Plasmodium knowlesi is increasingly recognized as a cause of human malaria in Southeast
Asia but there are no detailed prospective clinical studies of naturally acquired
infections.
In a systematic study of the presentation and course of patients with acute P. knowlesi
infection, clinical and laboratory data were collected from previously untreated,
nonpregnant adults admitted to the hospital with polymerase chain reaction-confirmed
acute malaria at Kapit Hospital (Sarawak, Malaysia) from July 2006 through February
2008.
Of 152 patients recruited, 107 (70%) had P. knowlesi infection, 24 (16%) had Plasmodium
falciparum infection, and 21 (14%) had Plasmodium vivax. Patients with P. knowlesi
infection presented with a nonspecific febrile illness, had a baseline median parasitemia
value at hospital admission of 1387 parasites/microL (interquartile range, 6-222,570
parasites/microL), and all were thrombocytopenic at hospital admission or on the following
day. Most (93.5%) of the patients with P. knowlesi infection had uncomplicated malaria
that responded to chloroquine and primaquine treatment. Based on World Health Organization
criteria for falciparum malaria, 7 patients with P. knowlesi infection (6.5%) had
severe infections at hospital admission. The most frequent complication was respiratory
distress, which was present at hospital admission in 4 patients and developed after
admission in an additional 3 patients. P. knowlesi parasitemia at hospital admission
was an independent determinant of respiratory distress, as were serum creatinine level,
serum bilirubin, and platelet count at admission (p < .002 for each). Two patients
with knowlesi malaria died, representing a case fatality rate of 1.8% (95% confidence
interval, 0.2%-6.6%).
Knowlesi malaria causes a wide spectrum of disease. Most cases are uncomplicated and
respond promptly to treatment, but approximately 1 in 10 patients develop potentially
fatal complications.