A new work that is still in the ‘preprint’ phase – that is, it has not yet passed peer review for publication in a scientific journal – suggests that it is possible to train dogs to detect positive cases of COVID-19 much faster than a PCR. According to its authors, people infected with SARS-CoV-2 have a distinctive odor that animals could detect.
Previous studies show that volatile organic compounds (VOCs) released in body odor change during respiratory infections. In this work, chemical analysis of VOCs associated with COVID-19 infection showed a clear distinction between infected and uninfected individuals, suggesting that it has a strong and distinctive odor. These findings could be used to develop a pseudo-odor for standardized dog training.
Double blind trial
To show that, with proper training, dogs can distinguish between the scent of people infected with SARS-CoV-2 and those who are not infected, the researchers designed a double-blind, randomized trial. The dogs were trained for several weeks on the scent samples from individuals who had tested positive for COVID-19, as well as control samples from people who had tested negative. The samples were presented to the dogs in a support system and the dogs were rewarded for correctly indicating a positive sample or for correctly ignoring a negative sample.
Then six dogs were taken to the “double blind” test in which both the dog, the technician and the handler were unaware which samples were positive or negative. This eliminates any risk of inadvertent bias or behavioral cues that the dog may pick up on to indicate the correct answer. The test was carried out with two hundred positive samples and two hundred negative samples.
The highest performing dogs in the test detected the odor of the virus in the samples with up to 94.3% sensitivity (meaning a low risk of false negative results) and up to 92% specificity (meaning low risk of false positive results).
The researchers, from the London School of Hygiene and Tropical Medicine (LSHTM), used more than 3,500 scent samples and combined the analysis of the same with the test with dogs and the development of a mathematical model that highlights the potential that it can have the use of dogs for rapid screening, for example, in airports and other places of flow of travelers. The model suggests that two trained dogs could examine 300 airplane passengers in half an hour. In this way, a pre-selection of people identified by the dogs would be made, who would be the ones that would require a confirmatory PCR test.
The researchers hope to move on to the next phase of the test, in which the dogs will detect COVID-19 in real people in real-world settings. The researchers also believe that these dogs could serve as visual deterrents to reduce the number of passengers traveling with counterfeit COVID-19 negative certificates , a fact that has already been anecdotally observed with drug and explosives detection dogs at public events. “Thinking that we can harness the amazing power of a dog’s nose to detect COVID-19 quickly and non-invasively gives us hope of returning to a more normal way of life through safer travel and access to public places, so that we can get back to socializing with family and friends, ”explains Claire Guest, Scientific Director of Medical Detection Dogs.
Limitations to work
This is not the first work to suggest the potential of using sniffer dogs to detect asymptomatic or mild COVID-19 infections: a few weeks ago the journal PLOS ONE published another study, in which researchers from the Faculty of Veterinary Medicine of the University of Pennsylvania, which showed how trained dogs could detect positives with 96% accuracy, but with a lower sensitivity – ability to avoid false negatives -.
Regarding the work of the LSHTM, it is necessary to remember that it has not yet been validated by peer review. Some experts consulted by the Science Media Center in this regard, while acknowledging that the design is well raised, show their doubts regarding the reliability of the results: “My concern would be the relatively small number of people examined. The results are good in the sample of patients they use, but breath signatures vary widely across the range of gender, age, ethnicity, and most importantly, with other diseases. Its sensitivity and specificity look good with their suite of patient tests, but I don’t know to what extent it can be extrapolated to large populations in less controlled settings , ”says Mick Bailey, professor of comparative immunology at the University of Veterinary School of Veterinary Medicine. Bristol board. “It could happen, for example, that the signature that the dogs are detecting is a general one associated with respiratory viruses such as the flu or a cold, rather than a specific one associated with SARS-CoV-2.”
The expert also raises ethical doubts about the use of dogs for this purpose: “The methods used (open training followed by double-blind tests with masks and socks) are appropriate for a laboratory validation study, but it may be worrying to expose to laboratory workers and dogs on the breath and clothing of persons known to be infected with SARS-CoV-2. The study was approved by the LSHTM Animal Welfare and Ethics committees, but I could not find any description of the methods to inactivate the virus in the samples. Infection of dogs with SARS-CoV-2 has been reported, both asymptomatic and associated with varying levels of disease such as in humans, so deliberately using dogs to sniff the breath and clothing of potentially infected people can put them at risk ”, He reflects.
Virologist and professor of molecular oncology at Warwick University School of Medicine, Lawrence Young, is more optimistic: “The study is very promising. It has been known for a long time that dogs are able to detect diseases ranging from cancer to Parkinson’s disease to the flu through their smell, sometimes even without humans being aware of it. This proof-of-concept study suggests that trained detector dogs could be used in places like airports, sports stadiums, and concert halls to quickly identify positive people who would then take a confirmatory PCR test. It is a carefully conducted study with a large number of samples incorporating a double-blind study and mathematical models ”.
“The big question is, will this approach work in the real world on people rather than on samples of socks and shirts? While the numbers given for the sensitivity and specificity of SARS-CoV-2 detection appear to be higher than those for lateral flow devices, there is dog-to-dog variation, particularly in specificity, which could result in a higher number of false positives. The data suggest that detector dogs can identify asymptomatic individuals with very low levels of virus in their swab samples. A more exhaustive analysis of the sensitivity of canine screening is necessary ”, he concludes.
