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Difficulty: EasyInfectious Diseases

A 3535-year-old man comes to the clinic with a rash on his right thigh. He first noticed it 33 days ago, and it has been expanding. He returned from a camping trip in Massachusetts one week ago. He feels well otherwise and denies fever, joint pain, or headaches. His temperature is 37.2∘C37.2^\circ\text{C} (99.0∘F99.0^\circ\text{F}), blood pressure is 120/80120/80 mmHg, and pulse is 7676/min. Physical examination shows a 1010-cm erythematous annular plaque with central clearing on the right thigh. Which of the following is the most appropriate next step in management?

  1. A
    Order Lyme disease serology
  2. B
    Prescribe intravenous ceftriaxone
  3. Prescribe oral doxycyclineAnswer
  4. D
    Prescribe oral cephalexin
  5. E
    Prescribe topical clotrimazole

Answer

The most appropriate next step is to prescribe oral doxycycline.
The patient's presentation of an expanding annular plaque with central clearing after camping in Massachusetts is highly characteristic of erythema migrans, the hallmark of early localized Lyme disease. The correct management is to immediately begin a 1010- to 1414-day course of oral doxycycline. In early localized disease, the diagnosis is purely clinical. Serologic testing is not recommended because the immune response takes weeks to mount, meaning tests are frequently falsely negative at this stage and waiting for them will inappropriately delay targeted empiric treatment.

Step-by-Step Solution

1
Analyze the patient's clinical presentation and exposure history.
The patient has an expanding annular rash with central clearing (erythema migrans) after camping in a Lyme-endemic area (Massachusetts).
Identifying the classic 'bull's-eye' rash establishes a clinical diagnosis of early localized Lyme disease.
2
Determine the required diagnostic evaluation.
No laboratory testing is needed because the presence of erythema migrans in an endemic area is diagnostic.
Lyme serology is often negative in the early stages and should not delay treatment.
3
Select the appropriate pharmacotherapy.
Oral doxycycline is the first-line treatment for early localized Lyme disease in non-pregnant adults.
Doxycycline effectively treats Borrelia burgdorferi and also covers potential co-infections like Anaplasma.

Key Concept

Early localized Lyme disease is a clinical diagnosis based on the presence of erythema migrans and should be treated immediately with empiric oral antibiotics, most commonly doxycycline.
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Topics

Infectious Diseases

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