I can tell from the responses (both on the web and off) that this case was far too easy for my U.S. friends, and a bit challenging for my U.K. friends. That's to be expected given that cases of Babesiosis are rare Great Britian, but common in parts of the the U.S.
Babesiosis must always be kept in the differential diagnosis with this microscopic picture and lack of travel to malaria-endemic areas. It is caused by over 100 species, but is mainly due to infection with B. microti and B. divergens.
In this case, your main differential diagnosis would be P. falciparum malaria caused by so-called "airport" malaria (malaria transmitted by mosquitoes that have stowed away on a plane from an endemic area). This is usually seen in patients who live near an international airport.
Babesia spp. and P. falciparum share the following features:
1. They have small, similar appearing ring forms
2. Multiple parasites per red blood cell are common
The main features that allow you differentiate Babesia spp. from P. falciparum are:
1. Babesia ring forms are irregular
2. No schizonts or gametocytes are present with Babesia spp.
3. Extracellular merozoites are commonly seen in Babesia infection (none shown in this case)
4. Babesia merozoites classically form a tetrad or "maltese cross", although it is actually rare to see this. Here is an image of how this would look (click on image to enlarge):
Shown with permission from Dr. Zandecki, Faculte de Medecine d'Angers