Abstract
ObjectiveThe main objective of this study was to compare the performance of nested PCR with expert microscopy as a means of detecting Plasmodium parasites during active malaria surveillance in western Thailand.MethodsThe study was performed from May 2000 to April 2002 in the village of Kong Mong Tha, located in western Thailand. Plasmodium vivax (PV) and Plasmodium falciparum (PF) are the predominant parasite species in this village, followed by Plasmodium malariae (PM) and Plasmodium ovale (PO). Each month, fingerprick blood samples were taken from each participating individual and used to prepare thick and thin blood films and for PCR analysis.ResultsPCR was sensitive (96%) and specific (98%) for malaria at parasite densities ≥ 500/μl; however, only 18% (47/269) of P. falciparum- and 5% (20/390) of P. vivax-positive films had parasite densities this high. Performance of PCR decreased markedly at parasite densities <500/μl, with sensitivity of only 20% for P. falciparum and 24% for P. vivax at densities <100 parasites/μl.ConclusionAlthough PCR performance appeared poor when compared to microscopy, data indicated that the discrepancy between the two methods resulted from poor performance of microscopy at low parasite densities rather than poor performance of PCR. These data are not unusual when the diagnostic method being evaluated is more sensitive than the reference method. PCR appears to be a useful method for detecting Plasmodium parasites during active malaria surveillance in Thailand.
Highlights
The detection of asexual parasites by light microscopy of Giemsa-stained thick and thin films remains the standard laboratory method for the diagnosis of malaria [1,2]
Detection of parasites in symptomatic patients reporting to local malaria clinics is the primary means used for malaria diagnosis in Thailand, use of active surveillance provides a tool for detection of patients with asymptomatic malaria and relatively low parasite rates
For Plasmodium vivax (PV), the mean density was 265.5/ μl (SEM = 66.5), with a range from 14–14,000/μl, while for Plasmodium malariae (PM), the mean density was 324.8/μl (SEM = 128.4), with a range from 28-1,134/μl. Prevalence rates for both Plasmodium falciparum (PF) and PV were highest at the start of the study and in general decreased over the 24 months that the study was conducted
Summary
The detection of asexual parasites by light microscopy of Giemsa-stained thick and thin films remains the standard laboratory method for the diagnosis of malaria [1,2]. In Thailand and many other malaria endemic regions, there are problems and limitations associated with reliance on microscopic diagnosis of malaria for both active and passive case detection [3,4,5]. These include lack of skilled microscopists, variation in individual training and/or experience, limited supply of microscopes and reagents as well as variation in equipment maintenance, and inadequate quality control [6]. When symptomatic patients with a relatively high parasitaemia (e.g., >500 asexual parasites/μl) report to a malaria clinic for treatment, microscopy can provide an accurate diagnosis that is used to initiate appropriate chemotherapy. Parasitaemia rates in asymptomatic patients are often quite low – in a recent study in western Thailand, the median asexual parasitaemia rate in 82 Plasmodium falciparum and 98 P. vivax-positive individuals was 848 and 155 asexual parasites/μl blood, respectively [7]
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