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Babesiosis (Babesia microti ) is the most frequently reported tick-borne pathogen transmitted by blood transmission in humans.At present, infection is best diagnosed morphologically, using Wright-or Wright-Giemsa-stained peripheral blood smears to evaluate these intraerythrocytic organisms. Four morphologic features are valuable in distinguishing Babesia from another intraerythrocytic protozoa (Plasmodium spp.), the cause of malaria. Infection of single RBCs by multiple Babesia organisms (polyparasitism, Panel A) is one finding; in malaria, by comparison, it is rare to observe more than two parasites per cell. Pleomorphism of Babesia parasites is another feature (Panel B; electron micrograph). The contorted shapes are possibly valuable to the organism in that they increase the amount of cell membrane and therefore feeding surface area. By comparison, malaria trophozoites are typically monomorphic. The presence of a white vacuole within an organism may also be diagnostic (Panel C). The vacuole represents residual membrane from the attachment of the parasite to host cytoplasm following erythrocyte invasion (Panel D; electron micrograph). Such vacuoles are not typically observed in Plasmodium trophozoites. Finally, Babesia replicate by budding, and as many as four nucleated daughter merozoites may remain attached to each other following division (Panel E). This grouped structure is known as a tetrad or Maltese cross, and its presence is pathognomonic of Babesia infection. However, tetrads are not universal to all Babesia species and are relatively scarce in B. microti infections compared to B. gibsoni (WA-1).

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