On a humid January morning in 2023, deep in the heart of Côte d’Ivoire’s Taï National Park, a baby monkey fell ill. Its skin erupted with angry red lesions, the kind scientists recognize immediately as a possible sign of monkey pox (mpox). Two days later, the infant was dead.
Then, more monkeys got sick. Some died. For a small band of researchers, this tragic scene opened a rare window into one of infectious disease’s longest-standing mysteries: Where does the monkeypox virus hide between outbreaks?
The surprising answer? Squirrels.
A Clue in the Canopy
The mpox virus was first discovered in 1958 in a Danish laboratory monkey, hence the original name. But scientists have long suspected that monkeys are just accidental hosts. The true reservoir—the animal or animals that carry the virus silently, transmitting it without falling ill—has remained elusive.
That changed when a team of international researchers, led by wildlife veterinarian Fabian Leendertz of the Helmholtz Institute for One Health, documented an mpox outbreak sweeping through a community of sooty mangabeys in Taï National Park. What set this investigation apart was the scale of the outbreak—as it affected nearly a third of the group—and its timing. “We were there when it happened,” Leendertz told Nature.
Years of daily monitoring meant the team had a treasure trove of samples: urine, feces, and even tissue from dead animals. One early fecal sample, dated December 6, 2022, came from a mangabey named Bako—the mother of the first infant victim. She tested positive for the mpox virus but never developed symptoms herself.
Her immune silence sparked a new question: Where did she get the virus?
The researchers traced it to a carcass found weeks earlier. It belonged to a fire-footed rope squirrel (Funisciurus pyrropus), a rather common species in the region. The virus extracted from the squirrel’s tissues was genetically identical to the one found in the mangabeys. In the same early fecal sample from Bako, scientists also detected DNA from F. pyrropus—a sign that she had eaten the squirrel.
“It’s unbelievable how well things fit together,” Leendertz said.
A Possible mPox Reservoir, But Not the Whole Picture
For a virus to persist in nature, it needs a reservoir host. That’s what makes this squirrel such an intriguing suspect.
“This study is a landmark contribution to understanding mpox dynamics,” Yap Boum, a biologist at the Africa Centres for Disease Control and Prevention told Science. Boum, who was not involved in the research, believes the work could guide proactive prevention efforts across Africa.
But not everyone is ready to close the case. Some scientists caution that the evidence, while compelling, doesn’t fully prove F. pyrropus is the definitive reservoir.
“To prove that a species is a reservoir host, there must be evidence that most of the animals can maintain and shed the virus without getting sick,” said Délia Doreen Djuicy, a disease ecologist at the Centre Pasteur of Cameroon. “But there is not yet proof of this for F. pyrropus.”
The fire-footed rope squirrel would make sense. A cousin species, Thomas’s rope squirrel (F. anerythrus), was the only animal found to carry the mpox virus in a 1985 survey in what is now the Democratic Republic of the Congo. And antibodies to the virus have turned up in a range of animals across Central Africa—including rodents, monkeys, and even shrews.
The Human Connection
Understanding the role of these squirrels could help prevent future outbreaks, especially in regions where contact between humans and wildlife is increasing.
The mpox virus made global headlines in 2022 when it triggered a major human-to-human outbreak, leading the World Health Organization to declare a global emergency. But in Africa, the virus has circulated for decades, with cases often linked to animal contact—especially through the hunting and consumption of wild meat.
“Consuming wild animals is popular in many parts of Africa for complex reasons, including tradition, subsistence, civil unrest, and commercial demand,” said Leendertz. That, coupled with waning immunity in humans since the end of smallpox vaccination in 1980, has left populations vulnerable.
“Are they being eaten? By who? Are there more contacts between humans and these animals today than before?” asked Martine Peeters, a virologist at the French National Research Institute for Sustainable Development. These are the kinds of questions she believes need urgent answers.
The research, posted as a preprint on the Research Square server, is still awaiting peer review. But it has already energized the scientific community. Whether the fire-footed rope squirrel is the lone culprit or part of a larger web of species remains to be seen.
