dog’s sense of smell is thousands of times stronger than a human’s. Their superior snoots are why canines are used in law enforcement; once they’re trained to detect certain scents, like narcotics and explosives, they can pick up traces that human noses could never notice. This same ability can be applied to medicine. Research shows that dogs can sniff out evidence of cancer and other diseases with impressive accuracy—and a recent study adds more evidence to suggest the same is true of SARS-CoV-2, the virus that causes COVID-19.
The study was published in the journal. PLOS ONE and builds on a smaller one published in 2020, dogs from French fire departments and the UAE’s Ministry of the Interior were trained to recognize the smell of compounds associated with SARS-CoV-2 in human sweat samples. “The welfare of the dogs was fully respected, with toy rewards and a total absence of work-induced physical or mental fatigue,” the authors write.
After collecting sweat from nearly 350 people who sought COVID-19 testing in Paris, the researchers took samples. About a third tested positive using PCR.
The dogs’ overall sensitivity (their ability to pick up on positive samples) was 97%, while their overall specificity (their ability to determine that a sample was negative) was 91%. That puts dogs’ diagnostic capabilities in the same league as gold-standard PCR tests, says study co-author Dominique Grandjean, a professor at France’s Alfort School of Veterinary Medicine.
Grandjean states that almost any breed of dog can learn to detect this kind of thing in just a few weeks. “We could have hundreds or thousands of dogs doing this for almost [no money], and they would be as efficient as PCR,” Grandjean says.
Potentially, dogs could be used in crowds, such as airports and conferences. They can also sniff out potential positive cases in places that don’t have traditional testing. They could also be used to test people who don’t respond well to invasive nasal swabs, such as those with autism or neurodegenerative diseases, Grandjean says.
There’s already some proof of concept. Finland’s Helsinki-Vantaa International Airport experimented with COVID-sniffing dogs in 2020, and a study published in May found their accuracy was around 98%, though there weren’t enough positive cases to draw firm conclusions about sensitivity. Miami International Airport and multiple Massachusetts schools also conducted COVID sniffing dog tests last year.
Dogs can also detect viral compounds in sweat from Long COVID patients up to 18 months after they first caught the virus, according to a study co-authored by Grandjean and posted in January on MedRxiv, a server for new research that hasn’t yet been peer-reviewed. Canines tested positive for the virus from sweat samples collected from Long COVID patients from 45 of them. The test was not performed on samples obtained from any other Long COVID patient. Emilie Seyrat, coauthor of the study and a Long COVID patient herself admits that this accuracy rate might be underestimated because samples were sent via mail in the summer heat.
These results, however, are fascinating. Scientists don’t fully understand why some people develop chronic symptoms after a case of COVID-19, but one of the main hypotheses is that remnants of the virus linger in some people’s systems and potentially continue to replicate, causing lasting health problems.
The fact that dogs can smell virus-related compounds more than a year after people initially got sick supports that idea, says Dominique Salmon-Ceron, an infectious disease specialist at Hôtel-Dieu Hospital in Paris and another author on the Long COVID study. “Our results add to the argument that the virus can probably persist” in the body, she says.
The study wasn’t designed to diagnose anyone with Long COVID, since participants were already known to have the condition. And detecting viral persistence isn’t the same as diagnosing someone with a complex, multi-system disease like Long COVID, Salmon-Ceron says.
But dogs can add to scientists’ understanding of the condition and validate patient experiences. Many Long COVID patients struggle to get proper care because they can’t prove they had COVID-19, particularly if they got sick early in the pandemic, before testing was widely available.
“Nobody was believing them, except some doctors,” Salmon-Ceron says—and, now, some dogs.
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