microti infection✓
CHuman granulocytic anaplasmosis
DRocky Mountain spotted fever
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.
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?
AWell-controlled type 2 diabetes mellitus
BSeasonal allergic rhinitis
CPrior surgical splenectomy
DHistory of iron-deficiency anemia
Reveal answer & full explanation
Correct answer: C — Prior surgical splenectomy
AWell-controlled type 2 diabetes mellitus
BSeasonal allergic rhinitis
CPrior surgical splenectomy✓
DHistory 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
Well-controlled type 2 diabetes mellitus — is not a major driver of severe babesiosis.
Seasonal allergic rhinitis — reflects atopy and has no bearing on parasite clearance.
History of iron-deficiency anemia — is unrelated to the splenic clearance of parasitized red cells and does not predispose to fulminant disease.
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Receipt of red blood cell transfusion from infected donor (no FDA-licensed screen until recently in endemic states)
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 history to endemic region, fever paroxysms; Plasmodium species on blood smear; cross-reaction possible — geography and exposure key
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): clindamycin 300-600 mg IV every 6 hours + quinine 650 mg PO every 6-8 hours for at least 7-10 days
Red blood cell exchange transfusion for parasitemia >10%, severe anemia (Hb <10), hemodynamic instability, end-organ failure
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 (6+ weeks) and have higher relapse rates
Atovaquone + azithromycin can be substituted for severe disease in some patients with less toxicity than clindamycin/quinine
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 outpatient mild disease; clindamycin + quinine is reserved for severe disease (and is poorly tolerated).
References
IDSA 2020 — Clinical Practice Guidelines by the Infectious Diseases Society of America (IDSA), American Academy of Neurology (AAN), and American College of Rheumatology (ACR) for the prevention, diagnosis, and treatment of Lyme disease (Lantos et al., Clin Infect Dis 2021) — includes co-infection with babesiosis
CDC — CDC — Babesiosis: epidemiology, diagnosis, and treatment
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