Dogs can be trained to detect COVID-19 by sniffing human sweat, study suggests

Detection dogs are already a common sight in airports and some other public places -- usually, they're sniffing out drugs, weapons or explosives.

But specially trained dogs have also been taught to detect infections and diseases, including colon cancer, malaria and Parkinson's disease.

Now, many countries worldwide are exploring the possibility of using dogs as a rapid, reliable and relatively cheap way to prescreen people for Covid-19 or perform rapid checking in certain circumstances, like at airports.

In the United Kingdom, a team from the London School of Hygiene and Tropical Medicine are training six dogs in the hope that they will be able to detect Covid-positive people -- even if they have no symptoms.

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In Finland, a group of sniffer dogs trained to detect Covid-19 began working at Helsinki Airport in September in an effort to identify those who have contracted the virus and in Chile police dogs are being trained to sniff out Covid-19 in humans.

In this new study, researchers based in France and Lebanon took sweat samples from the underarms of a total of 177 patients from four hospitals in Paris and one in Beirut -- 95 had tested positive for Covid-19, while 82 had tested negative. It was important that both the negative and positive samples came from the same hospitals so the dogs wouldn't be picking up on a particular "hospital odour," the researchers said.

Using some of the sweat samples, they trained 14 dogs that had been working as explosive detection dogs, search and rescue dogs or colon cancer detection dogs to take part in the study. The scientists did not use dogs trained to detect illicit drugs because they could not rule out whether study patients who had provided sweat samples had used prohibited substances.

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Six dogs -- Guess, Maika, Gun, Oslo, Bella and Jacky -- had their sniffing abilities formally tested in the study. Five of them were Belgian Malinois, a common breed for French working dogs, and Jacky was a Jack Russell terrier.

The researchers asked the animals to detect a positive Covid-19 sweat sample in one olfactory cone from a lineup of three to four cones that contained negative, or mock, samples. The scientists believe the dogs pick up on a specific scent produced by volatile organic compounds that are generated by catabolites, substances produced by the replication of the virus that escape the body through sweat.

During the testing period, the dogs did dozens of trials, with a success rate of between 76% to 100%. Jacky and Bella, the two dogs that specialized in detecting colon cancer, had a 100% success rate in the 68 tests they completed.

Moreover, during the testing, two samples collected from individuals who tested negative were repeatedly marked by two of the dogs. The relevant hospitals were informed, and in subsequent tests these individuals tested positive.

The study was published in the journal PLOS One on Thursday.

By filming the trials, the researchers were able to understand why sometimes the dogs were unsuccessful. In one case, a horse walked close to a testing site that was situated at a veterinary school. However, the reasons for detection failure weren't always obvious -- but the researchers said this wasn't necessarily a negative.

"Even if trained dogs are able to correctly discriminate symptomatic Covid-19 positive individuals from asymptomatic negative ones, they should not be considered a perfect diagnostic test -- but rather a complementary tool," the study said.

The study, the researchers said, was a "promising first step" in providing some evidence that dogs may be able to detect Covid-19 samples collected from sweat. However, the scientists said their study had some limitations, including that sweat samples had to be reused and researchers couldn't rule out that the dog was memorizing the scent -- although they didn't think memory played a major role. Further research was needed, they said.

In a few cases, dogs have been known to get infected with the coronavirus, the study said, but, according to the US Centers for Disease Control, there is no evidence that animals, including dogs, play any significant role in spreading Covid-19. For safety, the researchers did not use any samples for training or testing the dogs within 24 hours of collection.