As evidence mounts that dogs can be trained to detect Covid-19 infections, some companies and countries are deploying canine testers.

A new type of Covid-19 test comes with four legs and a wagging tail.

A growing body of research by scientists and dog trainers from the U.S. to the United Arab Emirates suggests that dogs can use their powerful sense of smell to sniff out Covid-19 infections, including in people without symptoms.

With more than 300 million scent receptors (compared with roughly five million in humans), dogs can do this with a high degree of accuracy by detecting compounds the human body releases in secretions like sweat and saliva as it reacts to the coronavirus, according to scientists.

Dogs have long been trained to detect odors associated with drugs or explosives and have also been used to identify diseases such as cancer, malaria and diabetes. But “this is the first time that dogs are able to detect a viral disease in humans,” said Dominique Grandjean, a professor at the National Veterinary School of Alfort in France and one of the first researchers to evaluate the potential of Covid-19-sniffing dogs.

Researchers say more research is needed to answer some unresolved questions, such as whether Covid-19-sniffing dogs can be confused by other viruses or by vaccinations. The University of Pennsylvania School of Veterinary Medicine Working Dog Center is conducting a study to test whether dogs are able to differentiate between people with Covid-19 and those who have been vaccinated against it.

The World Health Organization is coordinating an international task force of researchers to investigate the use of sniffer dogs. It said in a March report that Covid-19-sniffing dogs could complement conventional diagnostic tools because—unlike nasal swab tests, which require contact between people and waiting at least 15 minutes for a result—dogs can screen large numbers of people noninvasively in real time and at low cost.

One dog can screen 250 to 300 people a day, according to the WHO.

Prof. Grandjean calculated that dog screenings in France could cost as little as one euro, equivalent to about $1.20, per person, as opposed to roughly €75 for a polymerase chain reaction, or PCR, test, a highly accurate test that involves a nasal swab.

The UAE has invested heavily in training Covid-19-sniffing dogs and now has 39 dogs that are being used to screen people at malls and public events and in crowded living facilities, said Guillaume Alvergnat, an adviser to the interior ministry’s International Affairs Bureau. The UAE could quickly train dog handlers from other countries in its Dubai training facility, according to Mr. Alvergnat. In France, Prof. Grandjean said about 1,000 dogs are working for various government institutions and could quickly be trained for Covid-19 detection at places such as airports, concerts and sports venues.

Any dog can theoretically be trained in a few weeks, researchers say, and it could be faster in dogs already trained for medical or explosive detection.

Studies have shown that dogs can be trained to identify Covid-19 infections with roughly 82% to 99% sensitivity and 84% to 98% specificity, Prof. Grandjean said. A test’s sensitivity indicates its ability to correctly detect an infection, while its specificity shows how well it can avoid giving false positives.

Researchers at the University of Veterinary Medicine in Hannover, Germany, trained eight dogs for one week to detect respiratory secretions from infected patients with an average detection rate of 94%, according to a study published recently in the journal BMC Infectious Diseases

In a study of 21 dogs led by Prof. Grandjean, 15 of the animals were able to detect Covid-19 with a sensitivity of 90% or more, with six dogs showing a sensitivity of 71% to 87%. The study was published in April in the Open Access Journal of Veterinary Science and Research.

Such results mean dogs may be more precise than many rapid antigen tests, which correctly identify Covid-19 infections in an average of 72% of people showing symptoms and 58% of asymptomatic people, according to a recent review from Cochrane, a U.K.-based nonprofit that evaluates scientific research.

While sniffer dogs aren’t officially sanctioned as a diagnostic method for Covid-19, companies and national governments—including the National Basketball Association, construction firms in Finland, the Beirut airport in Lebanon and the UAE interior ministry—have begun deploying them for screenings.

“It’s an extra layer of protection…and it’s a lot more pleasant than having a swab stuck in your nose,” said Robert Meade, chief executive of Doctors Hospital of Sarasota, in Florida.

In April, the hospital started an experiment in which a Labrador retriever named Buffy sniffs visitors’ shoes. If she identifies someone with Covid-19, she sits at their feet and awaits a treat. The visitor must take a nasal swab test before entering. After 10 days, Mr. Meade said Buffy had detected one case.

Sports and entertainment events are driving demand in the U.S., said Jerry Johnson, president of Bio Detection K9, a firm in Anniston, Ala., which now has 13 Covid-19-detection dogs and 10 more in training. During a test the company conducted outside a Jacksonville, Fla., health clinic, dogs sniffed people waiting to be tested for Covid-19 and correctly identified their infection status about 93% of the time, Mr. Johnson said.

Bio Detection K9 later learned that at least two of the patients flagged by the dogs but who tested negative that day later tested positive for the virus, meaning the dogs might have been more sensitive to Covid-19 cases than the clinic’s PCR tests, Mr. Johnson said.

A Miami-based company called Sniff Screening supplied dogs to sniff fans and employees heading into games of the NBA’s Miami Heat between late January and early April.

Nascar has used Covid-19-sniffing dogs to screen drivers, mechanics and other employees at races at Atlanta Motor Speedway, Alabama’s Talladega Superspeedway and South Carolina’s Darlington Raceway, according to a Nascar spokesman.

Dogs will also be screening participants at this year’s Peachtree Road Race, a 10-kilometer foot race over the July 4 weekend, said Jay Holder, director of marketing and communications for the Atlanta Track Club, which organizes the race.

In addition to helping prevent new infections and curb the emergence of new variants of the virus, sniffer dogs can offer emotional comfort, people who have begun to deploy them say.

At the Doctors Hospital of Sarasota, Mr. Meade said Buffy has been helping soothe anxious staff as well as visitors. “It’s almost like pet therapy,” he said, adding that he had taken in Buffy to live with his family.

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