For several millennia dogs have been sharing our daily lives. They help us hunt, they watch over our belongings and they even find missing people. Their role in the medical field is less known but has been growing for the last few years: dogs can be used in cancer detection, seizure prevention or assisting people with handicaps such as visual or motor deficiency. Against the SARS-CoV-2 pandemic, it would not come as a surprise if dogs became precious allies again.
One of the keys in the struggle against the propagation of a virus such as SARS-CoV-2 is the capacity of healthcare systems to quickly detect and isolate contaminated people and people they interacted with in order to break the chain of transmission. The French Scientific Counsel summarizes this process in three words: “Test-Trace-Isolate”. In every announcement they disclose, they insist on a permanent reinforcement of this strategy, which is the only one allowing for an efficient control of the propagation of the virus 1. Numerous epidemiologists regularly warn of the importance of massively testing people whether they present symptoms or not 2. It is therefore essential to find solutions to increase our means to detect contaminated people in order to reduce the sanitary, economic and social impacts of this crisis.
A first option to increase our means of detection is the use of antigenic tests. These tests give a result in 30 minutes, whereas reference RT-PCR tests take 1 to 3 days. These quick tests are based on the detection of viral proteins and are now widely used. Obtainable in drug stores and easy to use, they are however less sensitive than RT-PCR tests 3. A less expected second option is to call in dogs to help. How can animals, even as close to us as dogs are, be able to outperform current technologies?
The answer lies in dogs’ extraordinary olfactory abilities. It is considered that their detection abilities are equivalent to being able to find 1 grain of sand in an Olympic-size swimming pool 4. Their olfactory abilities, extraordinary when compared to those of humans, already make dogs an excellent detection tool in many situations. From the detection of drugs and explosives in train stations and airports to finding missing people during natural disasters, dogs have also been used since the 1990s as a powerful detection tool for several diseases. They can detect cancer, pathogens, neurodegenerative diseases, they can prevent seizures or control blood sugar for people with chronic diabetes 5, 6.
Some “sniffer” dogs can even be more efficient than standard detection methods. The smell which these dogs sense is composed of a set of specific volatile organic compounds (VOC) as these molecules can easily become gaseous. This set is named the volatilome. As a complex association of exogenous substances or substances produced by our body (ingested foods, hygiene products, etc), the volatilome can be excreted in several ways: exhaled air, sweat, urine, faeces or even milk. Dogs’ olfactory capabilities allow them to identify specific smells in the volatilome, which can be linked to physiological states (such as blood sugar), neurological states (seizures) or the presence of parasites, bacteria or viruses. What dogs precisely detect in the volatilome is still unknown, however current research is trying to identify these molecules in order to use them as biological markers.
It was thus only natural that scientists were interested in dogs’ skills to discriminate between a healthy patient and a SARS-CoV-2 contaminated patient who could potentially be contagious. Among them Pr. Dominique Grandjean, a veterinarian, researcher and teacher at the veterinarian school of Alfort, along with his multi-disciplined team (composed of veterinarians, physicians, biologists and dog handlers) already study dogs’ ability to detect some cancers.
Working in close collaboration with firefighters from Seine-et-Marne, Ajaccio and the French-Lebanese University of Beirut, they launched the Nosaïs-Covid19 project in May 2020. Their goal is to provide an answer to the crucial question: can dogs be a tool to detect SARS-Cov-2? The first results announced as early as June 2020 are promising 7. After a three-week training, eight “sniffer” dogs initially trained for searching for explosives, human rescue and colorectal cancer detection were able to discriminate between sweat samples from healthy and virus-contaminated patients. These tests proved themselves fairly accurate with an accuracy varying from 83% to 100% depending on the dog considered. These numbers are on par with PCR and antigenic tests, with even 4 dogs achieving 100% success rates!
