Babesia

Babesia microti (main US species) infects red blood cells, causing intraerythrocytic parasitism visible on Giemsa/Wright-stained thin blood smears as ring forms resembling Plasmasodium falciparum, but classically forms the pathognomonic 'Maltese cross' tetrad (four merozoites budding within an RBC), which distinguishes it from malaria. Transmission occurs via Ixodes scapularis tick bite (same vector as Lyme disease), blood transfusion, or transplacentally. Clinical presentation ranges from asymptomatic to febrile hemolytic anemia (fever, chills, fatigue, hemolysis with jaundice, dark urine, hepatosplenomegaly), particularly severe in asplenic, elderly, or immunocompromised patients—can be life-threatening in these groups. Diagnosis is via peripheral blood smear (ring forms, Maltese cross), PCR, or serology. Treatment is atovaquone plus azithromycin (mild-moderate) or clindamycin plus quinine (severe disease); exchange transfusion may be needed in severe parasitemia. Key USMLE associations: co-infection with Lyme disease/Borrelia burgdorferi and Anaplasma phagocytophilum (same Ixodes tick vector), splenectomized patients at risk for fulminant disease, and the classic teaching point of distinguishing Babesia (Maltese cross, no exoerythrocytic liver stage) from Plasmodium (has liver stage, banana-shaped gametocytes in P. falciparum).