Plasmodium knowlesi from archival blood films: Further evidence that human infections are widely distributed and not newly emergent in Malaysian Borneo

Kim-Sung Lee,Janet Cox-Singh,George Brooke,Asmad Matusop,Balbir Singh

doi:10.1016/j.ijpara.2009.03.003

Kim-Sung Lee, Janet Cox-Singh + Show 3 more

Open Access

https://doi.org/10.1016/j.ijpara.2009.03.003

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Abstract

Human infections with Plasmodium knowlesi have been misdiagnosed by microscopy as Plasmodium malariae due to their morphological similarities. Although microscopy-identified P. malariae cases have been reported in the state of Sarawak (Malaysian Borno) as early as 1952, recent epidemiological studies suggest the absence of indigenous P. malariae infections. The present study aimed to determine the past incidence and distribution of P. knowlesi infections in the state of Sarawak based on archival blood films from patients diagnosed by microscopy as having P. malariae infections. Nested PCR assays were used to identify Plasmodium species in DNA extracted from 47 thick blood films collected in 1996 from patients in seven different divisions throughout the state of Sarawak. Plasmodium knowlesi DNA was detected in 35 (97.2%) of 36 blood films that were positive for Plasmodium DNA, with patients originating from all seven divisions. Only one sample was positive for P. malariae DNA. This study provides further evidence of the widespread distribution of human infections with P. knowlesi in Sarawak and its past occurrence. Taken together with data from previous studies, our findings suggest that P. knowlesi malaria is not a newly emergent disease in humans.

Highlights

Plasmodium knowlesi, a malaria parasite of Old World monkeys (Garnham, 1966), is one of the five malaria species known to cause human malaria (Cox-Singh and Singh, 2008)
Our prospective molecular epidemiological studies conducted on 960 samples collected from malaria patients in Sarawak between 2000 and 2006 showed that by using nested PCR assays 266 were diagnosed as P. knowlesi and only four were P. malariae
We present further evidence of the past incidence and widespread distribution of human P. knowlesi infections in the state of Sarawak based on a retrospective study using DNA extracted from archival malaria blood films diagnosed as P. malariae by microscopy

Summary

Introduction

Plasmodium knowlesi, a malaria parasite of Old World monkeys (Garnham, 1966), is one of the five malaria species known to cause human malaria (Cox-Singh and Singh, 2008). Plasmodium knowlesi malaria in humans is routinely misdiagnosed by microscopy as Plasmodium malariae malaria due to the morphological similarities between the two species and the only reliable diagnostic method to correctly distinguish between the two species is two nested PCR assays (Cox-Singh and Singh, 2008). Our prospective molecular epidemiological studies conducted on 960 samples collected from malaria patients in Sarawak between 2000 and 2006 showed that by using nested PCR assays 266 were diagnosed as P. knowlesi and only four were P. malariae cases, 312 had been diagnosed as P. malariae by microscopy (Singh et al, 2004; Cox-Singh et al, 2008). We present further evidence of the past incidence and widespread distribution of human P. knowlesi infections in the state of Sarawak based on a retrospective study using DNA extracted from archival malaria blood films diagnosed as P. malariae by microscopy

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