Malaria is sinister even when it’s not making people sick.
Some people infected with the Plasmodium parasite that causes the disease never develop any symptoms, but are still capable of transmitting the illness to others (usually through mosquitoes). Because these people feel perfectly healthy, they have no reason to get tested or get treatment. It’s one of the primary reasons malaria is such a persistent human illness, and kills about 400,000 people annually.
If public health officials could identify infections early on, they could, in theory, stop some outbreaks before they start. However, this has traditionally required blood tests, which are timely and invasive, or treating entire communities, which would include some people who don’t need it. So now, scientists are turning to dogs for alternatives. More specifically, to their hyper-sensitive noses.
Earlier this week, Steven Lindsay, an entomologist from Durham University in the northern UK presented the initial results of a study showing that dogs can be trained to sniff out malaria infections simply by smelling the socks worn for 12 hours overnight—enough time for odors undetectable by humans to work their way into the fabric. His team in Durham collaborated with researchers based in London and The Gambia. According to World Bank data, in 2015 for every 1,000 Gambian residents at risk, 208 had malaria, making the country one of the parts of the world where the disease is most common.
“People with malaria parasites generate distinct odors on their skin,” Lindsay said in a statement. “Our study found dogs, which have an incredibly sensitive sense of smell, can be trained to detect these odors even when it’s just on an article of clothing worn by an infected person.”
The Bill and Melinda Gates Foundation awarded Lindsay with a Grand Challenge grant to fund his work, and three dogs were donated by a charity called Medical Detection Dogs, which has in the past trained canines to sniff out other conditions, including prostate cancer.
Lindsay and his team provided socks to about 600 children in The Gambia, according to the Washington Post (paywall). These children were given basic medical exams, were tested for malaria (and presumably treated if they needed), and were asked to sleep with the socks on. Socks from children with fevers, indicating that they may have had malaria, were not included, but researchers were able to retrieve 200 socks from healthy children, and 30 from children who were asymptomatic but had Plasmodium in their blood. These socks were frozen and shipped to London, where they were preserved for over a year, while the dogs were trained to identify the specific odor from a malaria infection.
Eventually, the trained dogs were able to find those from children with malaria about 70% of the time; they correctly ignored socks from healthy children 90% of the time. Lindsay believes that some of the dogs’ errors were due the fact that the smell of the Plasmodium parasites may only be detectable when it reaches a certain maturity level. Because the parasite grows and changes form once it makes its home in the human body, it could be the case that Plasmodium in some of the children’s bodies were more mature than those in others.
This work has yet to be peer reviewed or published; Lindsay presented it at the 2018 American Society of Tropical Medicine and Hygiene meeting in New Orleans, Louisiana, this week. But it was a proof-of-concept study that shows it may be possible to non-invasively detect malaria in large populations—particularly in places where asymptomatic people may be traveling, unaware that they carry the disease, like at airports. More work and larger trials need to be done, but in the future, it could be possible that malaria-sniffing dogs could scan large groups of people and signal to individuals that they need treatment before they spread Plasmodium further.