Simian malaria in a U.S. traveler–New York, 2008

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.