Answer to Case 461

Answer: Plasmodium vivax infection

This case has several features that are consistent with P. vivax/P. ovale infection; specifically, the size of the infected red blood cells (RBCs) are slightly larger than the neighboring uninfected RBCs, and the timing indicates a relapsed infection which is only seen with these 2 species. We also know that both species are found in Uganda where this patient was from.

Differentiating between P. vivax and P. ovale can then be done by looking at a number of features. The CDC DPDx website has a nice table that compares these features (see the Laboratory Diagnosis panel). In this case, there are some features that are consistent with P. ovale and P. vivax:
1. Some cells have an oval shape suggestive of P. ovale.
2. One field shows an infected cell with a jagged edge suggestive of fimbriations. However, this could just reflect crenations from processing (seen in some of the neighboring RBCs).

3. Many mature schizonts with up to 24 merozoites are seen, consistent with P. vivax. Given this final feature, we make a final identification of P. vivax infection.

In this case, PCR was also positive for P. vivax and did not detect P. ovale, P. falciparum, and P. malariae, thus confirming our morphologic identification.