A middle-aged female presented to the emergency department after experiencing a fall and loss of consciousness due to syncope. Upon presentation, the patient endorsed an almost four-week history of fevers, chills, abdominal discomfort, night sweats, and dizziness. She also reported poor oral intake and recent unintended weight loss since the onset of her symptoms. When asked, she noted she had returned from a month-long trip to Italy and Ghana two months prior to presentation. She initially presented to an outside hospital with generalized weakness, body aches, and a fever where she was treated with antibiotics for a urinary tract infection. She then presented to a different outside hospital with similar symptoms. There, she confirmed she had not taken malaria prophylaxis and was bitten by mosquitos on her recent trip. Blood was taken for a peripheral blood smear review but no Plasmodium sp. were observed.
At her current presentation, the patient denied a history of seizures but continued to endorse recurrent fevers, malaise, nausea, and vomiting. She was mildly tachycardic, afebrile, and bloodwork revealed normocytic anemia (hemoglobin 10.3), and elevated creatinine. Given the uncertainty surrounding her syncopal episode, the patient was admitted for further workup. After admission, she spiked a fever up to 103°F and the infectious disease service was consulted. As part of her workup, blood was again drawn for Giemsa-stained peripheral blood smears which were read in the microbiology laboratory.
Upon receipt of the patient’s blood, Giemsa-stained thick and thin smears and an immunochromatographic assay for the detection of malarial antigens (BinaxNOW® Malaria, Abbott Laboratories, Abbott Park, IL) were performed. The BinaxNOW® assay was positive for the detection of pan-malarial antigen (T2), but not the histidine-rich protein II antigen specific to P. falciparum (T1). These findings were suggestive of infection with a non-falciparum Plasmodium species (Image 1). Analysis of the Giemsa-stained thin smear revealed several Plasmodium parasites at various stages of development. Importantly, parasites (and particularly ring forms) were only rarely encountered (Image 2, Image 3A). “Basket” (Image 3B) and “Band” (Image 3C) trophozoite forms were observed, as well as schizonts with 6-12 merozoites typical rosette patterns around central pigment (Image 3D). In the context of a positive antigen test, the patient was definitively diagnosed with a Plasmodium malariae infection based on morphology with a calculated parasitemia of less than 0.1%.
Plasmodium malariae is one of the five species of Plasmodium (along with P. falciparum, P. vivax, P. ovale and P. knowlesi) which cause human malaria. Infection begins when sporozoites are injected from the salivary glands of the female Anopheles mosquito into the host upon taking a blood meal. Sporozoites migrate to the liver where they infect hepatocytes and develop into schizonts which eventually rupture, releasing infectious merozoites. These merozoites enter the circulation and infect erythrocytes, subsequently developing into immature ring form trophozoites (Image 2A). Ring form trophozoites develop into either mature trophozoites or become gametocytes which can be taken up by another mosquito upon feeding (Image 2B). Mature P. malariae trophozoites adopt unique morphologies not seen with other Plasmodium species including “band” (Image 3B) and “basket” (Image 3C) forms. Mature trophozoites then develop into schizonts (Image 3D) which rupture, releasing 6-12 merozoites which perpetuate the erythrocytic cycle of infection. P. malariae elaborates fewer merozoites than other Plasmodium species which are often arranged in a “rosette” pattern around centrally localized pigment in the schizont (Image 3D).
The P. malariae infectious cycle has several unique hallmarks compared to that of other Plasmodium species. Unlike P. vivax and P. ovale, the P. malariae lifecycle does not include a latent hypnozoite form, and thus is devoid of classical relapse. P. malariae also preferentially infects older erythrocytes, as opposed to P. vivax which prefers younger cells. Additionally, the infected erythrocyte does not enlarge or fimbriate when infected with P. malariae as opposed to P. vivax and P. ovale, respectively. Patterns of erythrocyte infection and lysis lead to elevated parasite burden, characteristic cyclic fevers and anemia. However, the time needed for development from ring trophozoite to rupturing schizont is different among malarial parasites: P. knowlesi exhibits the most rapid development (24-hours), followed by P. falciparum, P. ovale, and P. vivax (48-hours), and then P. malariae (72-hours).
P. malariae has a global distribution overlapping with P. falciparum. While P. falciparum is the primary species causing reported infection in Ghana, P. malariae infection is encountered less frequently. Associated parasitemia are characteristically lower in P. malariae infections compared to other species due to fewer merozoites produced during infection, an extended 72-hour developmental cycle, and the preference for the infection of older erythrocytes. This can complicate microscopic diagnosis as well as lead to more indolent symptomology. Indeed, patients can often remain asymptomatic for months to years after leaving endemic areas. In this patient’s case, definitive diagnosis was made months following her travel to an endemic region. The patient completed a 5-day course of artemether/lumefantrine with complete resolution of symptoms prior to discharge.
-Clare McCormick-Baw, MD, PhD is an Assistant Professor of Clinical Microbiology at UT Southwestern in Dallas, Texas. She has a passion for teaching about laboratory medicine in general and the best uses of the microbiology lab in particular.
-Andrew Clark, PhD, D(ABMM) is an Assistant Professor at UT Southwestern Medical Center in the Department of Pathology, and Associate Director of the Clements University Hospital microbiology laboratory. He completed a CPEP-accredited postdoctoral fellowship in Medical and Public Health Microbiology at National Institutes of Health, and is interested in antimicrobial susceptibility and anaerobe pathophysiology.