Dogs can be trained to detect more than 90 percent of COVID-19 infections even when patients are asymptomatic, according to research published Monday, which authors hope could help replace the need to quarantine new arrivals.

Using their remarkable sense of smell – which can pick up the equivalent of half a teaspoon of sugar in an olympic-sized swimming pool – dogs have already shown that they can sniff out maladies such as cancer, malaria and epilepsy.

Several previous studies have shown proof-of-concept that dogs can detect SARS-CoV-2.

Researchers from the London School of Tropical Medicine wanted to see if dogs could detect a distinctive odor given off from chemical compounds associated with someone who is COVID-19 positive but doesn't show symptoms.

They gathered samples of clothing and face masks from people who had tested positive for mild or symptomatic SARS-CoV-2.

Samples of the socks of 200 COVID-19 cases were collected and arranged in lab tests for six dogs that had been trained to indicate either a presence or absence of the chemical compound.

The dogs needed to be trained not to identify "false positives" in a bid to hack their reward system and obtain treats even if there were no COVID-19 samples in a given test.

"This means that the dog fully understands and gets a reward for a correct negative as well as a correct positive," said Claire Guest, from the school's Faculty of Infectious and Tropical Diseases.

Overall, the dogs were successfully able to identify between 94 and 82 percent of SARS-CoV-2 samples.

The researchers then modeled how effectively these success rates, combined with traditional PCR tests, could help detect mild or asymptomatic COVID-19 cases.

They found that using dogs to screen arrivals at termini such as airports could detect 91 percent of cases, resulting in a 2.24 times lower rate of transmission than with PCR tests alone.

'Important start'

Authors of the research, which has yet to be peer-reviewed, said they hoped it could eventually replace the need for travelers to quarantine – which necessarily disrupts every arrival even though the vast majority are not COVID-19 positive.

"The key thing is that dogs are significantly quicker than other tests," said co-author James Logan.

"What we're suggesting is that dogs would give the first initial screening, and then those (arrivals) that were indicated as positive would then receive a complimentary PCR test."

The team said that out of a plane full of arrivals – around 300 people – less than one percent were statistically likely to be carrying SARS-CoV-2.

Under current quarantine regulations employed by some countries, all 300 would need to isolate, causing significant inconvenience.

But given the sensitivity of trained dogs, a maximum of 35 people on board would be indicated as positive, the paper said.

Of these, only around 3 would be expected to return a positive PCR test.

"This is a really important start and could lead to a useful, usable system," said Mick Bailey, professor of Comparative Immunology at the University of Bristol, who was not involved in the research.

"But there's a lot more validation needs to be done before we could be confident that the dogs can reliably and specifically detect asymptomatic SARS-CoV-2 infection in people in airports and train stations." 

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