Answer to Case 595

Answer to Parasite Case of the Week 595: Plasmodium vivax with exflagellated microgametocytes. Free microgametes are seen.

The following excellent description was written by Marc Couturier and Blaine Mathison, the contributors of this case.

There are several features of this slide that are supportive for P. vivax.

First, many of the infected cells are reticulocytes (which are larger than mature RBCs), which P. vivax and P. ovale have predilection to infect. Second, the gametocytes are enlarged and fill almost the entire space within the parasitized RBC. Some of the infected RBCs also take the shape of neighbouring RBCs (distorted, pleomorphic); particularly in the areas where cells are dense. Third, the mature schizonts contained upwards of 18-20 merozoites with yellowish brown pigment present, which strongly points toward P. vivax, since the range of P. ovale tends to reach 14 merozoites at maximum. Fourth, some developing trophozoites were grossly amoeboid. Plasmodium ovale usually demonstrates some amoeboid characteristics, but it is usually more exaggerated with P. vivax. Notice also none of the infected RBCs were fimbriated (which, when present supports P. ovale but the absence of which does not rule-out P. ovale).

In the images provided, there was an absence of Schüffner’s stippling. There were some parasitized cells that did display stippling (not shown), but the majority did not. It is important to remember that stippling can be helpful to support an identification, but the absence of stippling does not rule-out the species that usually present with stippling (P. vivax and P. ovale).  Stippling is a pH dependent feature and Giemsa stained slides are the best to display this due to the optimized pH (Wright and Wright-Giemsa are suboptimal for this). 

Overall the RBC morphology was quite good, suggesting that the blood was processed soon after collection. However, we suspected that there was a delay in slide processing which likely led to exflagellation of microgametocytes, with free microgametes seen among the RBCs: 

Exflagellation of microgametocytes in blood specimens post-collection can occur due to multiple factors including decreased temperature, decreased CO2, increased O2 and subsequent increased pH. Free microgametes can be confused with other blood parasites such as Trypanosoma or relapsing fever Borrelia spp. (RFB). The key differences between these organisms are as follows: RFB are thin, faint staining, and in thin films will typically maintain a sinusoidal amplitude. The staining is uniform throughout the length of the bacteria and the width of the spirochetes is also consistent along the length of the organism. As can be seen with the free microgametes, there is irregular amplitude, and chromatin visible within the structure (causing heterogenous thickness).  Trypanosoma spp. on the other hand are irregularly shaped compared to RFB; however, they should be identified based on the presence of a characteristic large nucleus in the central part of the organism, a posteriorly located kinetoplast, a flagellum, and an undulating membrane (not always visible).  The long, slender nature of the microgametes might also be suggestive of microfilariae; however, microfilariae would never be this small. Here is an image of these structures shown side-by-side for comparison: