test

When mpox (formerly monkeypox) started to surge in Ontario back in 2022, I set up a study to try to look at possible human-to-pet transmission of the causative virus (known as MPXV), because we have very little information about the potential risks to pets, and their potential role in transmission. By the time we got all the animal care and ethics approvals, the outbreak had already slowed down considerably, and I was ultimately able to test a grand total of one (1) potentially exposed dog (it was negative).

The World Health Organization has once again declared mpox a global health emergency in 2024. With mpox is surging again, there (unfortunately) might be more opportunities to once again study transmission of MPXV between people and pets here in Canada and elsewhere. Recruiting pet owners and pets for testing in situations like this is tough, as we’ve seen before with SARS-CoV-2 and H5N1 influenza, but it’s data we really need to inform decisions and recommendations to control the spread of the virus.

Information on human-to-pet mpox transmission is still very limited. Two potentially infected animals were reported during the global outbreak in 2022 – a dog in France and a puppy in Brazil. However, there was some debate about whether positive results were from true infection or just contamination of the pet’s skin with viral bits from the infected humans in the same households.

Fortunately, we now have some new information from the US. An ahead-of-print article in Emerging Infectious Diseases (Morgan et al. 2024), describes an investigation in the US between July 2022 and March 2023. They collected skin swabs and blood samples from pets of people with mpox from 21 households. They sampled 24 dogs, 9 cats and 1 rabbit, obtaining a total of 191 swabs from animals and 56 swabs from the animals’ environments (e.g. beds, bedding, toys, bowls). The results were interesting:

  • MPXV DNA was identified by PCR from five animals (4 dogs, 1 cat) from 4 households. Two positive dogs were from the same household. That’s an overall positivity rate of 17% in dogs and 11% in cats sampled.
  • Multiple positive results were obtained from all of the positive animals. Overall, 12% (22/191) of animal samples were positive.

Breaking it down by body site, positives were from:

  • 29% (4/14) of skin lesion samples
  • 16% (6/37) of ventral skin/fur (stomach) samples
  • 12% (4/33) of dorsal fur (back) samples
  • 11% (4/35) of periocular (around the eyes) samples
  • 8% (3/36) of anorectal (butt) samples
  • 3% (1/36) of oral samples

Researchers also tested for RNase-P as an indicator of human DNA contamination, and all PCR-positive samples were also positive for RNase-P. While that doesn’t mean that the MPXV DNA all came from direct human contamination, it means there was evidence of human contact with the same sites at the same time.

Seven animals had skin lesions, but five of those had skin lesions before the owner got sick, so those cases were likely unrelated. Two pets developed skin lesions after the owner got sick (or at least no one noticed them before that) and both were PCR-negative for MPXV.

Twenty-nine percent (29%) of households had positive animal-associated environmental samples (e.g. dog beds, toys, bowls). All households with positive animals had positive environmental sites, but so did two households without positive animals. Environmental samples are always hard to assess in situations like this, because a positive result could be from contamination from the person or the pet. Environmental samples also don’t tell us risk, since PCR testing will also detect dead virus. It’s more of an indicator of the viral load in the area, and provides some suggestions of contamination routes and what sites might potentially be of concern.

PCR is a really sensitive testing method that can pick up even small bits of dead virus, so the researchers also followed up with viral culture, which is more time consuming, specialized and hazardous, while also being less sensitive – which is why it’s not typically uses it for initial testing. Culture is useful for figuring out if there truly was an active infection, because it only detects live virus. All PCR-positive samples in this study were negative on culture. That’s a bit hard to interpret with confidence.

  • It could mean that infection levels were low, since culture isn’t sensitive.
  • If sampling was done late in infection (which would be common in studies like this, since researchers can’t usually get into households really quickly, then active infection may have waned by the time of sampling.
  • It could be that samples didn’t pick up much virus.
  • It could also mean that PCR-positives were from contamination with dead virus.

Testing for MPXV antibodies in the blood was also performed in 20 dogs at the time of the first site visit, and all were negative. More importantly, 22 dogs were tested 3-4 months later and still had no detectable antibodies. That’s more important because initial negative results might just be because the body hadn’t yet produced detectable antibodies to the virus, but by 3-4 months, antibodies should be present if the animal was previously infected. It’s not a complete guarantee that the dogs weren’t infected, since we don’t know how good the antibody test is or how robust antibody production might be if infection was mild and transient. However, it provides more evidence that PCR-positive results could be due to contamination or a very low (and maybe irrelevant) level of transient infection.

Does this mean dogs and cats can or cannot be infected with MPXV by infected owners?

What this shows is that pets are definitely being exposed from their owners (not surprising) and that it’s possible that some are getting infected. However, it’s unclear whether these were true infections, and even if they were true infections, the low viral load and lack of clear mpox lesions would suggest that the transmission risk posed by pets is probably very low to negligible.

We still need more studies to learn about transmission of this virus to animals. If human-to-pet infection is possible but uncommon, a study of this size might not detect it. Nonetheless it was a great start, and people may underestimate the amount of work that no doubt went into it. My tentative assessment would be that clinically relevant MPXV transmission to pets is uncommon, the health risks to pets from owners with mpox is probably very low, and the risk from exposed pets is close to negligible. But I want more data before I get too confident in that assessment.

What are the challenges with this type of work?

Timely recruitment is a big challenge, because the pet needs to be sampled around the time of active human infection. That means someone has to develop mpox, find out about the study, contact the study team, and then someone needs to go to the household to collect samples, all in a pretty short period of time. For our study, the household also needs to be in close proximity to where we’re based, since I don’t have the budget to fly across the country to get samples. We sometimes get assistance from veterinarians for surveillance studies, but we don’t want infected people taking potentially infected animals into clinics just for sampling, as that creates transmission risks that are hard to justify (versus a person going to the house to collect the samples, while wearing all the appropriate personal protective equipment, of course). So these studies largely rely on active, in-household sampling.

What should be done with pets in households with people who have mpox?

It’s still wise to assume that human-to-pet transmission of MPXV can occur, and therefore take reasonable infection control precautions. The paper sums it up quite reasonably:

  • “More work is needed to determine the susceptibility of companion animals to clade-IIb MPXV. Thus, CDC still recommends that companion animal owners with mpox limit their interactions with their pets while infectious, particularly if they are sharing smaller living spaces. That precautionary measure is recommended until more information is available about the susceptibility of common mammalian companion animal species to mpox.
  • In conclusion, no strong evidence yet exists to suggest that common companion animals, such as dogs or cats, are susceptible to infection with clade IIb MPXV. Given high likelihood for exposure among most of these animals, the paucity of evidence indicating infection might indicate resistance to infection. Nonetheless, to prevent further viral spread and potential evolution and establishment of new endemic areas, during public health emergencies caused by emerging zoonotic diseases, responders should apply a One Health approach to investigate potential spillback of human infections to animals, including pets.”

There is one other important factor to consider: the MPXV strain we’ve been dealing with in humans outside Africa since 2022 is clade II. There recent increase in cases in Africa includes the more transmissible clade Ib virus. The original clade I virus still predominates, but if this more transmissible clade Ib continues to increase in people, there will be more opportunities for it spread outside Africa, and more opportunities for exposure of other animals, including pets. Time will tell whether that becomes and issue; if it does, we’ll need to repeat some research, since we can’t assume clade Ib and clade II have the same risk of infection of animals.