Babesia parasites in red blood cells on a stained blood smear. (CDC Photo: DPDx)

In symptomatic people, babesiosis usually is diagnosed by examining blood specimens under a microscope and seeing Babesia parasites inside red blood cells.

If babesiosis is being considered, examination of blood smears should be specifically requested. Multiple smears may need to be examined to detect low levels of parasites.

Sometimes it is hard to distinguish Babesia species from Plasmodium falciparum (malaria) by blood-smear examination. Also, some Babesia species (such as B. microti and B. duncani) look identical: they cannot be distinguished from each other by microscopy. To be sure the diagnosis/species is correct, consider having a reference laboratory confirm the diagnosis—through blood-smear examination and, if indicated, by other means (for example, by serologic and molecular methods).