A portable, magneto-optical device that rapidly detects the early stages of malaria infection has been created by Andrea Armani of the University of Southern California and colleagues. It analyses blood samples by moving magnetic nanocrystals created by the parasite away from a laser beam, detecting changes in the light passing through the sample if the magnetic substance is present. The scientists say this technique allows them to identify early changes in the blood of someone infected with malaria, regardless of the parasite strain.
Malaria is a major global health problem. In 2016 there were an estimated 216 million cases in 91 countries, resulting in 445,000 deaths, according to the World Health Organization. Most of the burden of this mosquito-borne infectious disease, caused by Plasmodium parasites, falls on the developing world. In 2016 Africa was home to around 90% of malaria cases and deaths.
Early diagnosis is key to effective malaria treatment and helps reduce transmission. The two most common tests are microscopy and antibody-based diagnostic tests, but both have their limitations. Identifying malaria-causing parasites in blood samples using a light microscope is the diagnostic standard, but this can be slow, and it relies on good-quality equipment and well-trained technicians. Poorly done microscopy is a well-known problem in malaria diagnostics.
Rapid diagnostic tests (RDT) detect malaria antigens in a small sample of blood placed on a test strip, giving results in around 15 min. There are many different RDTs, however, and they vary in quality and some ideally need to be kept refrigerated. Also, most do not test for all strains of malaria – they target different antigen combinations depending on the local epidemiology.
Armani and colleague’s magneto-optical device detects haemozoin – a by-product created by all species of malaria parasite. The team says by focusing on haemozoin, their test can rapidly detect the early stages of infection for all malaria strains.
The prototype, described in ACS Sensors, can detect levels of a haemozoin mimic in rabbit blood that correspond to around 26 parasites/μL. According to the researchers, this is below the level at which there would be symptoms of disease, and comparable to, or better than, current malaria tests.
The device is the size of a shoebox, weighs less than 5 kg and can be powered by a battery for 8 h. It also works with whole blood samples without the need for added chemicals, which may have strict storage requirements. The researchers say that these attributes make it ideal for use in low-resource environments.
We design with the goal of making instruments that can be dropped from planes, operated with minimal power and are very lightweightAndrea Armani
“All of my research is funded by the military, so my lab takes a very different approach to designing diagnostic systems,” Armani explains. “We design with the goal of making instruments that can be dropped from planes, operated with minimal power and are very lightweight.”
Haemezoin is produced when the malaria parasite digests haemoglobin, a component of red blood cells and its primary nutrient source. As the parasite feeds on haemoglobin it creates haem, an iron-containing compound that is toxic to the parasite. To tackle this, the parasite converts haem into an insoluble crystalline form called haemozoin. This substance is a prime target for a magneto-optic diagnostic system because it is good at blocking light. It is not usually present in blood and crucially, unlike all other components of blood, it is magnetic.
Pulled to one side
The device comprises a laser, a light detector and a magnet. When a blood sample is placed in the device, the detector measures how much light from the laser passes through the blood. The magnet is then switched on and if haemozoin is present it is pulled out the laser beam. This causes the amount of light reaching the detector to increase.
Armani told Physics World that the team is now looking to reduce the sample volume required for the test from 5–7 drops of blood to 1–2 drops. This would allow finger prick testing, as well as working on further trials as a precursor to field tests.
“The current prototype demonstrated can detect early-stage infection,” Armani says. “Conservatively, the earliest part of the early-stage infection window begins 48–72 h after infection. However, our ultimate goal is to detect within the first 24–48 h.”
This is not the first time that a magneto-optical malaria detector has been proposed. In 2014 physicists in Hungary developed a prototype that used a somewhat different detection process.