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Explained: How big a role can sniffer dogs play in detection of coronavirus?

A series of studies has shown how effectively dogs can detect coronavirus in sweat or urine samples of infected individuals. But can they replace conventional RT-PCR tests? Not yet.

Written by Mehr Gill , Kabir Firaque , Edited by Explained Desk | New Delhi |
Updated: May 26, 2021 11:04:58 am
(From left) Marlow, Tala, Millie, Lexi, Kyp and Asher, trained to detect coronavirus from odour samples.(Source: Neil Pollock via Medical Detection Dogs, UK)

In the last one month, two groups of researchers have reported separate findings on sniffer dogs detecting coronavirus infection in humans with remarkable accuracy. The role of detection dogs has been the subject of a number of studies and experiments during the pandemic — and even before, with dogs trained to detect various other diseases, as well as drugs and explosives.

So, are dogs the future in coronavirus testing? The evidence so far indicates they can be useful in identifying potentially infected people in crowds — but those identified would still need to undergo conventional tests such as RT-PCR.

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What are these new studies?

In April, researchers from University of Pennsylvania and collaborators published a study in the journal PLOS One describing how nine trained dogs — eight Labrador retrievers and a Belgian Malinois — identified urine samples from patients who were positive for SARS-CoV-2, discerning them from samples that were negative for the virus. They detected positive samples with 96% accuracy, but their ability to detect false negatives was lower.

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And last week, researchers from the London School of Hygiene and Tropical Medicine (LSHTM) and collaborators published a preprint of the findings of a year-long study funded by the UK government. They reported that six trained dogs could identify odour samples from infected people with accuracy up to 94% (comparable with 97.2% for RT-PCR) and correctly ignore uninfected samples with accuracy up to 92%.

Does coronavirus have a smell?

Waste products from infected people have distinctive smells, various studies have found. Our urine, saliva and sweat release chemicals called volatile organic compounds, which can have different odours depending on whether a person has an infection or not.

Last December, French scientists had published yet another study on dogs detecting coronavirus, in PLOS One. “When the virus replicates or make the cell produce its ‘toxic’ molecules, specific molecules are produced and they have to leave the body as metabolites or catabolites,” Professor Dominque Grandjean of the National Veterinary School of Alfort, France, who led the French study, told The Indian Express by email in December. “It has been demonstrated that in the exhaled air we could find molecules that are specific to SARS-CoV-2, like an olfactory signature…” he said.

Besides dogs, the UK study also used sensors — called organic semi-conducting sensors — that could distinguish between odours from people with asymptomatic or mild symptoms, and uninfected individuals.

Why dogs?

According to Medical Detection Dogs, which trained the dogs for the UK study, a dog’s sense of smell is elevated due to the complex structure of its nose. Humans cannot detect the odours that dogs do.

A 2004 study in BMJ reported that trained dogs correctly identified urine samples of patients with bladder cancer on 22 out of 54 occasions. Medical Detection Dogs cites evidence that dogs may be able to detect Parkinson’s disease years before the onset of symptoms. A 2019 study in The Lancet described how dogs identified malaria-infected children in Gambia from their foot odours.

For coronavirus, Dubai Airport last year became the world’s first to deploy dogs to detect such infection among passengers. Finland and Lebanon have conducted trials with detection dogs at airports.

How were the new studies carried out?

In the US study, the dogs were trained to respond to urine samples from SARS-CoV-2 positive patients and discern positive from negative samples. After three weeks, all nine dogs were able to identify SARS-CoV-2 positive samples with high accuracy. However, they sometimes responded to negative samples too.

In the UK study, odour samples were collected from over 3,000 individuals; 325 positive and 675 negative samples were chosen for training and testing in a double-blind trial.

“The dogs were trained over a number of weeks by introducing them to the odour samples from individuals that had tested positive for Covid-19, as well as control samples from people who had tested negative. Samples were presented to the dogs on a stand system and the dogs were rewarded for correctly indicating a positive sample, or for correctly ignoring a negative sample,” LSHTM’s Professor James Logan, who led the project, said by email.

If a stand contained a positive sample, the dog would normally sit or point its nose towards the stand, he said. “They correctly ignored any samples that were negative.”

Mathematical modelling based on the findings showed that in a real-life setting, trained sniffers could screen over 300 passengers in an airport within 30 minutes.

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Does that mean that dogs can be an alternative to RT-PCR tests?

The authors of the UK study offer it as a complement rather than a substitute. The modelling found that using trained dogs followed by a confirmatory PCR test can detect nearly 91% of infections in both symptomatic and asymptomatic carriers.

“We do not mean for the dogs to replace PCR or LFT tests,” Professor Logan said. “The major benefit of these dogs is how quickly they can detect the odour of the infection. Our modelling suggested that the best use of the dogs is as a rapid mass screening tool with a confirmatory PCR for anyone who is indicated as positive by the dogs, and therefore could reduce the number of PCR tests required.”

Again, these studies were conducted in a trial setting where the dogs were trained in a controlled environment. Beyond the modelling in the UK study, the effectiveness of trained dogs is yet to be determined in a real-world setting.

Last November, Nature published an article on the possible role of sniffer dogs in detecting coronavirus. It quoted veterinary neurologist Holger Volk, who is leading such a study in Germany, as saying: “No one is saying they (dogs) can replace a PCR machine, but they could be very promising.”

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