Stephanie Seifert felt a wave of concern when she learned of the first dog known to have contracted monkeypox from a human. “I have dogs. So I was like, ‘Well, that’s terrible,'” she says.
But Seifert, a viral ecologist at Washington State University in Pullman who studies how viruses jump between species, also understands the case’s potential significance. In the months since the surge in global monkeypox cases that began in May, she and her colleagues have been waiting to hear reports of animals picking up the virus from humans. The first case of human-to-dog transmission was reported in August. The Italian greyhound in France had shared a bed with a couple who had symptoms; the viral DNA from the dog matched that of one of the owners. That same month, the Brazilian Ministry of Health announced a case of a puppy infected with the virus by a human.
The problem is not the odd case of human-to-dog transmission, says Malachi Okeke, a virologist at the American University of Nigeria in Yola. Sick pets can be isolated at home. Scientists are more concerned about the scenario where the monkeypox virus establishes itself in wild animals, such as rodents, outside its usual range in West and Central Africa. Such animal reservoirs can then transmit the virus back to humans. “Then we’re in trouble,” says Okeke. Controlling the spread in wildlife populations would be extremely difficult, he explains, making the virus “impossible to eliminate.”
Monkeypox is known to infect more than 50 species of mammals, according to data compiled by researchers at the University of Liverpool in the United Kingdom. But scientists don’t know the exact reservoir of the virus — the animal or animals that continually carry and spread the virus without getting sick from it. Evidence so far suggests that rodents and other small mammals in Africa – including Gambian marsupials, tree squirrels, rope squirrels and target rats – are responsible for keeping the virus circulating in the wild there. Epidemics of monkeypox among humans have been occurring in parts of Africa for decades.
Many more people have been infected in the past few months than in previous outbreaks, increasing the chances of the virus interacting with animals. World Health Organization figures show the number of weekly reported cases peaked at nearly 7,500 in August; more than 3,400 new cases were confirmed last week.
If the virus has established itself in a rodent population outside of Africa, that could spell trouble, according to a modeling study published on Sept. 111. The model, which mimics how monkeypox spreads, predicts an outbreak in a hypothetical urban area. When the model took into account the existence of a mouse reservoir, it predicted that animal transmission would lead to much earlier peaks and multiple waves.
When human-to-animal and animal-to-human spread are factored into the transmission process, things get much more complicated, says disease model Huaiping Zhu, director of the Canadian Center for Disease Modeling at York University in Toronto and lead study author . Without understanding how animals change transmission dynamics, scientists will struggle to control the spread of the virus and prevent future outbreaks, he says.
Part of the reason scientists don’t know the reservoir of the virus is the lack of active, long-term surveillance for monkeypox in the wild, Okeke says. But there is also a lack of interest. “Because this virus is endemic to so-called resource-poor countries, people didn’t take it seriously,” he adds. “I feel ashamed to say this, but this is the reality.
With little data from the field to show how animals might influence the course of the current epidemic, some scientists are taking other approaches. For example, predicting which species might be more susceptible to infection than others would help officials know where to step up surveillance, says virologist Marcus Blagrove of the University of Liverpool.
Blagrove and his colleagues collected a huge amount of data, including the genetic structure of monkeypox and 62 other poxviruses, as well as characteristics of nearly 1,500 mammals, including their diet, habitat and daily activities. They then trained machine learning algorithms to analyze the information and find potential monkeypox hosts.
Their results, which were published on bioRxiv’s preprint server on August 15 and have not been peer-reviewed2, suggest that two to four times more animal species may be susceptible to infection with the virus than currently known, primarily rodents and primates. “There are many potential hosts around the world,” including in Africa, but also in regions such as Europe, China and North America, Blagrove says.
It is difficult to predict whether these creatures can become reservoirs and shed the virus. Scientists lack important data, such as information about the immune response of potential hosts and direct evidence of the virus moving from these animals to another species, which would suggest the host is a reservoir, Seifert said.
The best way to prevent the monkeypox virus from spreading to more animals and possibly creating a reservoir outside of Africa, Seifert adds, is to stop the spread between people. And the best way to do that is to increase the distribution of vaccines. “That’s how we reduce the likelihood of these rare events happening, we protect people,” she says.