Four species of intraerythrocytic protozoa of the genus Plasmodium (P. falciparum, P. vivax, P. ovale, and P. malariae) are known to cause
malaria in humans. However, recent reports from Asia suggest the possibility that a fifth
malaria species, Plasmodium knowlesi, is emerging as an important zoonotic human pathogen. Although more than 20 species of Plasmodium can infect nonhuman primates, until recently, naturally acquired human
infections of simian
malaria were viewed as rare events lacking public health significance. When viewed by light microscopy (the gold standard for laboratory diagnosis of
malaria), many of the simian species are almost indistinguishable from the four Plasmodium species that cause
infection in humans. Molecular techniques, such as polymerase chain reaction (PCR) amplification and microsatellite analysis, are needed for definitive species determination. This report describes the first recognized case of imported simian
malaria in several decades in the United States, diagnosed in 2008 in a patient from New York who had traveled to the Philippines. Atypical features of the parasite seen on light microscopy triggered further molecular testing, which confirmed the diagnosis of P. knowlesi. To date, all simian
malaria species have been susceptible to
chloroquine treatment. Molecular analysis of certain
malaria parasites isolated from ill travelers returning to the United States from Asia or South America can more accurately assess the burden of simian
malaria parasite infections in humans.