Differential Prevalence of Plasmodium Infections and Cryptic Plasmodium knowlesi Malaria in Humans in Thailand

BackgroundA case of human infection with Plasmodium knowlesi has been recently discovered in Thailand. To investigate the prevalence of this malaria species, a molecular-based survey was performed MethodsBlood samples from 1874 patients were tested for Plasmodium species by microscopy and nested polymerase chain reaction. P. knowlesi was characterized by sequencing the merozoite surface protein 1 gene (msp-1) ResultsOf all Plasmodium species identified, P. falciparum, P. vivax, P. malariae, P. ovale and P. knowlesi contributed 43.52%, 68.08%, 1.37%, 1.03%, and 0.57%, respectively. Mixed-species infections were more common in northwestern and southwestern regions bordering Myanmar (23%–24%) than in eastern and southern areas (3%–5%). In northwestern and southwestern regions, mixed-species infections had a significantly higher prevalence in dry than in rainy seasons (P