Walsh & Hoyt: Diagnosis

Although babesiosis is a rare disease, it should be considered in any splenectomized or immunocompromised patient who develops a severe febrile illness. Babesiosis is usually diagnosed by microscopic examination of Giemsa -or Wright-stained blood smears. Most of the trophozoites seen in the blood are within erythrocytes, although organisms may be seen outside erythrocytes in patients with severe infection. The round or ring-shaped appearance of the trophozoite stage of the organisms usually resembles that of P. falciparum; however, several special features help distinguish babesiosis from malaria. First, larger rings may contain a central white vacuole not seen in patients with malaria. Second, Babesia species lack the synchronous stages -shizonts as well as gametocytes seen in patients with malaria. Finally, a rare diagnostic feature of babesiosis is tetrads of merozoites within erythrocytes.