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P. knowlesi malaria circulating in Thailand for more than a decade
Jongwutiwes S. Emerg Infect Dis. 2011;doi:10.3201/eid1710.110349.
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Researchers from Thailand identified Plasmodium knowlesi in human malaria cases from blood samples obtained in 1996, suggesting that this parasite is not a newly emergent pathogen, according to new findings published in Emerging Infectious Diseases.
“To date, it was unknown whether human infections with P. knowlesi in Thailand were caused by a new emergence of this parasite species or whether the parasite had been circulating cryptically with other human malaria parasites,” Somchai Jongwutiwes, MD, PhD, of the department of parasitology at Chulalongkorn University in Bangkok, Thailand, and colleagues wrote in the study.
Jongwutiwes and colleagues set out to assess the prevalence of P. knowlesi malaria in malaria-endemic areas of Thailand. The researchers assessed data from a 2008-2009 study that included 3,300 febrile patients with malaria and a retrospective analysis conducted in 1996 of malaria in 210 human blood samples gathered from one endemic area in Tak province.
Overall, P. knowlesi represented 0.67% of human malaria cases in the 2008-2009 study and 0.48% in blood samples obtained in 1996.
Most malaria cases obtained in the 2008-2009 study were P. falciparum (51.55%), followed by P. vivax (48.21%).
Most blood samples obtained in 1996 tested positive for P. vivax (45.71%), followed by P. falciparum (26.19%). Mixed species were identified in 28.1%. Specifically, 50 samples tested positive for P. falciparum and P. vivax; seven contained both P. vivax and P. malariae; and one contained both P. vivax and P. knowlesi.
“In conclusion, human malaria caused by P. knowlesi has occurred in Thailand for more than a decade,” the researchers wrote. “Despite variations in the prevalence of all four human malaria species, P. knowlesi has shown stable prevalence rates, which suggests different transmission cycles. Human infections with P. knowlesi in Thailand could result from macaques, as shown by identical Pkmsp-1 gene sequences of human and macaque origins.”
Disclosure: This study was supported by the National Research Council of Thailand and CU-Cluster-Emerging Fund from Chulalongkorn University (Grant H-6-66-53).
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