Exflagellating <i>Plasmodium vivax</i> in Peripheral Blood

A 3-year-old boy originally from Pakistan developed a high fever (temperature, 40°C) accompanied by rigors and chills. The boy had moved to Canada with his family 1 year before presentation. He had previously been well. There was no history of recent travel, no other family members were ill, and immunizations were up to date. The findings on physical exam were noncontributory.A vial of EDTA-anticoagulated blood was submitted to the laboratory approximately 50 minutes after the blood was drawn from the boy in the emergency department. A peripheral smear and complete blood cell count were immediately generated. The count revealed mild anemia and moderate thrombocytopenia: hemoglobin, 8.2 g/dL (age-specific reference range, 10.7–13.1 g/dL); hematocrit, 23.6% (reference range, 33.0%–39.0%); platelet count, 88 × 103/μL (reference range, 180–440 × 103/μL). Review of the peripheral smear identified occasional spherocytes and large platelet forms. The most significant finding was a parasitemia, with 1% to 2% of erythrocytes containing malarial ring forms and macrogametocytes. These were morphologically speciated as Plasmodium vivax. In addition, exflagellating microgametocytes were easily found (Figures 1 and 2, Giemsa stain, original magnification ×100).The life cycle of Plasmodium organisms in humans, or schizogony, is characterized by asexual reproduction. Sporozoites are injected into the circulation from the salivary glands of a carrier mosquito. After an extraerythrocytic (hepatic) merozoite stage, the parasite infects erythrocytes. Maturing into the trophozoite (ring form) stage, the parasite may divide still further to form more merozoites, or may undergo gametogony, to form male microgametocytes and female macrogametocytes. The classic Plasmodium falciparum banana-shaped gametocyte is one example. Gametocytes are typically ingested by the female Anopheles mosquito as part of a blood meal, and the parasite's sexual reproductive phase (sporogony) occurs within the mosquito gut. One of the first steps in the sexual phase is the exflagellation of microgametocytes. Up to 8 long slender flagella (Figures) extend out of each male microgametocyte within approximately 20 minutes of ingestion by the mosquito1 and subsequently detach as flagellar microgametes. Although other factors are contributory, exflagellation is thought to be stimulated primarily by a rise in pH,1 as may occur in the blood within the mosquito proboscis en route to the mosquito gut. The sexual phase of the Plasmodium life cycle is completed with the fertilization of a macrogamete by a flagellar microgamete, and the subsequent generation of sporozoites.Exflagellated microgametes, which are usually found only within the mosquito, have been occasionally noted in the peripheral blood of patients with malaria. As was noted by Manson2 as early as 1894, flagellated forms are not identified in freshly prepared peripheral blood smears, but may instead be seen after the blood has been exposed to air for several minutes. As the CO2 level in the blood quickly falls, to equilibrate with the surrounding air, the pH of the plasma rises, and exflagellation may begin.1 In usual clinical laboratory conditions (in vitro), it is not common to find the high blood pH (>7.7) that is thought necessary to allow exflagellation.3 Sufficiently high pH levels may be obtained in the plasma of blood samples collected into EDTA-containing tubes, if the samples are left unstoppered and unagitated overnight.3 However, exflagellation may be readily observed in a drop of Plasmodium-infected blood left exposed to air on a microscope slide.1There is no clinical significance attached to the presence of exflagellated malarial forms in peripheral blood, apart from the possibility of misdiagnosis. Although their respective morphologies are quite different, exflagellated microgametes may potentially be mistaken for other flagellated parasites such as Trypanosoma cruzi.3

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