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Difficulty: MediumInfectious Diseases

A 54-year-old man presents to the clinic with a 5-day history of fever, severe fatigue, drenching sweats, and dark urine. He recently returned from a 2-week camping trip in Nantucket, Massachusetts. Two days ago, he was evaluated at an urgent care center where a 6-cm expanding erythematous plaque with central clearing was noted on his right thigh. He was prescribed oral doxycycline for presumed early localized Lyme disease. Despite compliance with the medication, his fever and fatigue have persisted and worsened. His temperature is 38.9°C (102.0°F), blood pressure is 116/74 mm Hg, and pulse is 108/min. Physical examination reveals pale conjunctivae, mild scleral icterus, and a fading erythematous patch on the right thigh. There is no respiratory distress. Laboratory studies show:

Hemoglobin: 9.2 g/dL
Platelets: 88,000/mm³
Leukocytes: 4,500/mm³
Total bilirubin: 3.4 mg/dL
Indirect bilirubin: 2.8 mg/dL
Lactate dehydrogenase: 650 U/L

A peripheral blood smear demonstrates pleomorphic intraerythrocytic ring forms and occasional tetrads. Which of the following is the most appropriate pharmacotherapy for this patient's current condition?

  1. A
    Intravenous ceftriaxone
  2. Atovaquone and azithromycinAnswer
  3. C
    Intravenous clindamycin and oral quinine
  4. D
    Oral chloroquine and primaquine
  5. E
    Oral amoxicillin

Answer

Atovaquone and azithromycin
The patient's presentation of hemolytic anemia (elevated LDH, elevated indirect bilirubin, jaundice) and thrombocytopenia following a tick exposure in an endemic region (Northeastern United States) is classic for babesiosis. The diagnosis is confirmed by the peripheral blood smear demonstrating intraerythrocytic tetrads (often referred to as 'Maltese crosses'). Because the Ixodes scapularis tick transmits both Borrelia burgdorferi and Babesia microti, coinfection is common. The standard first-line treatment for mild-to-moderate babesiosis in a hemodynamically stable patient is a combination of atovaquone and azithromycin.

Step-by-Step Solution

1
Analyze the clinical presentation and geographic history.
The patient has a history of a tick bite in an endemic area (Nantucket) causing an erythema migrans rash (Lyme disease), but now presents with new-onset fever, jaundice, and dark urine despite appropriate Lyme treatment.
Identifying symptoms that do not align with simple early localized Lyme disease suggests a possible tick-borne coinfection.
2
Interpret the laboratory findings and peripheral blood smear.
Labs reveal a hemolytic anemia (low hemoglobin, elevated indirect bilirubin, elevated LDH) and thrombocytopenia. The blood smear shows intraerythrocytic tetrads (Maltese crosses).
These findings are pathognomonic for Babesiosis, caused by the Babesia microti parasite, which is co-transmitted with Borrelia burgdorferi by the Ixodes scapularis tick.
3
Determine the most appropriate pharmacotherapy.
The patient is hemodynamically stable without respiratory distress, indicating mild-to-moderate disease, which is best treated with atovaquone and azithromycin.
While clindamycin and quinine are used for severe babesiosis, their high toxicity profile makes them inappropriate for stable patients.

Key Concept

Diagnosis and management of babesiosis as a tick-borne coinfection with Lyme disease.
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Topics

Infectious Diseases

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