Answer: Babesia spp. As I noted previously, this patient was from Missouri, and was found to have the newly described MO-1 strain. Notice the multiply infected RBCs and atypical forms which are characteristic for infection with Babesia spp. Several viewers suggested Plasmodium falciparum as an alternative. This is definitely in the differential diagnosis, since they can both demonstrate high parasitemia, multiply infected RBCs, and small delicate rings on peripheral blood smear. Also, intermediate stages (e.g. schizonts) are almost never seen in P. falciparum infection, and never seen in Babesia infection. This overlap in morphologic features can create quite a challenge in distinguishing the 2 infections.
However, babesiosis can be distinguished from malaria (P. falciparum infection) by the following features:
1. Babesia has frequent extra-cellular forms.
2. Size variability of ring-forms is often marked.
3. Rings are often atypical, and include spindle, oval, round, and double rings forms. I've also noticed a high frequency of "tennis racket-shaped" rings, where a "handle" is present on the ring.
4. Finally, the rarely seen tetrad, or maltese cross, is diagnostic of babesiosis. This was also seen in this case, but I had excluded this image originally so I wouldn't give the answer away immediately!
Despite the newly describes Babesia variants, the most commonly reported species in the United States is stillBabesia microti.