Covid breakthrough: Trained sniffer dogs accurately pick out travellers with virus

Camilla’s coronavirus sniffer dog appeal detailed by experts

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Dogs have an extraordinary sense of smell, being able to pick up scents at particle levels as low as one part per trillion — far exceeding all mechanical sensors, let alone human noses. According to scientists, our canine friends can detect the diagnostic volatile organic compounds released during various metabolic processes in the body. These include those that are generated by viral, bacterial and parasitic infections.

In their study, infectious disease specialist Professor Anu Kantele of the University of Helsinki and her colleagues trained four sniffer dogs — who had experience detecting either cancer, dangerous goods or illicit drugs — to sniff out coronavirus.

Over the course of seven trial sessions, the dogs were randomly presented with skin swab samples collected from 402 volunteers, of whom 114 had tested positive for SARS-CoV-2 in a PCR swab test.

The team found that the dog’s diagnostic accuracy was 92 percent overall, with only minor variations.

On average, they had a 92 percent “combined sensitivity”, which is the accuracy of detecting those individuals with the infection, and 91 percent “combined specificity”, the accuracy of detecting those who weren’t infected.

Furthermore, the team noted, that the dogs correctly identified infections in 25 people who had tested positive but were not manifesting any symptoms of the infections.

In the second phase of their study, the team put the four dogs to work sniffing samples taken from incoming passengers at the Helsinki-Vantaa International Airport between September 2020–April 2021 — with each traveller also given a PCR swab to verify what the dogs determined.

According to the researchers, the PCR and sniffer dog results matched in 98 percent of the 303 real-life samples.

Because the prevalence of SARS-CoV-2 among the airport passengers was relatively low — at less than half-a-percent of travellers — the dogs were also presented with samples from 155 people who had tested positive on a PCR test.

The sniffer dogs were able to correctly identify just under 99 percent of these as positive, meaning that had they represented an influx of infected people into the airport in the real-life study, the dogs’ performance would have reached a sensitivity and specificity of 97 and 99 percent, respectively.

Finally, the team used their data to calculate the proportion of true positive results and true negative results in two hypothetical scenarios, one in which SARS-CoV-2 had a population prevalence of 40 percent and the other of just 1 percent.

For the former, they calculated a proportion of true positive results of 88 percent and true negative results at 94.5 percent — meaning that the information provided by the sniffer dogs increased the chance of detection to around 90 percent.

For the population prevalence of one percent, meanwhile, the researchers estimated a proportion of true positive results of just under 10 percent and a proportion of true negative results of just under 100 percent.

According to the team, the high probability of true negative results backs the use of sniffer dogs for screening — with the aim of excluding people who don’t need a PCR swab test.

They said that dogs could be used “in sites of high SARS-CoV-2 prevalence, such as hospitals, to pre-screen patients and personnel.”

However, they added, they could be as useful “in low prevalence sites, such as airports or ports, to pre-screen passengers.”

The researchers did concede, however, that there was a risk that dogs that had previously been trained to sniff out other substances might accidentally flag these as being SARS-CoV-2 positive.

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The researchers also found that the dogs were less successful at correctly identifying the Alpha variant of SARS-CoV-2, as they had been trained to detect the “wild type”, or background strain, of the virus that contains no major mutations.

However, they said, this just speaks to how sensitive dogs’ noses really are.

They concluded: “This observation is remarkable as it proves the scent dogs’ robust discriminatory power.

“The obvious implication is that training samples should cover all epidemiologically relevant variants.

“Our preliminary observations suggest that dogs primed with one virus type can in a few hours be retrained to detect its variants.”

The full findings of the study were published in the journal BMJ Global Health.

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