Novel Molecular Tool Detects Asymptomatic Malaria-Infected Individuals

Malaria control programs aiming at decreasing local transmission and achieving pre-elimination status require proper detection of Plasmodium parasites in the peripheral blood, both in febrile patients and asymptomatic carriers.

Asymptomatic carriers, representing the human reservoir contributing to the transmission of the parasites, generally have much lower parasite densities compared to symptomatic individuals, at detection thresholds of less than 200 parasites/μL, below which the standard diagnostic tools such as microscopy and rapid diagnostic tests (RDTs) become less reliable.

A team of collaborating scientists led by those at Medical Research Council Unit (Banjul, The Gambia) collected blood from 341 subjects whose median age was 9 years (range 1 to 68 years), and screened for malaria. From a single finger prick, blood samples were collected for RDT, thick blood film (TBF) and dried blood spots (DBS). The RDTs were read immediately in the field while the DBS and microscopy slides were taken to the field station at the end of each day for further processing. A novel molecular tool, the loop-mediated isothermal amplification (LAMP), targeting the apicoplast genome of Plasmodium falciparum was evaluated for the detection of asymptomatic malaria-infected individuals.

P. falciparum malaria prevalence was 37 % (127/341) as detected by LAMP, 30 % (104/341) by microscopy and 37 % (126/341) by the SD Bioline Malaria Antigen P.f RDT (Standard Diagnostics, Inc.; Yongin-si, Republic of Korea). Compared to the reference polymerase chain reaction (PCR) method, sensitivity was 92 % for LAMP, 78 % for microscopy, and 76 % for RDT; specificity was 97 % for LAMP, 99 % for microscopy, and 88 % for RDT. Turn-around time for the entire LAMP assay was approximately three hours and 30 minutes for an average of 27 ± 9.5 samples collected per day, compared to a minimum of 10 samples an hour per operator by RDT and over eight hours by microscopy.

The authors concluded that with a shorter turn-around-time and high throughput processing, LAMP results would be available much faster than by microscopy and more reliably than with RDT, supporting the use of this diagnostic test for mass screening and treatment (MSAT) or focused screening and treatment (FSAT) campaigns. The apicoplast genome that this novel LAMP assays targets is unique to the parasite, thus ensuring high specificity. The study was published on October 9, 2015, in the Malaria Journal.

Related Links:
Medical Research Council Unit 
Standard Diagnostics