A 45-year-old man presents with a 2-week history of fever, night sweats, and fatigue. He underwent splenectomy 10 years ago for trauma. Blood film shows ring-form intraerythrocytic parasites, some tetrads ('Maltese cross' or Babesia cross), and occasional extracellular merozoites without banana-shaped gametocytes or hemozoin pigment. What is the MOST likely causative organism?
- A Plasmodium falciparum (malaria)
- B Plasmodium vivax (malaria)
- C Babesia microti transmitted by Ixodes tick ✓
- D Trypanosoma brucei (sleeping sickness)
Explanation
Babesiosis (Babesia microti in the northeastern US) is transmitted by Ixodes scapularis ticks and causes intraerythrocytic infection. The diagnostic features distinguishing it from Plasmodium are: tetrad forms ('Maltese cross' or 'clover-leaf' tetrad unique to Babesia), absence of hemozoin pigment, presence of extracellular merozoites, and no banana-shaped gametocytes. Asplenic patients are at high risk for severe, potentially fatal disease. P. falciparum shows ring forms and banana-shaped gametocytes with hemozoin. P. vivax shows enlarged erythrocytes with Schüffner's dots. Trypanosomes are extracellular flagellated protozoa found in blood and lymph, not intraerythrocytic ring forms.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.