Answer: Babesia sp.
Interestingly, this case was identified as Babesia divergens/MO-1 strain by PCR testing. The currently unnamed MO-1 strain (found in Missouri) is closely related to B. divergens, a pathogen of cattle in Europe. Since the description of the MO-1 strain, other B. divergens-like organisms have been detected in Kentucky and Washington states.
The morphology of the B. divergens and B. divergens-like organisms is slightly different than that of B. microti, in that the organisms are usually oval or elliptical and tetrad-forms are more commonly seen. The clinical picture is also slightly different in that patients are generally asplenic and have severe to fatal infections.
Although it would be difficult to differentiate B. divergens-like parasites from B. microti based on morphology alone, the presence of predominantly small elliptical forms with prominent tetrads should raise the suspicion for the former parasite.