Human Plasmodium knowlesi infection in Ranong province, southwestern border of Thailand

Blood samples collected from 171 Thai and 248 Myanmese patients attending a malaria clinic in Ranong province, Thailand, located near the Myanmar border were investigated for P. knowlesi using nested PCR assays. Positive samples were also investigated by PCR for Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae and Plasmodium ovale, and were confirmed by sequencing the gene encoding the circumsporozoite protein (csp).Two samples, one obtained from a Thai and the other a Myanmese, were positive for P. knowlesi only. Nucleotide sequences of the csp gene derived from these two patients were identical and phylogenetically indistinguishable from other P. knowlesi sequences derived from monkeys and humans. Both patients worked in Koh Song, located in the Kawthoung district of Myanmar, which borders Thailand.This study indicates that transmission of P. knowlesi is occurring in the Ranong province of Thailand or the Kawthoung district of Myanmar. Further studies are required to assess the incidence of knowlesi malaria and whether macaques in these areas are the source of the infections.There are more than 200 species of Plasmodium that infect a variety of hosts, including reptiles, birds, rodents, primates and other mammals [1]. However, only four species (Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae and Plasmodium ovale) are well-known infectious agents that cause malaria in humans. Recently, humans infected with Plasmodium knowlesi, a simian malaria parasite [1], have been described in a number of Southeast Asian countries including Malaysia [2,3], Singapore [4], Myanmar [5], Vietnam [6], Indonesia [7], the Philippines [8] and Thailand [9]. Under the microscope, the early trophozoites of P. knowlesi cannot be distinguished from those of P. falciparum and the other blood stages are identical to P. malariae. Therefore, molecular detection methods are required for the accurate identification of P. knowlesi.In Thailand, the first case of a human P.