Differential prevalence of Plasmodium infections and cryptic Plasmodium knowlesi malaria in humans in Thailand.
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.
Putaporntip
Chaturong
C
Molecular Biology of Malaria and Opportunistic Parasites Research Unit, Department of Parasitology, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand.
Hongsrimuang
Thongchai
T
Seethamchai
Sunee
S
Kobasa
Teerayot
T
Limkittikul
Kriengsak
K
Cui
Liwang
L
Jongwutiwes
Somchai
S
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000595:Amino Acid Sequence; D000818:Animals; D002648:Child; D002675:Child, Preschool; D005260:Female; D005786:Gene Expression Regulation / Q000502:physiology; D014644:Genetic Variation; D006801:Humans; D007223:Infant; D008288:Malaria / Q000453:epidemiology / Q000469:parasitology*; D008297:Male; D020066:Merozoite Surface Protein 1 / Q000737:chemistry / Q000235:genetics; D008875:Middle Aged; D008969:Molecular Sequence Data; D010802:Phylogeny; D016790:Plasmodium knowlesi / Q000235:genetics / Q000302:isolation & purification* / Q000378:metabolism; D015995:Prevalence; D012621:Seasons; D013785:Thailand / Q000453:epidemiology; D055815:Young Adult
D020066:Merozoite Surface Protein 1