From the March 13 MMWR:
In the recent U.S. case, a woman aged 50 years with no previous history of malaria who was born in the Philippines but had lived in the United States for 25 years, returned to her home country to visit friends and relatives on October 17, 2008. While there, she stayed on the island of Palawan in a cabin located at the edge of a forested area known to be a habitat for long-tailed macaques. She had not taken malaria chemoprophylaxis and had not used any mosquito-avoidance measures, both of which are recommended preventive measures for travelers to this area.
The woman returned to the United States on October 30, 2008, and noted the onset of a headache. Fever and chills ensued, and symptoms persisted for several days, after which she sought medical attention. In the emergency department, she was noted to be hypotensive and to have thrombocytopenia. Examination of thick and thin malaria smears (Figure 1) was ordered, and an initial, erroneous diagnosis of babesiosis was made by a laboratory technician. Upon review by the laboratory supervisor the following morning, the diagnosis was reassessed as malaria with 2.9% of red cells parasitized. However, the atypical appearance of the Plasmodium sp. seen in the smears prevented a species-specific diagnosis. The woman was treated successfully with atovaquone-proguanil and primaquine for Plasmodium of undetermined species.
An ethylenediaminetetraacetic acid (EDTA) blood tube and two stained smears were sent to New York state's Wadsworth Center Parasitology Reference Laboratory for confirmation of malaria and molecular determination of species by PCR. The Wadsworth Center confirmed the presence of atypical rings and schizonts of a Plasmodium species (Figure 1), but conventional PCR targeting the small subunit (SSU) of rRNA did not yield a product consistent with any of the four species of Plasmodium known to infect humans. The specimen also was negative for the variants of P. ovale, which are commonly seen in Southeast Asia. However, primers specific for the SSU rDNA of the genus Plasmodium yielded a 1,055-bp PCR product that was sequenced and noted to be a 99% match over its full length to the SSU rRNA gene from P. knowlesi (H strain) (9). These data confirmed that the infection was caused by P. knowlesi.