Canines seem to detect coronavirus infections with remarkable accuracy, butresearchers say large-scale studies are needed before the approach is scaledup.
Asher is an eccentric, Storm likes sunbathing and Maple loves to use herbrain. All three could play a part in controlling the COVID-19 pandemic, butthey are not scientists or politicians. They are dogs.
And they are not alone. Around the world, canines are being trained to detectthe whiff of COVID-19 infections. Dog trainers are claiming extraordinaryresults — in some cases, they say that dogs can detect the virus with almostperfect accuracy. Scientists involved with the efforts suggest that caninescould help to control the pandemic because they can screen hundreds of peoplean hour in busy places such as airports or sports stadiums, and are cheaper torun than conventional testing methods such as the RNA-amplification techniquePCR.
But most of these findings have not yet been peer reviewed or published,making it hard for the wider scientific community to evaluate the claims.Researchers working on more conventional viral tests say that initial resultsfrom dog groups are intriguing and show promise. But some question whether theprocess can be scaled up to a level that would allow the animals to make ameaningful impact.
On 3 November, groups working with the animals met in an online workshopcalled International K9 Team to share preliminary results from experiments andto improve how their research is coordinated.
“No one is saying they can replace a PCR machine, but they could be verypromising,” says veterinary neurologist Holger Volk at the University ofVeterinary Medicine Hanover in Germany, who is leading an effort to train andstudy COVID-sniffing dogs and did not speak at the event.
Sense of wonder
Humans have taken advantage of canines’ superior sense of smell for decades.Dogs’ noses bear 300 million scent receptors, compared with humans’ 5 millionor 6 million. That enables them to detect tiny concentrations of odour thatpeople can’t. Sniffer dogs are already a familiar sight in airports, wherethey detect firearms, explosives and drugs. Scientists have also trained dogsto detect some cancers and malaria, but the animals are not routinely used forthis purpose. Researchers don’t know for sure what the dogs are smelling, butmany suspect that these illnesses cause the human body to let off a distinctpattern of volatile organic compounds (VOCs). These molecules readilyevaporate to create scent that dogs can pick up. Previous work with non-COVIDviruses has suggested that viral infections might also cause the body to dothis.
Many sniffer-dog scientists turned their attention to COVID-19 early in thepandemic. They have trained their canines to smell samples, most often ofsweat, in sterile containers, and to sit or paw the floor when they detectsigns of infection. Trials at airports in the United Arab Emirates, Finlandand Lebanon are using dogs to detect COVID-19 in sweat samples frompassengers; these are then checked against conventional tests. According todata presented at the K9 meeting, dogs in Finland and Lebanon have identifiedcases days before conventional tests picked up the virus, suggesting that theycan spot infection before symptoms start.
Riad Sarkis, a surgeon and researcher at Saint Joseph University in Beirut, ispart of a French–Lebanese project that has trained 18 dogs. Sarkis used thebest two performers for the airport trial in Lebanon. The dogs screened 1,680passengers and found 158 COVID-19 cases that were confirmed by PCR tests. Theanimals correctly identified negative results with 100% accuracy, andcorrectly detected 92% of positive cases, according to unpublished results.“This is very accurate, feasible, cheap and reproducible,” says Sarkis, whohas been approached about using the dogs in schools, banks and prisons, and isworking with a shopping mall to offer COVID-19 testing using the animals.
Low-income countries with limited lab space could particularly benefit fromthe approach, says Isabella Eckerle, a virologist at the University Hospitalsof Geneva in Switzerland.
But there is just one published journal article on dogs’ efficacy at sniffingout COVID-19, by Volk’s group; he describes it as a pilot study1. Theresearchers trained eight dogs on samples taken from the mouths and windpipesof seven people hospitalized with COVID-19 and seven uninfected people. Thedogs identified 83% of positive cases and 96% of negative ones.
The false positive and negative rates of the standard PCR lab test varydepending on the brand of test used and the timing of the test. A systematicreview published as a preprint2 on medRxiv found the false-negative rate ofRT-PCR tests to be 2–33% if the same sample is tested repeated times.Up to 4%of UK PCR test results could be false positives, according to governmentdocuments.
Critics say the German dog study used samples from too few patients. The dogscould be learning to identify the specific scent of the samples rather than ofCOVID-19, says Cynthia Otto, who leads the Penn Vet Working Dog Centre at theUniversity of Pennsylvania in Philadelphia and is also working with COVID-19sniffer dogs.
In her work, which is also unpublished, she has found that the dogs can tellthe difference between samples of either urine or sweat from people withCOVID-19 and those from people without the disease. She is working withchemists to understand which VOCs the dogs are picking up; a paper describingthis is under review. “The dogs can do it. The challenge is the ignorance thatwe have as humans as to what can confuse the dogs,” she says. And in an effortto gather a large data set, her team is collecting sweat samples from 1,000T-shirts worn overnight by people who have tested positive and negative forCOVID-19.
A group in France, led by veterinary scientist Dominique Grandjean at theNational Veterinary School of Alfort near Paris, posted its work3 on thepreprint server bioRxiv in June. The researchers, who included Sarkis, trained8 dogs to detect COVID-19 in 198 sweat samples, around half of which were frompeople with the disease. When these were hidden in a row of negative samples,the dogs identified the positive samples 83–100% of the time. The paper doesnot say how well the dogs identified negative test results. The research isnow under review at a journal, but Grandjean says the process has not beeneasy. “To publish papers on detection dogs is very difficult because mostreviewers do not know anything about working dogs,” he says.
The data in that study look promising, says Fyodor Urnov, a gene-editingscientist who is working on COVID testing at the University of California,Berkeley. But he would like to see larger data sets on how well dogs identifypositive and negative samples. He also notes that there is variation in howwell individual dogs perform. In Grandjean’s study, for example, 2 dogsidentified 68 out of 68 positive samples, whereas one missed 10 out of 57cases.
Groups need to boost their sample sizes before the wider scientific communitycan evaluate how useful the dogs might be, agrees James Logan, an infectious-disease researcher at the London School of Hygiene & Tropical Medicine who istraining and studying COVID-19 dogs, including Storm, Maple and Asher. “It’simportant not to go out too early with grand claims and small data sets,” hesays.
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