Malaria is considered to be the leading parasitic cause of morbidity and mortality worldwide. Despite nearly full eradication in many parts of the world, the WHO’s latest World Malaria Report, says, “There were an estimated 216 million cases of malaria in 2016, an increase of five million cases over the previous year. Deaths stood at approximately 445,000, a similar number to the previous year”.
While there are large-scale programmes to control the disease, an important part of the drive is detection of malaria, both for treatment as well as for prevention of spread. The usual method of detection, however, is to take blood samples, which are examined in a laboratory by trained persons and a microscope. Even other methods, called “Rapid Detection Tests”, which are now available, have limitations of cost and delivery at remote places.
It is in this context that Professor Steve Lindsay, department of biosciences, Durham University, UK, and his group, report that the domestic dog can be trained to detect the presence of malaria simply by sniffing the used socks of affected persons. At the annual meeting of American Society of Tropical Medicine and Hygiene, in New Orleans, US, Lindsay described his team’s trials in the Upper River Region of The Gambia in West Africa, where dogs had been trained to detect the presence or absence of malaria in schoolchildren with 70 to 90 per cent accuracy.
What the finding underlines is that malaria infection leads to the release of odours, which can be detected. Giving off traces of molecules and the ability to detect them, have formed the means of communication in many parts of the natural world. That mite-sized insects can detect mating signals over distances of several kilometres is well known. While birds are known to announce readiness to mate through plumage and behaviour, in most of the animal world, the signal is one based on smell.
Other evidence has also been reported of the malaria parasite signalling its presence. Xiunan Wang and Xiao-Qiang Zhao, from University of Newfoundland, St John’s, Canada, had reported a tendency of mosquitoes to prefer malaria-affected persons to source their blood meal.
This was apparently the result of an adaptation of the parasite to ensure its best proliferation. As malaria affects the red blood corpuscles, blood of infected persons is easier to feed on. The mosquito appears to find it rewarding to choose malaria-infected persons and feeding time in these cases is found to be significantly reduced.
While the study of Wang and Zhao was about the factors, including ambient temperature and the frequency of mosquito bites that affect transmission dynamics, it was observed that the mechanics of how mosquitoes select where they will feed could be a means of malaria control. The mechanism is clearly olfactory, in the sense that the malaria parasite induces an odour, which can be detected by the mosquito, to emanate from the parasite’s host.
While the mosquito has clearly adapted to be able to detect the odour and benefit from a meal of malaria-infected blood, the Durham University group experimented with our best-known master of following scents to see if it could sniff out a case of malaria. Even the human nose is impressive in its ability to differentiate smells —it is said that we can tell apart a trillion different smells. But unlike humans, whose senses, and intelligence, have evolved beyond the sense of smell, in most animals, smell is an important part of sensing the environment.
The nasal cavity of the dog consists of membrane that is packed with folds and its surface area is about that of human palm, some fifty times greater than the area in the human nose, which is about the size of a thumbprint. With tens of millions of more nerve endings than humans have, and an area in the brain dedicated to smell, forty times that of humans’, the dog’s nose is said to be a hundred thousand times more sensitive.
Police dogs and tracking dogs pick up scents of individual persons days after they have been at a place. As any dog owner would tell you, a dog never ceases to amaze by responding to known persons long before they come into view or earshot. It is clear that the dog can detect and differentiate vapours or molecules in the air even at vanishingly small concentrations.
The method used to train dogs is to repeatedly expose them to mixed stimuli, with reward when they react to selected stimuli. A simple example is of training a dog to respond to its name, or to follow commands like, “sit”, “stay” or “roll over”. Seemingly more complex tasks, like responding to the smell of gunpowder, or drugs, also take only the same extent of training. Hence, the widespread use of dogs to detect contraband at airports or the entry to places of high security.
The Durham experimenters used the same methods to train a pair of dogs, a Labrador- Golden Retriever cross called Lexi and a Labrador called Sally, to distinguish between the scent of children infected with malaria parasites and those who were uninfected. The trained dogs were then tested with the scent of a group of apparently healthy children from West Africa. Nylon socks used by 175 children, 30 of whom had been found to be infected with malaria were shipped from Africa to the UK to see if the dogs could identify malaria in the foot odour carried by the socks.
It was found that the dogs could correctly identify 21 of the 30 infected children and 130 of the 145 disease-free children. This is an impressive performance, for a first trial, of identifying the greatest part of the children in need of treatment and then, most of the children on whom no resources need be spent.
Co-author Professor James Logan, head of the department of disease control, at the London School of Hygiene and Tropical Medicine, said, “Worryingly, our progress on the control of malaria has stalled in recent years, so we desperately need innovative new tools to help in the fight against it.” Co-author Claire Guest said, “This is the first time we have trained dogs to detect a parasite infection and we are delighted by these early results”, the press release says.
The writer can be contacted at response@simplescience.in