People infected with SARS-CoV-2 emit a distinct odour that can be detected by specially trained dogs, UK scientists have reported.
Results from a phase 1 trial involving six dogs found a sensitivity range of between 82.1% and 94.3%, and a specificity range of 76.4% to 92.0%.
Results 'Exceeded Expectation'
The study, which is yet to be peer-reviewed, found that the dogs could detect asymptomatic cases, and that accuracy was consistent even when the viral load was low.
The researchers said further trials could result in dogs being used to rapidly identify people with suspected COVID-19 in crowded spaces, such as airports, backed up with confirmatory PCR testing.
The study was led by the London School of Hygiene & Tropical Medicine (LSHTM) in collaboration with the charity Medical Detection Dogs and Durham University.
"The results of this study far exceeded my expectations," said Prof James Logan, head of the Department of Disease Control at LSHTM, who led the project.
Dogs have previously been trained by Medical Detection Dogs to 'sniff out' distinctive odours linked to some cancers, malaria, and Parkinson's disease.
When scientists began the latest investigation last May they had no evidence on whether human infection with SARS-CoV-2 produced a detectable smell. However, one year on, they say that odour from clothing worn by people with COVID-19 can be successfully identified by trained dogs.
Clothing Samples
"These were small sock samples – 50 pence piece size samples – that had been collected some time ago, often over 2 months ago, before the dogs had actually been given the chance to sniff them," said Claire Guest, chief scientific officer at Medical Detection Dogs to a briefing hosted by the Science Media Centre.
Aided by a grant from the Department of Health and Social Care, clothing was obtained from 1097 people infected with SARS-CoV-2, and 2031 individuals who did not have the virus.
The dogs were trained over a number of weeks, and six were selected for a double- blind trial in which neither dog trainer nor technician were aware of which samples were positive or negative. The dogs, Asher, Kyp, Lexi, Marlow, Millie, and Tala were tested using 200 positive samples and 200 negative samples.
Tala, a 4-year-old Labrador, returned the most accurate results, with 94.3% sensitivity and 92.0 specificity.
The lowest sensitivity was recorded by Marlow at 82.1%, and the lowest specificity by Kyp at 76.4%.
The dogs successfully identified positive samples from the original SARS-CoV-2 strain and the so-called 'Kent' variant. "What was great was that the dogs who had been trained on the original variant transferred to the new variant," said Prof Logan. "They could detect the new variant without any additional training."
The initial results suggested that trained dogs might even outperform lateral flow tests, the researchers said.
They also hoped artificially to recreate the distinctive odour of COVID for use as a dog training aid.
Could Dogs Screen Large-scale Gatherings?
The researchers emphasised that their findings were based on laboratory experiments and had yet to be tested in a 'real world' situation. However, combining a screening by dogs, backed up by a gold standard PCR test, might result in 91% of cases being detected, they claimed.
The researchers presented mathematical modelling to suggest that two dogs could perform initial screening of 300 people in 30 minutes. Dogs could also serve as a visual deterrent at ports for people with symptoms thinking of travelling abroad, they added.
Prof Steve Lindsay, from the Department of Biosciences at Durham University, said: "Dogs could be a great way to screen a large number of people quickly, and preventing COVID-19 from being re-introduced into the UK. Trained dogs could potentially act as a fast-screening tool for travellers, with those identified as infective confirmed with a lab test. This could make testing faster and save money."
Further Research Needed
Commenting on the findings, Lawrence Young, a virologist and professor of molecular oncology at the University of Warwick, said the findings were "not to be sniffed at". However, he pointed to uncertainties over whether "this approach work[s] in the real world on people rather than samples of socks and shirts".
Prof James Wood, head of the Department of Veterinary Medicine at the University of Cambridge, said in an airport situation, "if there were five infected people on the plane, while the dogs would likely be able to detect them, they might also falsely classify a further 20 to 25 individuals as positive and in need of further testing".
Mick Bailey, professor of comparative immunology at the University of Bristol, commented: "Infection of dogs with SARS-CoV-2 has been reported, both asymptomatic and associated with variable levels of disease, as in humans, so using dogs deliberately to sniff breath and clothes from potentially infected people seems a bit of a risk to the dogs, let alone their handlers."
