Plasmodium knowlesi Malaria in humans and macaques, Thailand

Abstract

Naturally acquired human infections with Plasmodium knowlesi are endemic to Southeast Asia. To determine the prevalence of P. knowlesi malaria in malaria-endemic areas of Thailand, we analyzed genetic characteristics of P. knowlesi circulating among naturally infected macaques and humans. This study in 2008-2009 and retrospective analysis of malaria species in human blood samples obtained in 1996 from 1 of these areas showed that P. knowlesi accounted for 0.67% and 0.48% of human malaria cases, respectively, indicating that this simian parasite is not a newly emergent human pathogen in Thailand. Sequence analysis of the complete merozoite surface protein 1 gene of P. knowlesi from 10 human and 5 macaque blood samples showed considerable genetic diversity among isolates. The sequence from 1 patient was identical with that from a pig-tailed macaque living in the same locality, suggesting cross-transmission of P. knowlesi from naturally infected macaques to humans.

Figure 1

Provinces of Thailand where blood samples were obtained and tested for malaria, 1996–2009. Tak: blue, n = 210 in 1996, n = 681 in 2006–2007, and n = 1,216 in 2008–2009; Prachuab Khirikhan: orange, n = 215 in 2006–2007; Yala: purple, n = 286 in 2006–2007 and n = 1,408 in 2008–2009; Narathiwat: yellow, n = 370 in 2006–2007 and n = 421 in 2008–2009; and Chantaburi: red, n = 261 in 2006–2007 and n = 401 in 2008–2009.

Figure 2

Maximum-likelihood tree inferred from the complete merozoite surface protein 1 gene sequences of Plasmodium knowlesi from humans (red circles) and macaques (blue circles). The tree is drawn to scale, and branch lengths are measured in number of substitutions per site by using MEGA version 5.01 (14). Bootstrap values >50% from 1,000 iterations are shown. Human isolates are from the following provinces: Narathiwat (NR280, NR234, and NR522); Yala (YL975 and YL978); Chantaburi (CT157, CT190, and CT273); and Prachuab Khirikhan (BMC151, MC128, and DQ220743). Isolates HB3, HB92, HB126, HB132, and HB149 are from macaques in Narathiwat Province. GenBank accession nos. are shown after isolate names.