Answer: Plasmodium falciparum infection
Lots of great discussion on this case! This was the highest percent parasitemia I have seen in a non-fatal case of malaria. It was calculated at 24% in our lab. Fortunately this rapidly dropped with IV artesunate therapy and the patient is now doing well.
The key diagnostic features of this case were:
1. Heavy parasitemia, involving all stages of RBCs (note that some reticulocytes were infected). Plasmodium vivax and P. ovale only infect reticulocytes, whereas P. malariae is restricted to older RBCs; for these reasons, infections with these species will not reach the high level of parasitemia seen in this case.
2. The predominant form seen is delicate rings with 1-2 chromatin dots. Usually only rings (early trophozoites) and, less commonly, crescent-shaped gametocytes are seen with P. falciparum infection. However, this was a special case in which schizonts containing 8-24 merozoites were also seen. This is consistent with a very heavy infection in which the later stage forms that are usually sequestered in the peripheral capillaries have spilled out into the peripheral blood.
Thick blood film:
The main differential diagnosis, as pointed out by Lee, is babesiosis, which also consists primarily of delicate ring forms and can present with very high parasite loads. However, babesiosis can be excluded by the presence of schizonts and malaria pigment (hemozoin).
Finally, as a bonus, I saw a few of the following structures which I think might be microgametes. This is a stage usually only seen in the mosquito, but can rarely be seen when microgametocytes undergo a process called exflagellation and release microgametes (male gametes). See if you agree with me!