A 32-year-old man comes to the office due to an acute febrile illness that developed after a deer-hunting trip in Arkansas. The patient recalls getting tick bites in the woods but initially had no symptoms. He began experiencing fever, chills, malaise, headache, and myalgia a week after his return. Physical examination shows a diffuse, maculopapular rash; clear oropharynx; normal lung and heart sounds; and mild, diffuse abdominal tenderness. Laboratory testing reveals leukopenia, thrombocytopenia, and elevated aminotransferase levels. Microscopic evaluation of the blood demonstrates mulberry-shaped, intracytoplasmic inclusions in monocytes. Which of the following is the most likely pathogen responsible for this patient's condition?
Ehrlichia chaffeensis is harbored by white-tailed deer in the southwestern, southeastern, and mid-Atlantic United States. It is transmitted to humans by tick bite (eg, lone star tick) and subsequently spreads to tissue rich in mononuclear cells (eg, bone marrow, lymph nodes, liver, spleen), leading to nonspecific symptoms (eg, fever, chills, myalgia, headache), maculopapular rash, and significant laboratory abnormalities (eg, lymphopenia, thrombocytopenia, elevated aminotransferases).
E chaffeensis replicates in membrane-bound vacuoles within the cytoplasm of monocytes and can be visualized on peripheral blood or buffy coat examination as mulberry-shaped, intraleukocytic inclusions (morulae). Treatment with antibiotics (eg, doxycycline) is usually curative.
(Choice A) Babesia is a protozoa transmitted by ticks in the northeastern and midwestern United States. It appears as a ring-shaped, intraerythrocytic parasite (similar to Plasmodium).
(Choice B) Borrelia burgdorferi causes Lyme disease and is transmitted by ticks (eg, Ixodes scapularis). Initial infection usually causes nonspecific symptoms (eg, fatigue, headache, myalgia) and a spreading, annular rash called erythema migrans. However, B burgdorferi is not typically visualized on microscopy and does not replicate in membrane-bound inclusions.
(Choice D) Francisella tularensis is an aerobic, gram-negative bacterium that is transmitted when humans come into contact with infected animals (eg, rabbits). It causes tularemia, which is typically characterized by tender, regional lymphadenopathy and a single, papulo-ulcerative lesion with eschar. This pathogen does not grow within membrane-bound vacuoles within leukocytes.
(Choice E) Rickettsia rickettsii is a tick-borne, gram-negative, obligate, intracellular bacterium that attacks vascular endothelial cells and causes Rocky Mountain spotted fever. Patients typically develop nonspecific symptoms (eg, fever, headache, myalgia) followed by a blanching, erythematous, centripetal rash that begins on the ankles and wrists.
Educational objective:
Ehrlichia chaffeensis is harbored in white-tailed deer and transmitted to humans by tick bite. It replicates in vacuoles within monocytes and forms mulberry-shaped, intraleukocytic inclusions (morulae). Manifestations usually include nonspecific symptoms (eg, fever, chills, myalgia), maculopapular rash, and significant laboratory abnormalities, particularly lymphopenia.