The smear was prepared from a 59-year old male who presented fever, chills, headache, sweats, fatigue, nausea and vomiting in a cyclic manner having returned from aboard for a week or so. His blood profile was: WBC 7.0 x 10⁹/L, RBC 4.57 x 10¹²/L, Hb 13.9 g/dL, Hct 0.409 L/L, MCV 89.6 fL and Plt 12 x 10⁹/L. The most remarkable feature of the smear is the presence of intra-cellular red cell inclusions, malaria parasites, in the red cell population. The malaria parasites are almost purely ring forms (trophozoites), possessing a delicate bluish cytoplasm with one to two small reddish chromatin dots (Figure 1). Some red cells are multiply infected with 2-3 ring forms (Figure 2). A few schizonts are also found. There is no red cell enlargement. The white cell differential is predominated by hypo-segmented neutrophils. Some display Pelger-Huët anomaly (Figures 3 - 4). Marked thromobocytopenia is also evident.

The first diagnosis is Plasmodium falciparum infection. Staining of the blood film in pH 7.2 may help reach a definite classification of the parasites. A few HKIMLSQAP participants reported a mixed infection with Plasmodium malariae. This is probably due to the presence of a few schizonts with a low merozoite count. The schizont of Plasomodium falciparum only fills up two-third of an RBC, while that of Plasmodium malaraie occupies the whole RBC. The numbers of merozoites in a mature Plasmodium falciparum are 8-24 while in a mature Plasmodium malaraie are 6-14. The numbers of merozoites are even less in an early schizont. Although rare, schizonts of Plasmodium falciparum can also be seen in peripheral blood when the infection is severe.

Pelger-Huët anomaly is a benign but dominantly inherited defect of terminal neutrophil differentiation, secondary to mutations in the lamin B receptor (LBR) gene. The characteristic features of the blood picture include leukocytes with dumbbell-shaped bi-lobulated nuclei (Figures 3 - 4); a reduced number of nuclear segments; and a coarse clumping of the nuclear chromatin in neutrophils, lymphocytes, and monocytes. Acquired or pseudo Pelger-Huët anomaly has been associated with blood disorders such as anemia, leukemia, myelodysplastic syndromes, drug therapy with colchicines or ibuprofen, infections such as mononucleosis and malaria. It is therefore important to distinguish the congenital benign anomaly from the acquired form.

Figure 1.  Ring-formed Plasmodium falciparum parasites possess a delicate bluish cytoplasm with one to two small reddish chromatin dots (1,000x magnification).

Figure 2.  Red cell infection with two ring-formed Plasmodium falciparum parasites is noted (1,000x magnification).

Figure 3.  A reactive neutrophil with dumbbell-shaped bi-lobulated nucleus characteristics of Pelger-Huët anomaly (1,000x magnification).

Figure 4.  Pelger-Huët anomaly in a hypo-segmented neutrophil with dumbbell-shaped bi-lobulated nucleus (1,000x magnification).