Good dogs could help identify carriers of malaria | Science

Malaria kills hundreds of thousands of people every year, but the parasite itself, Plasmodium falciparum, infects many more people than it makes sick. The majority of people with malaria at any time are perfectly healthy parasite plants and unknowingly, these healthy carriers can easily spread the disease to new areas and new people who might not be so lucky.

Fortunately, the man's best friend is here to lend a helping hand to the doctors.

A new study from the London School of Hygiene & Tropical Medicine suggests that dogs can identify the smell of malaria. In a small proof-of-concept study, two trained dogs were able to distinguish socks worn by children with malaria and socks at the feet of those who did not have them. The researchers presented their preliminary results today at the annual meeting of the American Society of Tropical Medicine and Hygiene.

Malaria is known to make people more attractive to mosquitoes. A team led by James Logan, head of the Department of Disease Control at the London School of Tropical Medicine, had already demonstrated that socks worn by infected children were more attractive to leeches, probably because odors clothes contained more chemicals called aldehydes. With tiny wires attached to mosquito antennas, the researchers were blowing them with chemicals to see what odors were producing a reaction. The compounds called heptanal, octanal and nonanal were the most exciting for mosquitoes and were found at higher levels in the socks of infected children.

"So, if mosquitoes can [smell differences in people]so why not dogs? Says Steve Lindsay, a public health entomologist at the University of Durham in the UK and principal investigator of the new study.

Lindsay and her colleagues have asked apparently healthy Gambian children to wear a pair of new socks for one night. In the morning, they picked up the socks – about 175 pairs – and tested malaria in children. (About 30 children tested positive, but showed no symptoms.) The researchers froze the socks to preserve the odors, wrapped them up and returned them to the UK by mail.

The nonprofit medical detection dogs then spent six months training two dogs, Sally and Lexi, to differentiate the socks of children with malaria from socks worn by unaffected children. The mutts – a Labrador retriever and a mixed breed of Labrador – sniffed every sample and froze if they detected malaria or otherwise went. After training with one sock from each pair, dogs were able to correctly identify 70% of children with malaria and 90% of healthy children by sniffing socks on the other foot. The dogs even selected infected children with a very low parasite load – 10 to 20 parasites per microliter of blood. (According to the World Health Organization, a malaria diagnostic tool should have 75% accuracy at 200 parasites per microliter – but again, they have never specifically mentioned dogs)

"I think it's really exciting," says Audrey Odom John, a pediatric medical specialist at the University of Washington's School of Medicine in St. Louis. Odom John, who has not participated in the new study, is developing a respiratory test to detect the odors of malaria in infected children. "It's definitely a good start," she says about dog success.

There are, however, certain limits to work. The sample size of 175 pairs of socks is smaller than what medical detector dogs would have liked, says Claire Guest, co-founder and general manager of the non-profit association. A better sample would have been 100 positive socks for malaria and 300 negative socks, she said.

Due to the small size of the sample, the dogs had to be trained with the same pairs of socks as those tested (the training was performed with one sock from each pair and the test was performed with the socks). ;other). The experience is not ideal because dogs could simply have learned to recognize individuals rather than detect the smell of malaria.

However, Guest and his team do not believe that dogs memorize individuals. When dogs failed to identify a child with malaria, it was often a child infected with the sexual stage of the malaria parasite, which, according to Lindsay, may have changed the smell. If dogs were only capturing people's smells, matching one sock to another, they should not have systematically made this mistake.

Overall, the team felt that the dogs were doing well, considering that they used "small pieces of worn socks [once] Logan recounts, "A child, then frozen for a moment, this leaves us more hope that if they felt a real person, they would do a lot better because the signal should be much more powerful."

Feeling a person rather than a sock could also eliminate possible confounders. For example, many children shared a bed with other members of their family and the socks could catch the odors of the sheets or other people. And "what little boys do with their socks, no one can say," says Lindsay. "We saw a boy wearing a sock. What happened to the other sock? "

Lindsay says that sniffer dogs could be useful in the ports of entry of countries that have eradicated malaria, but where Anopheles mosquitoes, which spread the parasite, are still present. You can track down healthy people who may be carrying the malaria parasite to prevent them from reintroducing the disease into an otherwise "clean" country. According to some estimates, up to 19 out of 20 people may carry the malaria parasite without becoming ill, a quick and non-invasive way to identify carriers would be a major asset for those struggling against the spread of the disease. disease.

Current diagnostic methods do not detect hundreds or thousands of people passing through an airport. To diagnose malaria, doctors can draw blood and use a microscope to identify parasites, but this requires training and "things that seem simple but not simple", such as clean glass slides, a microscope. good condition and reliable electricity, says Heidi Hopkins Associate Professor in Malaria and Diagnosis at the London School of Hygiene & Tropical Medicine.

Health workers can also use any number of "quick diagnostic tests," which involve dropping a drop of blood on a small device. Fifteen minutes later, a color bar appears if an antigen produced by the malaria parasite is present in the patient's blood. These tests are easy to use by non-professionals in any context. The disadvantage is that you can not force all international travelers to undergo a blood test. A dog, on the other hand, "could get off a line of people and be done in seconds," says Logan.

And if it's not sniffer dogs, an electronic "nose" could perhaps be used. A device could be designed to detect the same compounds as dogs and mosquitoes – but to do this, additional research on specific molecules is needed.

One of the unknown pieces of the puzzle is to understand why, exactly, people infected with malaria feel differently. It is not known if the parasites directly produce odors, if they modify a person's microbiome or if our body produces them in response to parasites. However, says Odom John, the malaria parasite has an organelle similar to that found on plants that produce odorous compounds – "which gives pine an odor of pine or lemon smell lemons". It is possible that the malaria parasite produces odorous compounds directly with its strange plant organelle.

Another avenue of research to explore is to determine if the odor is uniform in all populations. Lindsay plans to test people from all over Africa to see if dogs can recognize malaria among their smells.

A final complicating factor is that there is more than one type of malaria. P. falciparum is the most common and the deadliest, but other species of the parasite can cause debilitating relapses months after the initial infection.

Malaria is a complex disease that will not be eradicated easily, even with the help of our four-legged friends. But these puppeteers – and their human trainers – will be working hard, in the lab and on the ground, to make this oppressed story a success.