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Difficulty: HardInfectious Diseases

A 38-year-old man comes to the emergency department due to persistent high fever, drenching sweats, and dark-colored urine. Five days ago, he was evaluated at an urgent care clinic for an expanding, target-shaped erythematous rash on his right calf and a mild headache. The symptoms began a week after a hiking trip in Rhode Island. He was diagnosed with early localized Lyme disease and prescribed oral doxycycline.

Despite adherence to the medication, the rash has faded but his fevers have worsened. Over the past 48 hours, he developed profound fatigue and noticed his eyes appearing yellow. Vital signs show a temperature of 39.4°C (102.9°F), blood pressure of 112/70 mmHg, and heart rate of 115/min. Physical examination is notable for mild scleral icterus. The abdomen is soft, with a spleen tip palpable below the left costal margin.

Laboratory evaluation shows:
- Hemoglobin: 8.6 g/dL
- Platelet count: 88,000/mm³
- Total bilirubin: 3.5 mg/dL
- Direct bilirubin: 0.5 mg/dL
- Lactate dehydrogenase: 720 U/L

A peripheral blood smear reveals intraerythrocytic pleomorphic ring forms. Which of the following is the most appropriate modification to this patient's antimicrobial regimen?

  1. A
    Discontinue doxycycline and initiate intravenous ceftriaxone
  2. B
    Discontinue doxycycline and initiate intravenous piperacillin-tazobactam and vancomycin
  3. Continue doxycycline and add atovaquone and azithromycinAnswer
  4. D
    Continue doxycycline and add oral amoxicillin-clavulanate
  5. E
    Discontinue doxycycline and initiate intravenous meropenem

Answer

Continue doxycycline and add atovaquone and azithromycin
The correct answer involves continuing the current Lyme disease treatment and adding targeted therapy for babesiosis. This patient's presentation of fever, hemolytic anemia (elevated indirect bilirubin and LDH), thrombocytopenia, and intraerythrocytic ring forms is diagnostic of babesiosis. *Babesia microti* is a parasite transmitted by the *Ixodes scapularis* tick, which is also the vector for *Borrelia burgdorferi* (Lyme disease) and *Anaplasma phagocytophilum* (anaplasmosis). Coinfection is common in endemic areas like the Northeastern United States. While the patient's early localized Lyme disease is being appropriately treated with doxycycline, this antibiotic has no efficacy against *Babesia*. Therefore, a specific anti-parasitic regimen of atovaquone and azithromycin must be added to treat the babesiosis.

Step-by-Step Solution

1
Identify the primary presentation
The patient has clinical signs of hemolytic anemia (jaundice, high indirect bilirubin, high LDH, low hemoglobin) and a peripheral smear showing intraerythrocytic ring forms.
To establish the specific cause of the new and worsening systemic symptoms.
2
Relate findings to the epidemiological context
The patient has a recent history of early localized Lyme disease following a tick bite in Rhode Island, an endemic area for the Ixodes scapularis tick.
To evaluate for potential tick-borne coinfections.
3
Determine the appropriate targeted therapy
The findings are diagnostic of Babesia microti coinfection. Doxycycline covers Lyme disease but not Babesia; therefore, targeted anti-parasitic therapy with atovaquone and azithromycin must be added.
To select the correct, narrow-spectrum treatment indicated by the diagnostic data, avoiding inappropriate broad-spectrum antibiotics.

Key Concept

Patients with tick-borne illnesses from endemic regions can present with coinfections (e.g., Lyme disease and babesiosis). Babesiosis requires targeted therapy with atovaquone and azithromycin, as the standard treatments for Lyme disease (such as doxycycline) are ineffective against Babesia.
Estimated Time:1m 30s
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Topics

Infectious Diseases

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