In the summer of 2003, just weeks after an outbreak of monkeypox sickened about 70 people across the Midwest, Mark Slifka visited the “super-spreader,” he told me, “that infected half of Wisconsin’s cases “.
Chewy, a prairie dog, had already succumbed to the disease, which he almost certainly contracted in an exotic animal facility he shared with infected rats from Ghana. But his owners’ other prairie dog, Mono, named for the way he climbed up his cage, contracted the pathogen and survived. “I was a little worried,” said Slifka, an immunologist at Oregon Health and Science University. All the traits that made Monkey a charismatic pet also made him an infectious threat. He hugged and nibbled at the owners of him; when they left home, he wrapped himself in their clothes until they returned. “It was sweet,” Slifka told me. “But I was like, ‘Can Monkey be in his cage when we come over?'”
Slifka came home without smallpox, and the 2003 outbreak fizzled out. But that wave of cases was close: a chance for the virus to establish itself in a new animal host. A long-lasting species jump, similar to what SARS-CoV-2 has turned into white-tailed deer, and monkeypox will be “with us forever” in the US, says Barbara Han, a disease ecologist of the Cary Institute, in New York. In central and western Africa, where the virus is endemic, scientists suspect that at least a couple of rodent species intermittently splash it onto humans. And as the largest monkeypox epidemic in history outside of Africa continues to unfold (more than 2,700 confirmed and suspected cases have been reported in roughly three dozen countries), the virus is now receiving many more shots at the door. This time, we may not be so lucky; the geography of monkeypox could soon change.
Any new jump could reshape the future of this virus and ours. Experts consider the possibility unlikely: “Low risk, but it’s a risk,” says Jeffrey Doty, a CDC disease ecologist. Existing animal reservoirs make some diseases nearly impossible to eliminate; the appearance of new outbreaks could generate future outbreaks in places where they are not currently common. If researchers can identify some of those animals and keep them from mixing with us, we might be able to avoid some of those problems. But that’s a big Yes. With so many susceptible animals, discovering which ones harbor the virus could send researchers on a year-long race, with no clear goal.
Scientists first discovered monkeypox in the 1950s, in two species of monkeys housed in a Danish animal facility; hence the name, which will probably change soon. But in subsequent decades, the best evidence that the virus persists in animals came from rodents in central and western Africa, including rope squirrels, sun squirrels, Gambian rats and dormouse. All signs point to rodents being “responsible for maintaining this virus in the wild,” Doty told me, so he and his colleagues are most concerned with those mammals as they ponder which animals in non-endemic regions may represent the greatest risk in the future.
but a batch of rodents scurry across the planet: about 2,500 species, together making up about 40 percent of known mammals. Although not all species are capable of carrying monkeypox (for example, guinea pigs, golden hamsters, and common mice and rats are not), many of them can.
Building the case for an animal reservoir tends to require years of fieldwork, rigorous safety protocols, and a lot of luck. For some viruses, the reservoir narrative is relatively clear: Hendra virus, an often fatal respiratory infection, typically jumps from bats to horses to people; most hantaviruses, which can cause lethal fevers, establish themselves in one species of rodent each. Monkeypox, however, is much less demanding than that. Experts suspect multiple animals keep the virus seeping into the wild. How many, however, is anyone’s guess.
The gold standard for establishing a reservoir requires isolation of active virus, proof that the pathogen was photocopying within a viable host. But in the wild, “you can break your back and end up getting only five animals of a species,” Han, who has been using machine learning to try to predict possible reservoirs of monkeypox, told me. “And how much are five animals?” They may lack the virus in question, even if other members of their population harbor it; they may have been caught at an age, or during a season, when the pathogen is not present. And among the animals that harbor the virus, a reservoir may not always be the most obvious species: rodents may be among the most frequently detected carriers of monkeypox, but outbreaks in zoos and laboratory experiments have shown that the virus it is capable of infiltrating anteaters, rabbits, and a sizable handful of primates, along with mammals other than mice. In several of these species and others, scientists have found antibodies that recognize poxviruses, suggesting past exposures; they have even discovered the DNA of the virus. Yet only twice has anyone found active viruses in wild animals: a rope squirrel from the Democratic Republic of Congo in the 1980s and a sooty mangabey, found in the Ivory Coast about a decade ago.
Even those cases were not a cake walk. More is needed to “find out which one is a reservoir, versus which ones get infected, but aren’t really responsible for keeping the virus circulating in people,” Jamie Lloyd-Smith, a disease ecologist at UCLA, told me. The fact that an animal can transmit the virus to us does not mean that it will.
For that to happen, humans must have enough contact with animals that exposure is likely, for example in routine bushmeat hunting, or in fractured landscapes where animals forage in and around homes. people. Lloyd-Smith, who has been surveying Congo residents, said parsing out what is risky and what isn’t is harder than it sounds: Most of the people she talks to interact with forest creatures all the time. “It’s not like, ‘Oh, it was the people who ate the salmon mousse at church breakfast,’” he told me. To further complicate matters, wild and domesticated animals can act as intermediaries between humans and a true reservoir, says Stephanie Seifert, a disease ecologist at Washington State University. Researchers sometimes have to traverse webs of interaction, moving through Kevin Bacon-style degrees of separation, to identify the original source.
Revealing those natural origins is key to preventing the virus from moving into new real estate and perhaps breaking existing leases. In West and Central Africa, for example, where some people’s livelihoods depend on hunting and eating wild animals, “you can’t just say, ‘Don’t interact with rodents,'” Seifert told me. But with more research, says Clement Meseko, a veterinarian and virologist who studies the human-wildlife interface at the Nigerian National Veterinary Research Institute, perhaps experts could finally identify just a couple of species and then recommend sustainable alternatives instead. its place. Improved sanitation to keep rodent pests away from humans could also help. The same could happen with the distribution of vaccines to people living in high-risk regions of endemic countries, or perhaps to the wild animals of concern themselves. (Immunizing animals is a pretty lofty goal, but it may still be a better alternative to euthanizing animals, which “often doesn’t work,” Lloyd-Smith said.)
In the US, amid the current wave of monkeypox cases, the CDC has recommended that infected people avoid interacting with pets, livestock, and other animals altogether. Although no cat or dog is ever known to have contracted the infection, “basically we don’t know anything about monkeypox in common companion animals,” Doty said. For now, it’s best to play it safe.
And the most significant way to prevent the virus from developing into a new animal species, Han said, “is to control the human outbreak.” The range of monkeypox species is already formidable, and in today’s world, humans and animals collide more often. Amid the ongoing outbreak, Meseko, who is spending the year completing a fellowship in St. Paul, Minnesota, has been taking note of “how free squirrels are everywhere.” Whatever threat they may pose to us, “animals are also in danger from humans,” he told her.
After all, human activity brought monkeypox to the US in 2003 and a cabal of prairie dogs that included Chewy and Monkey. “They wouldn’t have been exposed geographically without us moving around this virus,” Seifert said. And the human desire for pets brought those prairie dogs into dozens of Midwestern homes. People mobilize disease; our species also represents an immense infectious threat to the planet. The current outbreak of monkeypox, for example, is larger and more human-centric than those documented in the past. And the more opportunities the virus has to infiltrate new hosts, the more opportunities it has to expand its species range. Leakage from animals may not be detected until it is too late; perhaps, some experts pointed out, it happened a long time ago, seeding a reservoir that helped spark the ongoing epidemic. “We don’t have evidence of that right now,” says Grant McFadden, a poxvirus expert at Arizona State University. “But that could change on a dime.”