QEH serves as a tertiary care hospital for the Malaysian state of Sabah, which has an estimated population of 3 million. It has a modern well-equipped intensive care unit with facilities for invasive ventilation, hemodynamic support, and renal replacement therapy.

All patients with microscopy-diagnosed malaria during December 2007–November 2009 were recorded from a prospective laboratory register, and those with P. malariae monoinfection or mixed infections were identified. Additional patients, for whom conditions had been diagnosed by microscopy as caused by other Plasmodium species but were identified as P. knowlesi infections by PCR, were also included. Case records were reviewed, and clinical information was entered into a standardized data collection form. Severe disease was classified on the basis of WHO criteria for severe falciparum malaria (22). National policy recommends that all patients with microscopy-diagnosed malaria be hospitalized until negative blood smears are obtained on 2 consecutive examinations. The study was approved by the Medical Research Ethics Sub-Committee of the Malaysian Ministry of Health and the Menzies School of Health Research, Australia.

Blood films were examined by experienced laboratory microscopists, and the parasite count was classified on a scale of 1 to 4 (1= 4–40 parasites/µL, 2 = 41–400 parasites/µL, 3 = 401–4,000 parasites/µL, 4 = >4,000 parasites/µL). Hematologic results (Sysmex XT1800 [Sysmex Corp., Mundelein, IL, USA] and CELL-DYN Sapphire [Abbott Diagnostics, Abbot Park, IL, USA]) and prothrombin and partial thromboplastin times (STA Compact Hemostasis Analyzer [Diagnostica Stago, Asnières sur Seine, France]) were obtained on site. Serum sodium, potassium, glucose, creatinine, bilirubin, albumin (Roche/Hitachi Modular Analytics EVO, Roche, Basel, Switzerland), and arterial blood gas levels (Radiometer ABL520, Radiometer, Brønchøj, Denmark) were also assayed on site. Blood cultures were performed with an automated system (Becton Dickinson, Franklin Lakes, NJ, USA) and dengue serology by ELISA (PanBio, Brisbane, Australia). In accordance with QEH policy, all slides indicating P. malariae monoinfection or mixed infections were sent for molecular testing at the Sabah State Reference Laboratory, along with ≈15% of other species. Parasite DNA was extracted, and nested PCR was performed for P. falciparum, P. vivax, P. malariae, P. ovale, and P. knowlesi by methods previously published (1,17).

Data were analyzed by using STATA version 9.2 (StataCorp LP, College Station, TX, USA). For continuous variables, intergroup differences were compared by Student t test or Mann-Whitney U test. For categorical outcome variables, intergroup differences were compared by using the χ² test or Fisher exact test. Logistic regression was used to determine the association between binary outcomes and other variables. A 2-sided value of p<0.05 was considered significant.

Included in the final analysis were 56 patients with PCR-confirmed P. knowlesi malaria. On the basis of WHO severity criteria (22), 22 (39%) had severe malaria, and 34 (61%) had uncomplicated disease (Figure). These patients were identified from a group of 74 patients with documented P. malariae malaria listed in the laboratory microscopy register: 54 had P. knowlesi monoinfection shown by PCR and medical records available for review (Figure). In addition, another 2 patients received a diagnosis by microscopy as being infected with other Plasmodium sp. but were found to have P. knowlesi monoinfections on PCR testing (Figure). All 24 patients with severe malaria had PCR performed; of these, 22 had P. knowlesi monoinfection (Figure). In the group with uncomplicated infection, 41 had PCR; 34/41 had only P. knowlesi monofection; 4 cases were mixed with P. knowlesi and other species; 2 cases were non–P. knowlesi, and no Plasmodium sp. was detected in 1 patient (Figure).

The baseline demographics and clinical symptoms of both groups with PCR-confirmed P. knowlesi monoinfections are detailed in Table 1. The mean age of the patients with severe malaria (57 years; 95% confidence interval [CI] 50–64 years) was significantly older than that of the uncomplicated group (37 years; 95% CI 34–42 years) (p<0.001); however, the proportion with chronic coexisting conditions did not differ significantly between groups (Table 1). In severe disease, the proportion of female patients (36%) was significantly higher than that in uncomplicated malaria (8.8%) (odds ratio [OR] 5.9, 95% CI 1.4–25.6; p = 0.02). Overall, 8 of 11 female patients had severe disease. Two women were pregnant; 1 had severe malaria, and the other had uncomplicated malaria. Two male patients had second uncomplicated infections during the study period; 1 became infected 10 months after the first, and the other had a mixed P. vivax/P. knowlesi infection 6 weeks after initial infection. Both patients had received chloroquine for the first P. knowlesi infection. One patient with uncomplicated disease also had HIV infection.

Almost all patients with severe and uncomplicated disease had a history of fever, and no significant difference was found in duration of fever or other clinical symptoms before the patient sought treatment (Table 1). However, patients with severe complications had a lower mean arterial pressure and an increased respiratory rate. Of those not already intubated on transfer or admission to QEH, patients with severe disease also had lower oxygen saturation at room air than did those with uncomplicated disease (Table 1).

