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Difficulty: MediumInfectious Diseases

A 35-year-old man presents to the emergency department in mid-July with a 4-day history of high fever, severe headache, myalgias, and profound fatigue. He recently returned from a week-long camping trip in the Ozark Mountains of Arkansas. He does not recall any specific insect bites. His temperature is 39.1°C (102.4°F), pulse is 112/min, blood pressure is 118/76 mmHg, and respirations are 18/min. Physical examination reveals a non-toxic appearing man with no skin rashes, nuchal rigidity, or palpable lymphadenopathy. Laboratory studies reveal:

Leukocyte count: 2,400/mm³
Platelet count: 85,000/mm³
Aspartate aminotransferase (AST): 135 U/L
Alanine aminotransferase (ALT): 142 U/L

A peripheral blood smear demonstrates basophilic, intracytoplasmic inclusions within monocytes. Which of the following is the most appropriate pharmacotherapy for this patient?

  1. Oral doxycyclineAnswer
  2. B
    Intravenous ceftriaxone
  3. C
    Oral amoxicillin
  4. D
    Oral atovaquone and azithromycin
  5. E
    Intravenous vancomycin and piperacillin-tazobactam

Answer

Oral doxycycline
The correct answer is oral doxycycline. This patient's clinical and geographic presentation (summer camping in Arkansas, high fever, myalgias), combined with characteristic laboratory findings (leukopenia, thrombocytopenia, elevated AST/ALT) and pathognomonic peripheral blood smear findings (intracytoplasmic morulae in monocytes), confirms the diagnosis of Human Monocytic Ehrlichiosis (HME). HME is caused by the obligate intracellular bacterium Ehrlichia chaffeensis, transmitted primarily by the Lone Star tick (Amblyomma americanum). Doxycycline is the specific, first-line therapy for HME across all age groups, as it effectively penetrates cells and inhibits bacterial protein synthesis.

Step-by-Step Solution

1
Analyze the patient's epidemiologic and clinical presentation.
The patient has an acute febrile illness, severe headache, and myalgias after a summer camping trip in Arkansas.
The geographic location (Ozarks) and summer season heavily favor exposure to the Lone Star tick (Amblyomma americanum).
2
Evaluate the laboratory findings.
Labs show leukopenia, thrombocytopenia, and elevated transaminases.
These hematological and hepatic findings are classic for Ehrlichiosis and Anaplasmosis, helping to distinguish them from typical bacterial sepsis.
3
Interpret the peripheral blood smear.
The smear shows basophilic, intracytoplasmic inclusions (morulae) within monocytes.
This specific morphological finding confirms the diagnosis of Human Monocytic Ehrlichiosis (HME) caused by Ehrlichia chaffeensis.
4
Select the appropriate pharmacotherapy.
Identify oral doxycycline as the treatment of choice.
Doxycycline effectively penetrates host cells to eradicate obligate intracellular bacteria. Beta-lactam antibiotics are inherently ineffective.

Key Concept

Human monocytic ehrlichiosis (HME) presents with high fever, myalgias, leukopenia, thrombocytopenia, and elevated transaminases. The presence of intracytoplasmic morulae in monocytes is diagnostic, and doxycycline is the definitive, targeted treatment.
Estimated Time:1m 0s
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Infectious Diseases

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