National Malaria Prevalence in Cambodia: Microscopy Versus Polymerase Chain Reaction Estimates

Abstract

Accurate information regarding malaria prevalence at national level is required to design and assess malaria control/elimination efforts. Although many comparisons of microscopy and polymerase chain reaction (PCR)-based methods have been conducted, there is little published literature covering such comparisons in southeast Asia especially at the national level. Both microscopic examination and PCR detection were performed on blood films and dried blood spots samples collected from 8,067 individuals enrolled in a nationwide, stratified, multistage, cluster sampling malaria prevalence survey conducted in Cambodia in 2007. The overall malaria prevalence and prevalence rates of Plasmodium falciparum, Plasmodium vivax, and Plasmodium malariae infections estimated by microscopy (N = 8,067) were 2.74% (95% confidence interval [CI]: 2.39-3.12%), 1.81% (95% CI: 1.53-2.13%), 1.14% (95% CI: 0.92-1.40%), and 0.01% (95% CI: 0.003-0.07%), respectively. The overall malaria prevalence based on PCR detection (N = 7,718) was almost 2.5-fold higher (6.31%, 95% CI: 5.76-6.89%, P < 0.00001). This difference was significantly more pronounced for P. falciparum (4.40%, 95% CI: 3.95-4.90%, P < 0.00001) compared with P. vivax (1.89%, 95% CI: 1.60-2.22%, P < 0.001) and P. malariae infections (0.22%, 95% CI: 0.13-0.35%, P < 0.0001). The significant proportion of microscopy-negative but PCR-positive individuals (289/7,491, 3.85%) suggest microscopic examination frequently underestimated malaria infections and that active case detection based on microscopy may miss a significant reservoir of infection, especially in low-transmission settings.

Figure 1.

Malaria prevalence estimated by microscopy and distribution of the individual clusters, Cambodia, 2007. Domains 1 and 2 are shown in dark and light gray, respectively. The color assigned to each cluster corresponds to the malaria prevalence (%) estimated by microscopy (all species). The number beside each colored cluster indicates the risk zone: risk zone 1 (0–250 m from forest), risk zone 2 (250–1,000 m), risk zone 3 (1–2 km), and risk zone 4 (2–5 km).

Figure 2.

Malaria prevalence by domain and risk zone, Cambodia, 2007. The malaria prevalences (any species) by domain and risk zone as measured by microscopy and polymerase chain reaction (PCR) are shown. Risk zones 1, 2, 3, and 4 correspond to distances from the forest of 0–250 m, 250–1,000 m, 1–2 km, and 2–5 km.

Figure 3.

Malaria prevalence by diagnostic methods (microscopy and polymerase chain reaction [PCR]), Cambodia, 2007. The malaria prevalence rates (any species) found by microscopic examination and PCR detection are shown.

Figure 4.

Distribution of microscopic parasitemias by age group, Cambodia, 2007. The microscopic parasitemias by age group for each domain are presented in Panel A for Plasmodium falciparum infection and in Panel B for Plasmodium vivax infection.

Figure 5.

Malaria prevalence by species. The estimated prevalences for the entire surveyed area for any Plasmodium species (Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale, and mixed P. falciparum/P. vivax), as measured by microscopy and polymerase chain reaction (PCR) are shown.