In patients with severe disease, hemoglobin concentrations and platelet counts were significantly lower, and leukocyte counts, prothrombin time, and partial thromboplastin time were elevated, compared with results for patients with uncomplicated malaria (Table 2). Sodium and albumin concentrations were also significantly decreased, and creatinine and total bilirubin levels increased in patients with severe disease (Table 2). Parasite counts were significantly higher in severe disease (Table 2). Blood cultures, performed in 12/34 patients with uncomplicated disease, were all negative. Dengue serologic testing was performed on admission for 11 patients with uncomplicated disease; 10 were negative for immunoglobulin (Ig) M and IgG, and 1 patient was positive for IgM. Blood cultures were performed for 17 patients with severe disease; 1 patient had Enterobacter cloacae bacteremia, 5 had coagulase-negative Staphylococcus infection (attributed to contamination), and samples from the remainder were negative. Dengue serologic tests for 10 patients with severe disease were all negative. Chest radiographs were obtained on admission for 30 patients (8 with uncomplicated disease, 17 with severe disease); none with uncomplicated disease were reported to have infiltrates, compared with 8/17 with severe disease (p = 0.01); all of those with severe disease had acute respiratory distress syndrome (ARDS) with hypoxemia.

Of patients with severe disease, 17 were referred from district hospitals and 5 were directly admitted. Twenty-one of these patients had complications on admission, and in 1 patient with acute renal failure, respiratory distress developed 3 days after the start of therapy. During the hospital course, 7 patients each had 1 WHO criterion for severity, and 15 each had >2 severity criteria (2, n = 6; 3, n = 5; 4, n = 1; 5, n = 1; and 6, n = 2) (Table 3). The mean age of the 6 patients who died was 64 years (95% CI 49–78 years), and the mean age of the 16 survivors (was 53 years (95% CI 45–61 years; p = 0.1). Decreased Glasgow Coma Scores (GCS 14 and 11) on initial hospital visit were seen in 2 patients who died, but other signs and symptoms did not satisfy WHO criteria for cerebral malaria. No cases of severe anemia (<6 g/dL) were found. Acute respiratory distress (n = 13), acute renal failure (n = 12), shock (n = 12), and hyperbilirubinemia (n = 9) were the most common manifestation of severe disease (Table 3). Seven patients had acidosis (on the basis of arterial blood gas analysis) (Table 3). Seven (32%) patients had significant elevations of the prothrombin and partial thromboplastin times, although none were reported to have clinically important bleeding. In patients with acute respiratory distress, the ratio of the partial pressure of oxygen to the fraction of inspired oxygen (PaO₂:FiO₂) was available for 11/13 patients. In this group, the mean ratio was 165 (range 101–250), with 10 meeting the criteria for ARDS (PaO₂:FiO₂ <200). Eight patients had cardiac function evaluated by transthoracic echocardiogram, and all had normal results, except for a 76-year-old woman with a left ventricular ejection fraction of 30%.

Seventeen patients required intensive care unit management, 12 received inotrope support, 11 required hemodialysis, and 10 received mechanical ventilation (Table 3). The median duration of intensive care stay was 6 days (range 1–11 days); for hemodialysis, 3 days (range 1–6); and for mechanical ventilation, 3 days (range 1–9). Eleven patients were transfused with erythrocytes, 2 with fresh frozen plasma, and 4 with platelets (Table 3).

Of 34 patients with confirmed, uncomplicated P. knowlesi malaria, 15 received oral chloroquine, 11 received oral quinine, and 8 received artemether-lumefantrine. Two patients from the quinine group received intravenous quinine for ≈24 hours before treatment was changed to oral therapy. Daily peripheral blood films were available for 10 patients who received chloroquine (mean admission parasitemia 2+), for 8 who received quinine (mean admission parasitemia 3+), and for 6 who received artemether-lumefantrine (mean admission parasitemia 2+). When we excluded patients who received intravenous therapy initially, the difference was significant in median parasite clearance times between those who received artemether-lumefantrine (1 day; range 0–3) and those who received chloroquine (2.5 days; range 1–3) or quinine (2.5 days; range 1–3); p = 0.01. The proportion with negative results for parasitemia by day 1 was 4/6, 3/10, and 1/8 for artemether-lumefantrine, chloroquine, and quinine, respectively (p = 0.1), and 5/6, 5/10, and 4/8 for each group on day 2 (p = 0.2). Among patients with uncomplicated disease, 11/34 patients received doxycycline, and 7/34 received other antimicrobial drugs during their hospitalization.

In December 2008, hospital policy changed, and the recommendation was made that patients receive intravenous artesunate, when available, rather than quinine for treatment of severe malaria. Of the 22 patients with severe P. knowlesi malaria, 16 received intravenous quinine, and 6 received intravenous artesunate. Daily peripheral blood films were available for 11 of the patients in the quinine group (mean admission parasitemia 3+) and 4 of the artesunate group patients (mean admission parasitemia 3+), with median parasite clearance time significantly shorter with artesunate (2 days; range 1–3) than with quinine (4 days; range 2–7) (p = 0.02). Of the 6 patients who died, 5 received quinine (median severity criteria 2; case-fatality rate 31%), and 1 received artesunate (median severity criteria 2.5; case-fatality rate 16.6%). Of patients with severe malaria, 13/22 received doxycycline, and 16/22 received other antimicrobial drugs during their hospital course.

Six (27%) of the 22 patients with severe malaria died; mean time from admission to QEH and death was 2.5 days (range 0–4). Of these, all had >3 severity criteria; 6 had ARDS, 5 had acute renal failure, and 4 had shock. All patients who died had a parasitemia level of 4+; survivors had a median level of 3+. None of the patients with uncomplicated disease died. The mean duration of hospital stay was 8.4 days (95% CI 6.3–10.5) for those with severe disease and 5.3 days (95% CI 4.1–7.4) for those with uncomplicated malaria.