Close-up of a grey and white Italian greyhound looking away while wearing a collar.

An Italian greyhound in France was the first dog reported to have caught monkeypox.Credit: Getty

Stephanie Seifert felt a surge of anxiety when she learnt about the first dog known to catch monkeypox from a human. “I have dogs. So I was, like, ‘Well, that’s awful,’” she says.

But Seifert, a viral ecologist at Washington State University in Pullman who studies how viruses jump between species, also understood the potential significance of the case. In the months since the surge of global monkeypox cases, which started in May, she and her colleagues waited to hear reports of animals catching the virus from people. The first case of human-to-dog transmission was reported in August. The Italian greyhound in France had slept in the same bed with a couple who had symptoms; viral DNA from the dog matched that from one of the owners. The same month, Brazil’s health ministry announced the case of a puppy catching the virus from a person.

The problem is not the odd case of human-to-dog transmission, says Malachy Okeke, a virologist at the American University of Nigeria in Yola. Sick pets can be isolated at home. Scientists are more worried about a scenario in which the monkeypox virus becomes established in wild animals, such as rodents, outside its usual range in West and Central Africa. Such animal reservoirs could then transmit the virus back to people. “Then we are in trouble,” says Okeke. Controlling the spread in wild animal populations would be extremely difficult, he explains, making the virus “impossible to eliminate”.

Animal carriers

Monkeypox is known to infect more than 50 species of mammal, according to data compiled by researchers at the University of Liverpool, UK. But scientists don’t know the virus’s exact reservoir — the animal or animals that continuously carry and spread the virus without becoming ill from it. The evidence so far indicates that rodents and other small mammals in Africa — including Gambian pouched rats, tree squirrels, rope squirrels and target rats — are responsible for keeping the virus circulating in the wild there. Monkeypox outbreaks in people have been cropping up in parts of Africa for decades.

Many more people have been infected in the past few months than in previous outbreaks, thereby increasing the chances of the virus interacting with animals. Data from the World Health Organization indicate that the number of weekly reported cases reached a peak of nearly 7,500 in August; more than 3,400 new cases were confirmed last week.

If the virus did establish itself in a rodent population outside Africa, that could mean trouble, according to a modelling study published on 11 September1. The model, which mimics how monkeypox spreads, forecast an outbreak in a hypothetical metropolitan area. When the model factored in the existence of a mouse reservoir, it predicted that animal transmission would drive much earlier peaks and multiple waves.

When human-to-animal and animal-to-human spread are factored into the transmission process, things become much more complicated, says disease modeller Huaiping Zhu, director of the Canadian Centre for Disease Modelling at York University in Toronto, and the study’s lead author. Without an understanding of how animals change the transmission dynamics, scientists will struggle to control the virus’s spread and prevent future outbreaks, he says.

Virus surveillance

Part of the reason scientists don’t know the virus’s reservoir is a lack of active, long-term surveillance for monkeypox in the wild, says Okeke. But there’s also the lack of interest. “Because this virus is endemic to the so-called resource-poor countries, people didn’t take it seriously,” he adds. “I feel ashamed to say this, but that’s the reality.”

With few data from the field to indicate how animals might affect the course of the current outbreak, some scientists are taking other approaches. For instance, predicting which species might be more susceptible to infection than others would help officials to know where to increase surveillance, says virologist Marcus Blagrove at the University of Liverpool.

Blagrove and his colleagues gathered massive amounts of data, including the genetic structure of monkeypox and of 62 other poxviruses, and characteristics of nearly 1,500 mammals, including their diet, habitat and daily activities. They then trained machine-learning algorithms to analyse the information and locate potential monkeypox hosts.

Their results, which were posted to the bioRxiv preprint server on 15 August and have not been peer reviewed2, suggest that two to four times more animal species might be susceptible to infection with the virus than are currently known to be, mostly rodents and primates. “There are a lot of potential hosts all over the world,” including in Africa, but also in regions such as Europe, China and North America, says Blagrove.

Whether these creatures could become reservoirs and shed the virus is hard to predict. Scientists lack important data, such as information about the potential hosts’ immune response and direct evidence of the virus passing from those animals to another species, which would suggest that the host is a reservoir, says Seifert.

The best way to prevent the monkeypox virus from spilling over into more animals, and possibly establishing a reservoir outside Africa, Seifert adds, is to stop the spread between humans. And the best way to do that is to ramp up vaccine distribution. “That’s how we reduce the likelihood of these rare events from happening, is we protect people,” she says.