The team responsible for the remarkable findings, published in the online journal BMJ Global Health, said sniffer dogs could be a valuable tool in the early stages of a pandemic when other resources for detection might not yet be available, as well as helping to contain an ongoing outbreak.
Dogs possess an exceptionally sharp sense of smell, and are able to discern a scent at levels as low as one part per trillion, which far exceeds any available mechanical alternatives, scientists say.
In order to test the how their abilities would translate to real-life conditions, the researchers trained four dogs to sniff out SARS-CoV-2 – the virus responsible for Covid – in the Spring of 2020.
The team picked out dogs which had previously undergone training to sniff out illicit drugs, dangerous goods or cancer.
To test the dogs’ comeptence, four skin swab samples from 420 volunteers were collected by the team. Each dog sniffed the skin samples from 114 of the volunteers who had tested positive on a PCR swab test and from 306 who had tested negative. The samples were randomly presented to each dog over 7 trial sessions.
During training, the dogs were able to detect infection with 92 per cent accuracy and those without with 91 per cent accuracy.
The Finnish researches said that only minor variation was seen among the dogs – the best performance reached 93 per cent for sensitivity and 95 per cent for specificity, while the worst reached 88 per cent and 90 per cent in the same categories.
Following the preliminary tests, the dogs’s detection skills were put to work sniffing out 303 incoming passengers at Helsinki-Vantaa International Airport, Finland, between September 2020 and April 2021. Each passenger also took a PCR swab test.
Miraculaously, the results from the sniffer dogs and PCR tests matched in 296 out of 303 (98 per cent) of the real-life samples.
The dogs correctly identified the samples as negative in 296 out of 300 (99 per cent) PCR negative swab tests and identified three PCR positive cases as negative, the researchers said.
Due to low prevalence of the virus among the airport passengers (less than 0.5 per cent), another 155 samples from people who had tested positive on a PCR swab test were also presented to the dogs.
They were able to correctly identify just under 99 per cent of them as positive. According to the researchers, had these ‘spike’ samples been included in the real-life study, the dogs’ performance would have reached a sensitivity of 97 per cent and a specificity of 99 per cent.
The study suggests that trained sniffer dogs would save considerable time and resource as they can be “used both in sites of high SARS-CoV-2 prevalence, such as hospitals (to prescreen patients and personnel), as well as in low prevalence sites, such as airports or ports (to prescreen passengers)”.
Another key finding was that the dogs were less successful at correctly identifying the Alpha variant as they had been trained to detect the wild type. But this just goes to show how good dogs are at distinguishing between different scents, the researchers said.
“This observation is remarkable as it proves the scent dogs’ robust discriminatory power,” the report said.
“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.”