More than 120 confirmed or suspected cases of monkeypox, a rare viral disease rarely detected outside Africa, have been reported in at least 11 non-African countries in the past week. The emergence of the virus in separate populations around the world where it does not usually appear has alarmed scientists – and sent them racing for answers.
“It’s great to see this kind of spread,” says Anne Rimoen, a UCLA epidemiologist who has studied monkeypox in the Democratic Republic of the Congo for more than a decade.
Known as monkeypox because researchers first discovered it in lab monkeys in 1958, the virus is thought to be transmitted instead from wild animals such as rodents to humans — or from infected people. On average, a few thousand cases occur in Africa, usually in the western and central parts of the continent. But cases outside Africa have been limited to a handful linked to travel to Africa or the import of infected animals. The number of cases detected outside Africa in the past week alone – which is sure to increase – has already exceeded the number detected outside the continent since 1970, when the virus was first identified as causing disease in humans. This rapid spread is what has scientists on high alert.
But Jay Hooper, a virologist at the Medical Research Institute of Infectious Diseases in Fort Detrick, Maryland, says monkeypox is not SARS-CoV-2, the coronavirus responsible for the COVID-19 pandemic. It is not easily transmitted from person to person, and because it is linked to the smallpox virus, there are already treatments and vaccines on hand to limit its spread. So while scientists are concerned, as any new viral behavior is worrisome – they don’t panic.
Unlike SARS-CoV-2, which spreads through tiny airborne droplets called aerosols, monkeypox is thought to spread from close contact with body fluids, such as coughing saliva. This means that a person with monkeypox is much less likely to infect close contacts than someone infected with SARS-CoV-2, Huber says. Both viruses can cause flu-like symptoms, but monkeypox also causes swollen lymph nodes and, eventually, fluid-filled lesions on the face, hands, and feet. Most people recover from monkeypox within a few weeks without treatment.
On May 19, researchers in Portugal uploaded the first draft genome of the monkeypox virus discovered there, but Gustavo Palacios, a virologist at the Icahn School of Medicine at Mount Sinai in New York City, confirmed that it’s still a very early draft, and there’s more work to be done. To be done before any final conclusions are drawn.
What the researchers can tell from this preliminary genetic data is that monkeypox virus is related to a viral strain found mostly in West Africa. This strain causes milder disease and has a lower mortality rate—about 1% in the poor rural population—than the disease prevalent in central Africa. But how different the strain causing the current outbreaks is from those in West Africa — and whether the viruses that have emerged in different countries are related to one another — remains unknown.
The answers to these questions can help determine whether the sudden rise in cases is caused by a mutation that allows this monkeypox virus to transmit more easily than those in the past, and if each of the outbreaks is due to a single origin, says Raina MacIntyre, An infectious disease epidemiologist at the University of New South Wales in Sydney, Australia. In contrast to SARS-CoV-2, a rapidly evolving RNA virus whose variants have regularly eluded immunity from previous vaccines and infections, monkeypox virus is a relatively large DNA virus. DNA viruses are better at detecting and repairing mutations than RNA viruses, MacIntyre says, meaning it’s unlikely that monkeypox virus could have suddenly mutated to be adept at transmitting infections from humans.
However, the discovery of monkeypox in people who had no apparent connection to one another suggested that the virus may have been spreading silently — a fact that Andrea McCollum, an epidemiologist who heads the US Centers for Disease Control and Prevention team, called “extremely concerning”.
Unlike SARS-CoV-2, which can spread asymptomatically, monkeypox does not usually go unnoticed when it infects a person, in part because of the skin lesions it causes. McCollum says that if monkeypox could spread asymptomatically, that would be especially troublesome because it would make the virus more difficult to track.
Another mystery is why nearly all of the case groups include men between the ages of 20 and 50, many of whom are gay, bisexual, and have sex with men (GBMSM). Although monkeypox is not known to be sexually transmitted, sexual activity certainly forms close contact, says Remoen. The most likely explanation for this unexpected pattern of transmission, MacIntyre says, is that the virus was accidentally introduced into the GBMSM community, and the virus continued to spread there. Scientists will have a better idea of the origin of the outbreaks and risk factors for infection once the epidemiological investigation is complete, which could take weeks and include rigorous contact tracing.
Scientists have been keeping an eye on monkeypox since the campaign to eradicate smallpox, its cousin virus, disappeared in the 1970s. Once smallpox was no longer a threat thanks to vaccinations around the world, public health officials stopped recommending smallpox vaccinations—which also kept monkeypox at bay. With every year that has passed since smallpox was eradicated, MacIntyre says, the number of the population with weakened or unimmunized to these viruses has grown.
There have been a few outbreaks since then. The Democratic Republic of the Congo, for example, has battled monkeypox for decades, and Nigeria has had a major outbreak, with about 500 suspected cases and more than 200 confirmed cases, since 2017, when the country reported its first case in more than 39 years. The United States also reported an outbreak in 2003, when a shipment of rodents from Ghana transmitted the virus to domestic prairie dogs in Illinois and infected more than 70 people.
However, public health authorities are not powerless to tackle monkeypox. As a precaution against bioterrorism, countries like the United States maintain supplies of smallpox vaccines, as well as an antiviral treatment believed to be highly effective against the virus. McCollum says the treatments probably won’t be widely disseminated. Health-care workers might instead use a method called “circular vaccination” to contain the spread of monkeypox: this would vaccinate close contacts of people who have had monkeypox to cut off any routes of transmission.
Based on the data I’ve seen so far, McCollum believes that current outbreaks probably won’t require containment strategies beyond circular vaccination. “Even in areas where monkeypox is common every day, infection is still relatively rare,” she says.