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 weak, small studies rushed out to be first, with inadequate depth and 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 is 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. The source of the father’s infection was unclear, but he noted that his dog had been sick (cough, fatigue), starting 11 days before the onset of his own illness. The dog had been taken to a vet and tested negative for some common respiratory pathogens, but false negatives are very common based on timing of sampling and our inability to test for all causes of respiratory disease.

The owner was then diagnosed with COVID, so the dog was tested, with samples taken 18 days after the onset of the dog’s respiratory disease. All 3 samples taken from the dog were positive for SARS-CoV-2.

Timeframe figure of SARS-CoV-2 infection in a dog and 4 family members, with the dog’s infection preceding human infections.

The timeframe was suggested as being consistent with dog-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 pretty resistant to clinical infection but are commonly infected from their owners. Yet, they don’t seem to be great hosts for the virus. Virus shedding 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 well over a week, and that’s unlikely. It would have also had to have been PCR positive for 18 days after the onset of disease, something I’d consider really unlikely in a dog.

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

Surprisingly, 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 have happened 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. He was PCR positive and antibodies were later detected in the dog’s blood.

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

Where did the dog get infected?

  • That’s an interesting and unresolved question since he had little outside contact given restriction that were in place at the time.
  • Maybe he picked it up from unreported outside contact.
  • Maybe a person in the household had an undiagnosed (potentially asymptomatic infection) with that strain.
  • Maybe he picked it up while visiting the vet 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 vet).

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

Dog image from:

Figure from Hoppe et al, Infection.