Trained sniffer dogs can accurately detect airport passengers infected with SARS-CoV-2, the virus that causes COVID-19, according to a study published in the journal BMJ Global Health. This method of detection is likely to be valuable, not only in the early stages of a pandemic when other resources might not yet be available, but also to help contain an ongoing pandemic, the researchers said.
A 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. This just goes to show how good dogs are at distinguishing between different scents, they said. It is thought that dogs are able to detect distinct volatile organic compounds released during various metabolic processes in the body, including those generated by bacterial, viral, and parasitic infections.
The researchers from the University of Helsinki and Helsinki University Hospital, Finland, trained four dogs to sniff out SARS-CoV-2 in 2020. Each of the dogs had previously been trained to sniff out illicit drugs or dangerous goods or cancer.
To test the dogs’ detection skills, 420 volunteers provided four skin swab samples each. The four dogs each sniffed the skin samples from 114 of the volunteers who had tested positive for SARS-CoV-2 on a PCR swab test and from 306 who had tested negative.
The samples were randomly presented to each dog over seven trial sessions. The diagnostic accuracy of all samples sniffed was 92 per cent: combined sensitivity — accuracy of detecting those with the infection — was 92 per cent and combined specificity — accuracy of detecting those without the infection — was 91 per cent.
Some 28 of the positive samples came from people who had had no symptoms, the researchers said. Only one was incorrectly identified as negative and two were not sniffed, meaning that 25 of the 28 (just over 89 per cent) were correctly identified as positive: the lack of symptoms did not seem to affect the dogs’ performance, they said.
The four dogs were then 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.
The PCR and sniffer results 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.
After re-evaluation with clinical and serological data, one was judged to be SARS-CoV-2 negative, one SARS-CoV-2 positive, and one a likely post-infectious positive PCR test result, they said. Similarly, the dogs indicated four PCR negative cases as positive.
These were all judged to be SARS-CoV-2 negative. Because the prevalence of SARS-CoV-2 among the airport passengers was relatively low (less than 0.5 per cent), 155 samples from people who had tested positive on a PCR swab test were also presented to the dogs.
The dogs correctly identified just under 99 per cent of them as positive. 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 researchers said.
Based on these results, they calculated the proportion of true positive results (PPV) and the proportion of true negative results (NPV) in two hypothetical scenarios reflecting a population prevalence of SARS-CoV-2 of 40 per cent and 1 per cent. For the prevalence of 40 per cent, they estimated a PPV of 88 per cent and an NPV of 94.5 per cent.
This means that the information provided by the dog increases the chances of detection to around 90 per cent. For a population prevalence of 1 per cent, on the other hand, they estimated a PPV of just under 10 per cent and an NPV of just under 100 per cent.
In both scenarios, the high NPV backs the use of sniffer dogs for screening, with the aim of excluding people who do not need a PCR swab test, the researchers said. ”Dogs could 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),” they said.
The researchers acknowledged that dogs trained to sniff out other substances may mistakenly identify these substances as SARS-CoV-2 positive. The required storage period of the training and spiked samples may also have affected the viability of the volatile organic compounds, they said.