Differential prevalence of Plasmodium infections and cryptic Plasmodium knowlesi malaria in humans in Thailand

Abstract

Background: A 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.

Methods: Blood 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).

Results: Of 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 < .001). P. knowlesi was found in 10 patients, mostly from southern and southwestern areas-9 were coinfected with either P. falciparum or P. vivax. Most of the P. knowlesi Thai isolates were more closely related to isolates from macaques than to isolates from Sarawak patients. The msp-1 sequences of isolates from the same area of endemicity differed and possessed novel sequences, indicating genetic polymorphism in P. knowlesi infecting humans.

Conclusions: This survey highlights the widespread distribution of P. knowlesi in Thailand, albeit at low prevalence and mostly occurring as cryptic infections.

Conflict of interest statement

Potential conflicts of interest: No conflict

Figures

Figure 1.

Map of Thailand showing the locations of the sample collection sites in northwestern (Tak Province), southwestern (Prachuab Khirikhan Province), southern (Yala and Narathiwat Provinces) and eastern areas (Chantaburi Province). The Pie diagrams represent percentage of single (filled) and mixed species (unfilled) malaria infections. Distribution of malaria species (F=P. falciparum, V=P. vivax, M=P. malariae, O=P. ovale and K=P. knowlesi) for each endemic area as determined by the species-specific nested polymerase chain reaction is shown in bar graphs.

Figure 2.

Neighbor-joining tree inferred from the type A sequences of the small subunit ribosomal RNA gene of Plasmodium knowlesi from Thai and Sarawak human isolates comparing with those from macaque origins (ATCC30191 and W1) using maximum composite likelihood model with the orthologous sequence of P. fragile as an outgroup. Percentage of 1000 bootstrap replicates supports over 50% is shown along the branches. Scale underneath the tree indicates the number of base substitutions per site. Isolate names are listed along with their respective GenBank accession numbers.

Figure 3.

Amino acid sequence comparisons of merozoite surface protein-1 of Plasmodium knowlesi from human origins (A1, BMC-51 and MC1-26) and a macaque origin (Malayan strain). Identical and deleted residues are indicated by dots and dashes, respectively. Boundaries of blocks follow those of the msp-1 gene of P. vivax [11]. Shaded and unshaded regions denote variable and conserved blocks, respectively.