Congratulations to all of my viewers who got this correct.
Babesiosis is a tick-borne disease, found in North America, Europe, and Asia. It is a mimic of infection with Plasmodium falciparum (which was the 2nd most common answer that viewers gave), since the infected RBCs are not enlarged, the rings are small (1/3 the size of the RBC or smaller), and multiple infections per cell are common. Fortunately, several features allow for differentiation of Babesia spp. from P. falciparum.
First, note that the size of the organisms within RBCs is variable, and that some forms are extremely small (<1/5 the diameter of the RBC). This is typical for babesiosis, and not for P. falciparum.
Second, Babesia spp. tend to produce atypical forms, with multiple chromatin dots, 'double' rings, and chromatin 'tails' such as seen here:
Babesia rings may also be seen outside of the RBC, in the extracellular space.
If you're really lucky, you will see the classic tetrad formation or "Maltese cross" formed when this organism undergoes binary fission. Although this is the image that is commonly shown in textbooks, it is much rarer in real life.
Finally, the travel/exposure history of the patient is very helpful. It wasn't initially available in this case, but we were later told that the patient had no travel history outside of the U.S. He lived in Connecticut which is endemic for Babesia microti.
This was a challenging case, so thanks to all who were brave enough to write in!
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