Cytometry in malaria: Moving beyond Giemsa

COMMENTARY The article by Bhakdi et al. in this issue of Cytometry Part A on optimizing flow cytometric detection of mouse malaria parasites (1) is, somewhat remarkably, one of only a few dozen publications in the literature (2–54) in which cytometry is applied to the diagnosis, treatment, or biology of this protean disease. The World Health Organization estimates that 350– 500 million cases of malaria occur annually, causing at least a million deaths. Of the 270–400 million cases of the severest form of the disease, due to the parasite Plasmodium falciparum, about 70% of these cases are in Africa and about 20% in southeast Asia (55). The diagnosis of malaria is primarily cell-based and involves visual detection of intraerythrocytic parasites by transmitted light microscopy in a peripheral blood smear stained with Giemsa’s stain, a mixture of eosin and methylene azure dyes first described over a century ago (56). Identification of the various stages of parasites depends heavily on morphologic information, requiring observation at high power. Although it has been known for many years that methods based on fluorescence microscopy, using acridine orange (57– 59) and other dyes (60–62), compare in accuracy with light microscopy (63) and may require less time and a less skilled observer, the required fluorescent microscope has, until recently, been too expensive for most laboratories in areas where malaria is most prevalent. If malaria were more common in affluent countries, we might expect that cytometry would, by now, have supplanted microscopy of Giemsastained smears for malaria diagnosis, just as it has for differential leukocyte counting and reticulocyte counting.