Characteristics of Subpatent Malaria in a Pre-Elimination Setting in Southern Zambia

Abstract

To achieve and sustain malaria elimination, identification and treatment of the asymptomatic infectious reservoir is critical. Malaria rapid diagnostic tests (RDTs) are frequently used to identify asymptomatic, Plasmodium-infected individuals through test-and-treat strategies, but their sensitivity is low when used in low transmission settings. Characteristics of individuals with subpatent (RDT-negative but polymerase chain reaction [PCR]-positive) Plasmodium parasitemia were evaluated in southern Zambia where malaria transmission has declined and efforts to achieve malaria elimination are underway. Simple random sampling based on satellite imagery was used to select households for participation in community-based, cross-sectional surveys between 2008 and 2013. Questionnaires were administered to collect information on age, gender, recent history of malaria symptoms, and recent antimalarial drug use. Blood samples were collected by finger prick for Plasmodium falciparum histidine-rich protein 2 RDT, blood smears for microscopy, and dried blood spots for molecular analysis to detect malaria parasites and their sexual stage. Of 3,863 participants with complete data, 102 (2.6%) were positive by microscopy, RDT, or PCR. Of these, 48 (47%) had subpatent parasitemia. Most individuals with subpatent parasitemia were asymptomatic (85%). Compared with individuals without parasitemia, individuals with subpatent parasitemia were significantly more likely to be aged 5-25 years. Approximately one quarter (27%) of those with subpatent parasitemia had detectable gametocytemia. These findings suggest that strategies based on active or reactive case detection can identify asymptomatic individuals positive by RDT, but more sensitive diagnostic tests or focal drug administration may be necessary to target individuals with subpatent parasitemia to achieve malaria elimination.

Figure 1.

Flow chart showing the selection of participants included in the analysis (A) and contingency table of rapid diagnostic test (RDT) and polymerase chain reaction (PCR) results (B).

Figure 2.

Parasite density distribution and detection of parasitemia by (A) microscopy or (B) rapid diagnostic test (RDT). Parasite densities quantified by pfcytb q-PCR are presented in log₁₀ scale. All q-PCR–positive samples were included regardless of symptomatic or asymptomatic status (N = 61).

Figure 3.

Parasite density distribution and detection of gametocyte-specific mRNA by reverse transcriptase polymerase chain reaction (RT-PCR). Of those pfcytb Quantitative PCR (q-PCR)–positive samples, 59 were tested for the detection of gametocyte-specific mRNA.