The first known monkeypox infection in a pet dog hints at spillover risk

The first recorded case of a person passing monkeypox to a dog could be a harbinger of other animals catching the sometimes disfiguring and deadly virus. If that happens, monkeypox could establish animal reservoirs outside of Africa for the first time.

Two men in France appear to have spread monkeypox to their Italian greyhound, researchers report August 10 in the Lancet. The men reported letting the dog sleep in bed with them.

Monkeypox can spread through skin-to-skin contact, such as the intimate contact that happens during sex. Even more casual contact such as dancing in close confines can spread the virus, an Aug. 15 study in Emerging Infectious Diseases suggests. So can contact with objects an infected person has used, including bedding and clothing. Infectious monkeypox viruses linger more often on such soft, porous materials than on hard surfaces, researchers report August 11 in Emerging Infectious Diseases. Some 60 percent of soft goods and 5 percent of hard surfaces tested still carried viable virus for at least 15 days, the team found.  

In the case of the dog, the animal developed pustules about 12 days after its owners reported symptoms. Viral DNA from one of the men matched that from the dog, suggesting that the human had given monkeypox to an animal.

Usually monkeypox goes the other way, from animals — especially rodents in some parts of Africa — to people in “spillover,” or zoonotic, infections. “This is a classic case of reverse zoonoses,” or spillback, in which a viral disease hops from humans back into animals, says Grant McFadden, a pox virologist at Arizona State University in Tempe.

Such spillback events are fairly common with other viruses; people are known to have given COVID-19 to dogs, cats and zoo animals, for instance (SN: 3/5/20; SN: 3/31/20). Some pox viruses, including cowpox, can infect a wide range of species, while others like smallpox and a rabbit pox virus called myxoma virus can infect only one or a few species.

How widely monkeypox can spread among nonrodent animal species isn’t known. Researchers have documented that the virus can infect 51 species, including apes and monkeys, and other animals including anteaters, porcupines and opossums.

Right now, monkeypox is endemic in some parts of Africa. But some scientists worry that the global outbreak, which has infected more than 36,000 people so far, creates more chances for the virus to jump from humans to animals. If that happens, the virus could become established in animal populations around the world, setting up new reservoirs that could cause repeated infections in humans and animals.

Preliminary research suggests that monkeypox may be able to infect two to four times more species than previously thought, researchers from the University of Liverpool in England report August 15 in a preprint on bioRxiv.org. The team used machine learning trained to consider the genetic makeup of the virus, the number of species of animals in a genus known to be infected by pox viruses, diet composition of potential hosts, where the animals live and other factors that could contribute to a species becoming a new host for monkeypox, says virologist Marcus Blagrove.

About 80 percent of the potential new hosts for monkeypox are rodents or primates, the researchers predict. But domestic animals like dogs and cats were also predicted to be susceptible to infection. The researchers didn’t know about the case of the dog in France when they made the prediction, Blagrove says, so the report of the canine infection “was a quite nice validation that the method works.”

Red foxes and brown rats are two potential monkeypox hosts that the researchers say are particularly worrisome. Foxes (Vulpes vulpes) scavenge from garbage, which could bring them in contact with monkeypox-contaminated items. Brown rats (Rattus norvegicus) are already known hosts for cowpox. They are common in sewers in Europe and could get infected through feces containing monkeypox. Though the study emphasized the risk in Europe, where more than half the monkeypox cases in people in the current outbreak have been reported, the findings could be applicable more widely. Brown rats are found on every continent except Antarctica. Red foxes roam much of the Northern Hemisphere, including North and Central America, Europe, Northern Africa and parts of Asia.

The study also names three European rodents that could become reservoir species. The herb field mouse (Apodemus uralensis), yellow-necked field mouse (Apodemus flavicollis) and Alpine marmot (Marmota marmota) all have pockets of big populations that could be ideal for passing the virus around.

“These are examples of wild animals that might be a reservoir. We can’t say for certain, but they might be susceptible,” Blagrove says. Those species along with foxes and brown rats should be regularly surveyed for monkeypox to prevent new reservoirs from being established, he says.

But just because an animal can get infected with monkeypox doesn’t mean they can pass it on. “There is a difference between accidental hosts and a reservoir,” says Giliane de Souza Trindade, a pox virologist at the Federal University of Minas Gerais in Brazil. Accidental hosts are often dead ends for the virus. A true reservoir species must be able to pass the virus from animal to animal, and then sometimes to humans they encounter.  

If dogs can easily get monkeypox, they may be able to pass it to humans, other dogs or other animals through feces or saliva, Trindade says. Domestic animals that live with people who get monkeypox should be isolated from sick people and from other animals outside the home, she says.

Trindade and her colleagues are preparing to study pets of people who have monkeypox to see whether the virus passes easily to cats and dogs, she says. But she is more worried about live animal markets. “Animals are in cages very close together and people are passing by all the time.” Such settings are ripe for transmitting viruses between species. The COVID-19 pandemic probably got its start at a live animal market in Wuhan, China, researchers reported July 26 in Science.  

McFadden stresses that the dog’s case is still an isolated report. “We don’t know is this a rare thing or have we just not paid attention to it?” For now, he says, efforts should be focused on containing the outbreak among humans. While people with monkeypox should take care not to pass the virus to their pets, this case shouldn’t cause undue worry, he says. “We’re not at the panic button stage just yet.”

Scientists are also still learning how monkeypox spreads among people. Some people may have monkeypox, but not develop symptoms, researchers report August 16 in the Annals of Internal Medicine. It’s not known whether asymptomatic people can transmit the virus to others, but if they can, vaccinating close contacts of symptomatic people may not be enough to contain the outbreak, the researchers warn.