Other teams in Germany, Colombia and the United Arab Emirates came to similar conclusions: dogs are able to detect the presence of the virus with excellent sensitivity in various contaminated samples such as sweat, saliva and nasopharyngeal samples 8, 9. This detection is very quick, barely a few seconds, in the conditions of the study. This is a timeframe vastly inferior to the 30 minutes necessary for the antigenic tests and even more compared to the 72 hours for the RT-PCR tests. To learn to report the infected samples, the dogs are first acclimated to the smell of contaminated samples and trained to sit in front of the sample container. Once the smell is learned, the dogs go through several test sessions during which they have to detect contaminated samples among 4 to 10 samples (Figure 1).
These steps use the method of “positive reinforcement” which means that the dog receives their toy or a treat each time they succeed. This kind of motivation is necessary to get the most out of the dogs during training as well as out in the field. A question raises itself: can “sniffer” dogs be contaminated by the virus? Small viral loads were detected in dogs owned by sick people, but numerous studies concluded that the risk of transmission to dogs is near zero 10, 11. Furthermore, unlike with saliva, there is no risk of infection with sweat samples in the test equipment.
Figure 1
However, as with any other test, the use of dogs has its own limits. The first is the high variability in results depending on the race of the dog, and above all between individuals 12. This requires a refined selection of the “sniffer” dogs. A second limit is the impact of the olfactory environment. Between a clean room only dedicated to training and an outside location filled with numerous varied smells the difference can be an important one. Dogs already trained to work in diverse environments such as firefighters’ dogs should nonetheless have no difficulty in the field. Finally, the relationship between the dog and its owner as well as their emotional state have an important part in the detection capabilities, which means that periodic training is necessary to learn and maintain good performances.
With this in mind, are these limitations enough to question the relevance of “sniffer” dogs for the detection of the virus? For researchers, the answer is clearly no. Standard tests also have their limitations 13. Against this pandemic, it is important to use all the reliable ways to detect the virus at a large scale. As early as August 2020, the National Academy of Medicine and the French Veterinary Academy published a joint statement stressing the need to “finish the scientific evaluation and development of this new test in order to deploy it without undue delay” and to “encourage the creation of specialised teams (staff, dogs)” 14.
Several countries have already adopted dogs as a way to detect the virus: dogs have been deployed in airports in Finland and the United Arab Emirates with outstanding preliminary results. In addition to a nearly 100% accuracy, dogs could even detect contaminated people up to 5 days before any symptoms appeared 15. However, in France, not a single dog has been deployed yet even if 10 of them could already be used in the field 16. Pr Grandjean regularly decries the inertia of public authorities who rely on criticism raised by people in the medical field: data are still too scarce and results too variable. As much as this criticism is legitimate and well understood by researchers, they highlight that to have a clear answer data collected in the field are needed as well as funds and easier access to samples from positive patients. Nevertheless some progress needs to be noted.
Near the end of 2020, the World Health Organization endorsed the Nosaïs-Covid19 project, granting them a much needed 200 000 euros budget to continue their research 17. The beginning of 2021 was marked by a new paper published by their team about 21 dogs in the United Arab Emirates which confirms the dogs’ excellent “nose”. Indeed, 15 of them have an accuracy level superior to 90%, and only 3 of them have an accuracy level between 70% and 80% 18. Moreover, the team suggests that by increasing the training duration, the dogs could have an increased accuracy level, but other studies will be necessary to confirm this hypothesis. Finally, in France, dogs from the Handi’chiens association, which help handicapped people, should soon be trained for this task, then be deployed in nursing homes 16. The benefits would be significant: a cheap detection tool, a daily support and a companion for these isolated people who are particularly affected by all the sanitary measures.
In the end, more and more data show that “mankind’s best friend” can discriminate between healthy people and virus SARS-CoV-2 infected people who are potentially contagious, allowing the development of a new screening technique. Even if “sniffer” dogs cannot replace reference tests (RT-PCR) they can be on par with the antigenic tests. They can be a valuable complementary aid in the “Test-Trace-Isolate” strategy thanks to their speed, great sensitivity and accuracy. Their ease of deployment in situations in which people are already used to seeing them work are also a great asset, for instance in highly frequented locations (train stations, airports) and around people at risk.
References
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This article was specialist edited by Pr Herve Bourhy, copy edited by Elsa Charifou and translated by Emile Auria