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:

Case of the Week 595

This week's case was generously donated by Dr. Marc Couturier and Blaine Mathison. The patient is a young adult male from the South Central United States. No travel history is available. Review of his peripheral blood film reviewed the following:

Identification? What are the structures seen in the last 2 images?

Answer to Case 603

Answer to Case of the Week 603: Borrelia sp. spirochetes. The history is most consistent with a relapsing fever Borrelia sp., and subsequent PCR showed this to be Borrelia hermsii. 

As Florida Fan mentioned, this case is a great reminder that when we examine a Giemsa-stained blood film, we may find things other than parasites. In addition to spirochetal bacteria such as in this case, we can find intracellular clusters (morulae) of Anaplasma phagocytophilum and Ehrlichica spp. bacteria, and intracellular years (e.g. Histoplasma capsulatum, Talaromyces marneffei). Leishmania spp. amastigotes can also rarely be seen within phagocytic cells. I've shown Borrelia spirochetes on this blog before (see Case of the Week 502), but never on a thick film, so I thought this would be a nice case to include here.

Note that the spirochetes are uniformly slender with broad undulations. This is different than the microgametes of an exflaggelated Plasmodium sp. microgametocyte which I previously showed on Parasite Case of the Week 595. In that case, I created a composite image of 'squiggly things' seen on a blood smear which may be helpful in identifying and differentiating them:

Some readers asked if the Borrelia species that cause Lyme disease could also be seen on blood film. The answer to this is NO - with one small caveat. The main cause of Lyme in the United States and Canada is Borrelia burgdorferi, whereas in Europe, you can also see B. garinii and B. afzelii. These organisms have extremely low levels of spirochetemia, and therefore spirochetes are NOT seen on peripheral blood films. The only rare exception to this is Borrelia mayonii, an uncommon cause of Lyme disease restricted to the upper Midwestern United States. This organism is actually found in much higher concentrations in the blood, and occasionally a rare spirochete can be seen on peripheral blood smears. We published on this several years ago; you can read the article HERE. (Let me know if you would like a copy).

There were many other good comments posted on this blog case - both in the Comments section and on Twitter - so I would encourage you to read them to learn more about the experiences from our readers.