Molecular Assay Assesses Asymptomatic Malaria

In these areas, individuals frequently have no symptoms and go undetected by conventional microscopy or newer, rapid diagnostic tests (RDTs), but the parasites can be detected by polymerase chain reaction (PCR).

A study was carried out under the auspices of the London School of Hygiene and Tropical Medicine (UK), in a region of northeastern Tanzania to determine the relative prevalence of submicroscopic level parasite carriage and clonal complexity of infections over a range of endemicities. This study used 1,121 blood samples collected from a previously conducted cross-sectional malariometric survey during the short rainy season in 2001 from 13 villages at three different altitudes. Samples were analyzed by PCR for carriage of parasites and multiplicity of infection. The data was compared with other measures of transmission intensity collected from the same area. The PCR was carried out in a tetrad thermo-cycler (PTC-0240, The DNA engine Tetrad2 Thermal Cycler, Bio-Rad; Hercules, CA, USA), followed by gel electrophoresis to assign individuals as either parasite positive or negative.

A 2.5-fold difference was observed when parasite prevalence was compared between PCR at 34.7% and 13.6% by microscopy. This fold difference was relatively consistent at the different altitude bands despite a marked decrease in parasite prevalence with altitude. Multiplicity of infection (MOI) ranged from 1.2 to 3.7 and was positively associated with parasite prevalence assessed by both PCR and microscopy. There was no association of MOI and age. The difference between parasite prevalence by PCR was 3.2 in individuals aged between 15 and 45 years compared with 2.5 in those aged from one to five years.

The authors concluded that a large number of individuals carried submicroscopic parasites in a malaria endemic area of Tanzania and they are a potential reservoir of infection in the population. These infections are important as potential contributors to the infectious reservoir of parasites and need to be identified by control programs especially in this era where malaria elimination is a focus. High throughput standardized PCR approaches are needed to identify individuals who are malaria carriers. The study was published online on December 16 2011 in the Malaria Journal.

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London School of Hygiene and Tropical Medicine
Bio-Rad