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.

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.

Controlling the spread in wild animal populations would be extremely difficult, he explains, making the virus “impossible to eliminate”.

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.

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.

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.

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.

“Because this virus is endemic to the so-called resource-poor countries, people didn’t take it seriously,” he adds.

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 primatesJ

“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.

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.

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