Answer to May 2022 Photo Quiz

Figure 1A and B show small Plasmodium trophozoite ring forms about one-third of the diameter of the infected, normal-sized, erythrocyte. Figure 1A demonstrates multiple infection, and Fig. 1B demonstrates a “bird’s eye” appearance (not species specific). Figure 1C shows an immature schizont (mature Plasmodium knowlesi schizonts have elongated, segmented merozoites); in contrast, Plasmodium falciparum schizonts are rarely seen in peripheral blood except in severe cases. Figure 1D shows an oval-shaped gametocyte with scattered brown pigment similar to that of Plasmodium malariae; in contrast, P. falciparum gametocytes are sausage-shaped. (The observed acanthocytosis is artefactual and does not affect morphology of Plasmodium.)

Morphologic diagnosis of P. knowlesi is difficult due to overlapping features and geographical distribution with P. falciparum and P. malariae and because it is infrequently encountered outside regions where the disease is endemic. P. knowlesi-infected erythrocytes are normal sized, and early trophozoites resemble P. falciparum (double chromatin dots, multiple infection, appliqué forms), while older trophozoites may develop band forms resembling P. malariae (1). Due to the short 24-h asexual cycle of P. knowlesi, hyperparasitemia reminiscent of P. falciparum may occur, with levels as high as 27% reported (2). However, in P. falciparum (longer asexual cycle of 48 h), this instead occurs because all erythrocyte stages can be infected.

Aside from morphological similarities on microscopy, diagnosis of P. knowlesi infection may also be missed by rapid immunochromatographic tests due to suboptimal cross-reaction of “pan-malarial antibodies” against P. knowlesi antigens or low parasitemia counts (3). This underscores the importance of molecular diagnostic methods to allow definitive diagnosis of P. knowlesi malaria, including identifying coinfections. In our patient, definitive identification was obtained by both morphology and PCR (real-time PCR followed by nested PCR of positive samples for species identification, backed by confirmatory sequencing of the reverse transcriptase PCR [RT-PCR] amplicon) (4). He, fortunately, had low parasitemia (0.3% at diagnosis) and made a full recovery.

P. knowlesi is a simian zoonotic malaria endemic to Southeast Asia (5). The long-tailed, pig-tailed, and northern pig-tailed macaques and the banded leaf monkey are its natural hosts; the Anopheles leucosphyrus group is its vector. Once thought uncommon, it is now the preeminent malaria in Malaysia, even being reported in areas previously considered “malaria-free,” such as neighboring Singapore (6) and Brunei Darussalam (7) and as far as southern China (Yunnan) (5) and the Andaman Islands (8). A 2014 study showed that P. knowlesi accounted for nearly 80% of malaria cases in Johor state (which adjoins Singapore) where this patient’s infection was acquired, compared with 56.5% of malaria across Malaysia (9).