Wednesday, June 1, 2011

Answer to Case 163

Answer: Plasmodium vivax

Everyone who wrote in recognized these parasites correctly. As mentioned by Kanoot, there are multiple amoeboid trophozoites of P. vivax shown and a single gametocyte in image #2. Neuro_Nurse correctly mentions that failure to comply fully with malaria prophylaxis is one of the most common reasons for malaria in travelers. Although we don't know for sure, it is likely that this patient did not strictly adhere to the prescribed regimen.

After recognizing that a malaria parasite is present, it is then important to speciate the organisms and provide an estimation of percent parasitemia.
I use the following approach to determine the infecting species:

First, I look at the size of the infected red blood cell, in comparison to the uninfected neighboring cells. If the infected cells are consistently enlarged, then I narrow my identification down to either P. vivax or P. ovale. The presence of stippling (a.k.a. Schuffner's dots) as seen here helps to confirm this differential. I then look for characteristic features of the 2 species, such as amoeboid forms of P. vivax or a predominance of ovale-shaped infected red blood cells with fimbriated (jagged) edges that are associated with P. ovale infection. In this case, the enlarged amoeboid forms that fill most of the cytoplasm, and the irregular shaped infected red blood cells that seem to hug the contours of the neighboring cells allow for the diagnosis of P. vivax infection.

Back to our Patient:
Neuro_Nurse correctly states that the treating clinician should consider radical treatment with primaquine after ruling out G6PD deficiency in this patient. Primaquine is typically given in combination with other antimalarial treatment, since it has the most effect on the hypnozoite stages that remain dormant in the liver. This will prevent subsequent relapse due to activation of the hypnozoite forms. It is important to check for the inherited condition, G6PD (glucose-6-phosphate dehydrogenase) deficiency, before giving the patient primaquine, since it can lead to a serious, and even fatal, hemolytic anemia.

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