Sunday, February 24, 2013

Answer to Case 247

Answer:  Infection with Plasmodium vivax

There was a lot of great discussion on this case!  The definitive features for identification as some readers pointed out were:

1.  Enlarged infected RBCs (compared to uninfected cells)
2.  Stippling seen in all forms (a.k.a. Schuffner's dots)
3.  "Ameboid" shaped of some trophozoites. 

The presence of stippling and enlarged size of the RBCs automatically puts this into the P. vivax/P. ovale category, and the next step is to differentiate between the 2 species.  This can be done by considering the following features:

P. vivax late stage trophozoites have an ameboid appearance (vs. P. ovale late stage trophs which are more compact).  This can be seen in the following image:

(Note that the infected RBCs are enlarged and have fine stippling)

Also, the infected RBCs have a tendancy to 'hug' or mold to the neighboring RBCs.  Although this is a 'soft' diagnostic feature, it supports the diagnosis of P. vivax infection rather than P. ovale infection.

P. vivax cell 'hugging' 2 neighboring cells

Finally, some of you mentioned that the presence of a multiply-infected RBC indicates that this is a P. falciparum infection.  In fact, it is not always appreciated that P. vivax (and even other species) can also have multiply-infected cells.  Notice however, that in this case (as shown below), the infected RBC is enlarged, there is fine stippling throughout, and the rings are relatively large and thick; all three of these features would argue against P. falciparum and point instead to P. vivax or P. ovale infection. 

Here is an image from a previous case of P. falciparum for comparison, demonstrating the normal sized RBCs and thin, delicate, small ring forms (less than 1/3 of the size of the infected RBC).

And now for our excellent poem from Blaine Mathison:

While staring in a film for Bollywood
Bobbi got bit by a skeeter! That’s no good!
This vivax may relapse
in a few years perhaps
Better start gnawing on a piece of chinchona wood

(I'm Bobbi by the way, although I don't know about the Bollywood part!)

No comments: