Local transmission of monkeypox wasn’t on my 2022 bingo card, but I guess we’re learning to expect the unexpected. I commonly talk about monkeypox when discussing zoonotic diseases, especially those associated with importation of animals, as this was highlighted by an outbreak of monkeypox in the US associated with imported rodents from Africa in 2003.  However, in the past the focus has always been on zoonotic (animal-to-human) spread. The current situation is making us think about some other issues, and spillback of monkeypox virus into animals from people.

What is monkeypox?

Monkeypox is a potentially nasty disease caused by the monkeypox virus. The virus is in the same family as smallpox, but is nowhere near as transmissible or severe. It’s mainly found in tropical rainforest areas of Central and West Africa.

What is the reservoir of the virus?

You might say “duh… it’s monkeys” but it’s not. It’s another misleadingly named virus, like chickenpox (which doesn’t come from chickens, and isn’t even a poxvirus) or cowpox (the reservoir of which is actually rodents).  We don’t actually know for certain what species is the natural reservoir of monkeypox virus, but it’s likely one or more small mammals in Africa. It was named “monkeypox” because the disease was first reported in monkeys, but monkeys aren’t the true reservoir, they are just (like us) susceptible to infection.

What animal species are susceptible to monkeypox?

We know that a number of species are susceptible through natural or experimental studies. These include:

  • People (obviously)
  • Various non-human primates
  • Prairie dogs
  • Ground squirrels
  • Various rodents (including some pet species, such as rats, although susceptibility of rats seem to vary even by rat species, with some being highly susceptible and others resistant)
  • African hedgehogs

There’s also potentially susceptibility of other species, such as:

  • Rabbits.  There’s some mixed information, with very young rabbits clearly being susceptible and more variable susceptibility in older rabbits. It’s possible that older rabbits are not really susceptible to natural exposure, just artificial exposure like intravenous injection of the virus.
  • Pigs.  Pigs are on various lists of susceptible species, but I haven’t found much detailed information about this yet.

Not much is known about other large mammals either. There’s one report that suggests potential infection of monkeypox from a gazelle, and that gazelles with pox-like lesions were common in one area. However, the evidence is pretty limited.

Guinea pigs and golden hamsters seem pretty resistant to infection.

So, potentially susceptible species include a few domestic species and could include some wild rodents. Missing from the list are common pets such as dogs and cats, and other livestock such as cattle and horses.  A challenge here is differentiating “not susceptible” from “not known to be susceptible”. It’s probably a mix of both – we have species that are truly not susceptible but we also have species that haven’t been studied enough to really know. We also have to interpret older experimental studies with caution because of evolution of the virus. We can probably have confidence in limited or no susceptibility of many common domestic species that are present in areas of Africa where the virus is endemic, since it’s likely there would have been reports of infections in dogs, cattle or goats if they were highly susceptible.

So, realistically, the main concerns are probably with rodents, maybe ferrets, rabbits and potentially pigs.

Can people infect animals with monkeypox?

We don’t know. All the focus to date has been on transmission of the virus in the other direction. Monkeypox virus can be transmitted by droplets and direct contact, but it’s not highly transmissible, so close and prolonged contact is likely required. We have that kind of contact with pets, so we have to assume there’s some risk of exposure. Whether that’s enough to cause infection is the big question. I suspect the risk is low but not zero.

How do we answer some of these questions?

I’d love to look at spillback risks from infected people to their pets. I’ve been working the phones (email actually) to see if anything is being done or whether we can set up some surveillance. It’s not straightforward, as we need access to testing, a way to identify exposed animals, and various approvals. I got my first email about a potential animal exposure today and that’s presumably going to increase.

Hopefully we’ll get some information and not repeat the typical pattern where animal transmission risks get thrown out the window when the focus is on human infections. Spillback into domestic animals here is probably quite unlikely, but it would be nice to have some actual data to understand the risk and what/if control measures need to be used. With emerging infectious diseases, it’s best to over-react (within reason) at the start than to try to play catch up later on.

What can be done to reduce the risk of transmission to animals?

The most important thing is to control the disease in people, at least in the context of what’s going on in North America and Europe, where the issue is now human-to-human spread, versus infection from a wildlife reservoir.

If someone has monkeypox or is concerned they might have monkeypox, using basic control measures make sense including reducing direct contact with animals. Distancing and masking will help reduce droplet spread. Keeping pets out of the bedroom at night makes sense too, given the long contact time in a small airspace. I don’t think drastic measures are needed, just some easy, basic infection control practices that are not overly disruptive but likely reduce any risk that might be present.

What should veterinarians do if an owner calls saying they have monkeypox and are worried about their pet(s)?

At this point, I’d tell owners to stay isolated with their animals, and reduce contact with their animals within the household, to reduce any uncertain risks (but not freak out or do anything drastic). I’d tell them the risks are low for most pet species, but since we don’t know too much, it’s best to take some precautions. I wouldn’t want them to take the animal to a clinic unless it’s sick. That all sounds eerily like the myriad COVID-19 discussions, doesn’t it? It goes to show that infection control is a lot of basics that we just need to do right.

Image: Electron microscopic image of the mokeypox virion (source: CDC Public Health Image Library 22664)