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

Natthawan Sermwittayawong1*, Balbir Singh2, Mitsuaki Nishibuchi3, Nongyao Sawangjaroen1 and Varaporn Vuddhakul1

Author Affiliations

1 Department of Microbiology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla, Thailand

2 Malaria Research Centre, Faculty of Medicine & Health Sciences, University Malaysia Sarawak, Kuching, Sarawak, Malaysia

3 Center for Southeast Asian Studies, Kyoto University, Kyoto, Japan

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Malaria Journal 2012, 11:36  doi:10.1186/1475-2875-11-36

Published: 8 February 2012



Plasmodium knowlesi, a simian malaria parasite, has been reported in humans in many Southeast Asian countries. In Thailand, most of the limited numbers of cases reported so far were from areas near neighbouring countries, including Myanmar.


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.

Plasmodium knowlesi; Thailand; Myanmar; Circumsporozoite protein