This idea has a ‘whiff’ of the far-fetched about it, but it’s deadly serious. Dogs are already used to detect cancer cells, explosives, drugs, and people. From the UK to Australia, researchers are devising studies and training dogs to sniff out COVID-19. More work needs to be done but early results are promising. Being sniffed out by man’s best friend may be a silver lining to being virus positive.

How does it work?

Dogs have an extremely developed sense of smell, with the part of their brain that analyses scent proportionally around 40% more effective than ours. This results in a sense of smell 10,000 to 100,000 times better. To illustrate, if this was sight, it would be equivalent to them seeing 3000 miles to our 1/3 of a mile. 

Scientists know that people sick with certain diseases emit particular odours. As different infections affect different parts of the body, specific volatile compounds are released. Dogs have been shown to pick up these airborne chemicals, detecting people infected with malaria and even certain cancers with accuracy. What they smell is often not fully understood. However, just because we don’t yet fully understand the scenting doesn’t mean it’s not there. 

Researchers currently think that dogs don’t smell the virus itself, rather chemicals produced and released by the body when the virus infects cells. The virus would have to be active to emit these, so a dead virus would not be picked up, unlike with many lab tests. If true, it would also crucially not matter if the person had symptoms or not. 

How accurate is it?

Results of a small French study using 8 trained dogs and human sweat samples found that sniffer dogs detected the virus at least 83% of the time. Some dogs had 100% accuracy. They concluded that there is high evidence that sweat odour of COVID-19 sufferers was different to non-sufferers, with dogs able to detect that difference. These results need to be validated but the team plan to work with a French organisation to use dogs in care homes. In a German pilot study, 8 specialised scent dogs had an overall detection rate of 94%.

The London School of Tropical Medicine is working on a government-funded study to train coronavirus-sniffing dogs, which could be used at schools, airports and other public venues to reinforce existing nasal swab testing programs. A similar study is underway at the University of Pennsylvania.

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Finland, however, is the first country in Europe to enlist sniffing dogs. Four COVID-19 sniffer dogs are currently working at Helsinki airport in a state-funded pilot scheme. If the dog detects COVID-19, the passenger takes a standard nasal swab test to verify the dog’s result. There is a similar trial happening at Dubai airport.

What are the pros and cons of sniffer-dog detection?

These early studies show accuracy levels slightly lower than the currently mass-used COVID-19 antigen nasal swab tests but there is a need for an additional faster, reliable, non-invasive, and versatile screening tool, especially to identify people before they get symptoms or those people that are infected but actually never have symptoms. These ‘spreaders’ are currently not very well picked up.

The four-month trial at Helsinki airport trial cost £274,000, which is significantly less than laboratory-based testing methods. This may be especially important in the current economic climate, and for developing countries where funds are limited. 

Trained scent dogs can be resourced quickly, often only taking a few days or weeks to learn the skill. It’s much less obtrusive, taking seconds and with no uncomfortable nasal swab needed. 

Because of the costs, availability, and time-consuming nature of current tests, they often only target people showing symptoms. It’s thought the dogs detect live virus, whether symptoms are present or not. They could act as an early detection aid, finding people before the onset of signs or, as about 40% of people with the virus show no symptoms but continue to spread it to others, act as a method of identifying these ‘carriers’. It’s also thought that dogs may be able to identify COVID-19 from a much smaller molecular sample than traditional tests.

So what’s the catch?

This idea is not perfect, however. Virologists point out that many viruses infect the same cells as COVID-19 producing similar changes that dogs will scent. This is where it differs from sniffing out explosives and drugs where an odour difference is all that’s required.

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Are dogs detecting SARS-CoV-2 virus specifically, or also a broader array of similar viruses, like flu and colds? It’s thought different virus types may lead to different compounds coming from cells, so dogs may hypothetically be able to distinguish different viruses, but this is yet to be proven. 

It’s also not clear how strong any coronavirus odour smells to dogs. If sniffer dogs are sensitive to coronavirus smells, the dogs could be used for lines of people, many subjects at once. If not, then they may have to scan one person at a time. 

The small studies have many limitations. For example, the German study used samples from hospitalized coronavirus patients, sicker than many. This means we don’t know how far COVID-19 infection needs to progress before a dog can pick up the scent.

Looking to the future

While researchers are optimistic that dogs may play some role in helping fight the pandemic, they are clear that there’s a need for more evidence before large-scale plans can be considered. This takes time. Small projects are however moving forward, ahead of the evidence, perhaps due to the urgency of the situation. 

If further studies can iron out limitations, dogs could indeed be employed in public areas such as airports, sport events, borders or gatherings as an alternative or addition to laboratory testing, helping to prevent spread of the virus or further outbreaks. 

Personally, I know I would prefer to be sniffed by a friendly dog than have a swab stuck up my nose by a masked human. That’s a ‘lab test’ I can get on board with!

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