A colleague asked me about scent detection dogs the other day. My response was that I hadn’t heard much after all the initial buzz, which might suggest things weren’t going well. However, as opposed to the horrible pre-print about COVID-19-sniffing dogs I wrote about previously, a paper in BMC Infectious Diseases (Jendry et al. 2020) provides some more robust and interesting information. It’s a pilot study, so it’s small, preliminary and underpowered, but it shows potential. Whether that’s “potential for dogs to be able to detect SARS-CoV-2 under certain circumstances” or “potential for dogs to be an effective detection tool” isn’t clear, but that’s the big question.
Here’s a breakdown of the study and some commentary:
The researchers collected saliva samples and respiratory secretions from hospitalized COVID-19 patients, and healthy people who were PCR-negative for the SARS-CoV-2 virus.
- This may not be ideal, depending on the goal. My vision is using these dogs in the community to rapidly detect infectious people in high risk situations (e.g entrance to transit stations, public buildings, schools). In that case, people who are hospitalized with severe COVID-19 are likely not the best test population. A dog isn’t going to replace a PCR machine in the hospital. It’s simply not practical in most cases to collect a sample from a patient, take it to a dog as a quick screening test, and then submit the sample for definitive testing. We want dogs that can detect a mild case in the community, long before the patient needs to be hospitalized.
- They didn’t test samples for other human coronaviruses, like those that cause the common cold. It’s a potential limitation, but I don’t think it’s a big deal in this case.
- They also don’t explain where they got their negative samples. A clear description of the study populations is critical and it’s somewhat lacking here. We want to be sure the dogs were detecting SARS-CoV-2 and not something else unique to the positive sample population, like a smell associated with being from a hospital.
Because of the potential susceptibility of dogs to the SARS-CoV-2 virus, samples were inactivated prior to exposing them to the dog.
- That’s a reasonable step, but raises more issues of practicality and how the dogs could ultimately be used (e.g. can the dogs only be used to screen specimens collected from high risk patients, or can they be used to detect infection in someone walking by).
Eight dogs were trained using standard methods. They had a 2-week habituation process for the training system, then had 5 days of training until their rate of detection was greater than what would be expected by chance alone. They then started the study
- The sample size was small, but reasonable for a proof-of-principle study.
The ability of dogs to detect positive samples increased over time. There was some variation between dogs, but all of them were pretty good. The overall sensitivity (percentage of positive samples that the dogs correctly identified as positive) was 83%, ranging from 70-95%. The specificity (percentage of negative samples that the dogs correctly identified as negative) was 96%, ranging from 92-99%.
- For a screening test, we’d actually want the reverse, that is to say higher sensitivity at the expense of specificity. That would mean the dogs would catch most of the positives. Lower specificity is okay initially if the screening test (i.e. the dog sniffing) can be followed up with a more specific test, and if the implications of an initial false positive aren’t high. If a dog calling a person positive results in that person being sent home to self-isolate for 14 days, then a high false positive rate is a problem. If it just results in the person being pulled aside to have a swab collected for a lab test, that’s not as big of a deal (perhaps a bit of a hassle but maybe not a deal breaker).
- A low sensitivity and high specificity means you run into fewer hassles with false positives, but the test will miss more positive people. The fact that 17% of prime samples from people hospitalized with active COVID-19 were called negative is a concern in terms of the dogs being able to detect the virus in less severely affected people and from less voluminous and close samples (e.g. detection directly in someone walking by).
I’d file this in the “interesting but preliminary” folder. Anything that can help identify infectious people is useful. If dogs can do it, that’s great, but they also have to be able to do it from a distance, because a handler and a dog getting very close to large numbers of people might cause more problems than they fix.
In my perfect world:
- A SARS-CoV-2-sniffing dog would be parked at the entrance of schools, office buildings, transit stations, etc.
- The dog would be able to detect infected people from a short distance away (i.e. without direct contact).
- The dog would signal its handler when it detected a positive person.
- That person would then (discretely) be pulled aside for testing, which would (in my perfect world) be done quickly, right there (there is lots of work being done to develop a more rapid test like this that can be done on the spot, but we don’t have one yet).
- If positive, the person would be told right away and sent home to self-isolate. If negative, the person would be good to go (though maybe wondering why they smell like a coronavirus).
As I’ve said, it’s an interesting and useful preliminary study that shows potential. The key is to follow up preliminary studies with more detailed, rigorous work, which unfortunately often doesn’t get done. Nonetheless, I suspect media headline writers will jump on this and over-interpret the results. It’s also another example of the remarkable things a dog’s nose can do, but the potential practical applications (if any) are still very much up in the air. I’ll be a bit surprised if this ever becomes a common/useful tool, but I’d love to be wrong about that.