As a journal associate editor and reviewer, I’ve seen lots of papers about SARS-CoV-2 in animals. Some have been great, ground breaking papers. Many have been small, weak studies rushed out to be “first,” with inadequate depth and lacking critical assessment. Some have been a complete disaster. The latter two groups are a concern beyond just being bad science. Shortcuts can lead to bad conclusions when things aren’t studied properly.

A recent study (Hoppe et al, Infection 2023) highlights the need for proper study to avoid bad conclusions. It describes SARS-CoV-2 infection in a household where both people and a dog were identified as infected.

A human (the father) was the first identified case, and the rest of the family subsequently got COVID-19 as well. The source of the father’s infection was unclear, but he noted that his dog had been sick (i.e. coughing, lethargic) starting 11 days before the onset of his own illness. The dog had been taken to a veterinarian and tested negative for some routine respiratory pathogens, but false negatives are very common with these tests based on timing of sampling and our inability to test for all causes of respiratory disease.

After the owner was diagnosed with COVID-19, the dog was tested for SARS-CoV-2. Samples were taken 18 days after the onset of the dog’s respiratory disease, and all 3 samples from the dog were positive for SARS-CoV-2 at that time.

It was suggested that the time frame was consistent with dog-to-human SARS-CoV-2 transmission. I’d say that’s a bit of a stretch based on what we know about infection of dogs, but it’s not impossible. Dogs seem to be commonly infected from their owners, but seem to be pretty resistant to clinical infection and don’t seem to be great hosts for the virus. Virus shedding in dogs tends to be low and of short duration. With a fairly short incubation period in people, the dog would have had to have been infectious for well over a week to be responsible for the father’s infection, and that’s unlikely. The dog also would have had to have been PCR-positive for 18 days after the onset of disease, which I’d consider really unlikely.

However, “unlikely” doesn’t mean “impossible,” so more study was needed. Presumably, at this point, they thought the dog was the source of the family’s infection and were waiting for sequencing results to confirm that.

(Un)surprisingly, the sequencing results didn’t implicate the dog.

There were enough differences in the viral sequences between the dog and owner that the viruses were classified as two different lineages (B.1.1.29 and B.1.1.163, respectively). They concluded the infections were independent, since the series of mutations that would have to happen between the dog and person were “so unlikely, that secondary zoonotic transmission can virtually be excluded.”

It’s pretty clear that the dog was infected. The dog was PCR-positive and antibodies to SARS-CoV-2 were later detected in the dog’s blood.

It’s also pretty clear that the dog wasn’t the source of human infection. The dog’s infection was actually unrelated.

Then how did the dog get infected?

That’s an interesting and unresolved question, since the pet had little outside contact given restrictions that were in place at the time.

• Maybe he picked it up from unreported contact outside the household.

• Maybe a person in the household had an undiagnosed (potentially asymptomatic infection) with the strain that infected the dog.

• Maybe the dog picked up the virus while visiting the veterinary clinic.

It’s impossible to say. That highlights a challenge. Confirming interspecies transmission of a virus that’s widespread in humans is very difficult, and often a really unique set of circumstances need to be present (as was for the cat-human transmission of SARS-CoV-2 that occurred in a Thai veterinarian).

This is a good example of the need to investigate interspecies transmission but the need to do it right. 

Dog image from: http://www.quickmeme.com/meme/3oyz00

Figure from Hoppe et al, Infection 2023.