Enhanced responses with various approaches to Active Case Detection: Field trial of loop-mediated isothermal amplification (LAMP) for detecting low-level parasite density infections.

Background:

Eliminating the submicroscopic reservoir of Plasmodium infections in asymptomatic carriers is likely to play a critical role in the elimination of malaria. Until recently, asymptomatic carriage was considered to be insignificant in low-transmission settings but studies utilising high-volume PCR have been able to detect a significant burden of malaria parasites at levels well below the detection limit of RDTs and light microscopy in these settings. Assessment by calibration with spiked samples shows the limit of detection of these approaches and highlights the need for more sensitive detection of low-density parasites. The use of filter paper sampling for PCR has been assessed in the field as a means of improving sensitivity but maintaining ease of sampling with finger-prick blood samples. Such samples use approximately 5 μL of blood and can detect parasite densities of greater than 200 parasites/ml).

It is now well established that transmission can occur at parasite densities that are well below the threshold of detection by RDTs and LM, and that there is a need for tools with enhanced sensitivity to identify individuals with low parasitaemias who may be at risk of progressing to clinical disease but who also contribute to a parasite “reservoir” and ongoing transmission.

Once established, such tools could be used to directly implement either Focal Screen and Treat (FSaT) strategies in which only parasite positive individuals are treated. However the surveillance data can also be used to implement broader Mass Screen and Treat (MSaT) strategies or to guide a targeted Mass Drug Administration (tMDA) approach. Thus the development of these diagnostic tools is important in their own right, irrespective of the responses that follow subsequently.

It is unclear, which technical approaches are ideal for detecting low density parasitaemias especially in remote settings, and how they can be implemented in a cost-effective and timely manner. Most approaches are seeking to assess some form of nucleic acid amplification test (NAAT), most commonly polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP). LAMP has particular appeal because it does not require the use of a thermocycler and can theoretically be established with only minimal infrastructure and training. It has been well validated in Africa and South America but there have been no comprehensive assessments in Vietnam, Cambodia, or Laos. LAMP methods utilise whole blood samples or dried blood spot (DBS) samples collected on filter papers. The former is likely to yield greater sensitivity due to the higher volume of blood used, but the latter is likely to be quicker and more easily deployed in the field. These two approaches have not been compared in a field setting. These methods will be compared with real-time PCR and light microscopy.

Overall goal:

The overall goal of this activity is to evaluate the feasibility of implementing LAMP-based testing in rural Vietnam.

Project methodology:

Two staff will independently read the LAMP results visually (turbidity and UV fluorescence) and using the LA-500 turbidimeter. The rationale for the visual read is to try and ultimately overcome the need for high-cost technologies like the turbidimeter.

Samples will be sent for real-time PCR and light microscopy to establish the performance of the LAMP assays against these widely accepted standards. At this stage RDT will not be included because the additional expense and because the literature clearly indicates that they will not sensitive enough in this context.

At the end of study, the different LAMP methods will be compared with the light microscopy and real-time PCR methods to establish parasite prevalence using this approach to Re-Active Case Detection.

Diagnostics
Measurement of Transmission
Modeling

Oct 2015 — Dec 2016

Vietnam

Malaria Control and Outreach for Mobile and Migrant Populations along the Vietnam-Cambodia Border

Mass Test and Treat (MTaT) strategies