When the first cases of monkeypox were identified in early May, European health authorities were stunned. The virus was not known to spread easily between people, let alone infect dozens – and soon hundreds – of young men.
The origins of the outbreak are now becoming clear. Genetic analysis shows that while the monkeypox virus spreads rapidly outdoors, it has been quietly circulating among humans for years.
Health authorities have already identified two versions of monkeypox in US patients, suggesting at least two different chains of transmission. Researchers in several countries have found cases with no known source of infection, indicating undetected community spread. And a research team argued last month that monkeypox has already crossed a threshold of sustained human-to-human transmission.
Genetic information available to date signal Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle, said the virus has become better at spreading between people over the years.
“Genomic patterns suggest this happened around 2018,” said Dr. Bedford.
Once the virus has adapted to accept humans as hosts, monkeypox outbreaks can be more frequent and more difficult to control. In countries outside of Africa that have been fighting this problem for decades, there is a risk that monkeypox can be transmitted from infected people to animals – most likely rodents. The virus can persist in infected animals and sporadically cause new infections in humans.
“We can also transfer it back to animals, which can transmit disease in wildlife and to humans,” said Sagan Frant, an anthropologist at Pennsylvania State University who has studied human-animal relationships in Nigeria for nearly 15 years. “
dr Fryant said the longer it took to get the virus under control, the more likely it was that people or animals would find permanent new homes.
As of Wednesday, the United States had identified 156 cases in 23 states and the District of Columbia. The global toll has surpassed 3,400 confirmed cases and a further 3,500 cases are being evaluated, triple the number from two weeks ago.
In Africa, eight countries had reported more than 1,500 suspected cases and 72 deaths as of June 10, most of them in the Democratic Republic of the Congo.
Monkeypox is a large double-stranded DNA virus about seven times larger than the coronavirus. DNA-based viruses can correct their mistakes when they replicate their genetic material. They may only accumulate one or two mutations a year, compared to 20 to 30 mutations for RNA viruses like the coronavirus.
But the monkeypox virus appears to have carried an unexpectedly high number of mutations — about 50, according to a preliminary analysis, compared to a variant circulating in 2018.
Of the 47 mutations identified in one analysis, 42 have specific signatures from an enzyme called ApoBEC3. This enzyme, first discovered by HIV researchers, is what’s called a host defense factor — an immune system weapon that animals and humans use to inactivate viruses like monkeypox.
The enzyme essentially forces viruses to make mistakes when trying to replicate, causing them to self-destruct. Mice only carry one version of this enzyme, while humans have seven. dr Bedford said the rapid accumulation of mutations characteristic of the enzyme since 2018 that monkeypox may have altered humans as hosts.
It is not clear how the mutations can alter the virus. According to the UK Health Protection Agency, 21 of the 48 mutations identified in the UK can affect transmission of the disease, its severity and its response to a treatment called tecovirimat.
But since the mutations introduced by the enzyme ApoBEC3 are designed to damage the virus, their level alone is not of concern, said Michael Malim, a virologist at King’s College London who discovered ApoBEC3 in 2002. The effect of the mutation is “more likely to be weakened,” he says. Comparing the current version of the virus with samples from years past can help understand its evolution, but that information is scarce. Nigeria was unable to sequence genetic material until 2017.
Since then, scientists there have analyzed the sequences of about 50 cases of monkeypox, according to Dr. Efedayo Adetifa, Director of the Nigeria Center for Disease Control. But without the special equipment or expertise needed for rapid analysis, scientists have yet to complete their work, he said.
Although the researchers have made multiple requests for data from outside Nigeria, Dr. Adetifa, they would wait for teams with more resources to publish their work to prevent them from overtaking them and gaining recognition.
What’s to know about the monkeypox virus?
Map 1 of 5
What is monkeypox? Monkeypox is a virus found in parts of Central and West Africa. It is similar to smallpox but less severe. It was discovered in 1958 after an outbreak in monkeys kept for research, according to the Centers for Disease Control and Prevention.
What are the symptoms? Monkeypox causes a rash that starts with a flat red spot that rises and fills with pus. Infected people may also have fever and body aches. Symptoms usually appear after six to 13 days but can last up to three weeks and last two to four weeks. Health officials say the smallpox vaccine and other treatments could be used to control the outbreak.
How contagious is it? The virus is transmitted primarily through body fluids, skin contact, and respiratory droplets, although some experts suggest it can occasionally be airborne. It does not usually cause major outbreaks, although this year it has spread in unusual ways and among populations that have not been weakened in the past.
should i be concerned? The virus is more likely to be transmitted through sexual contact, but the risk of transmission through other methods is low. Most people have mild symptoms and recover within weeks, but the virus can be fatal in a small percentage of cases. Studies also suggest that older adults may get some protection from the decades-old smallpox vaccine.
“I’m all for open data sharing and all that,” he said. “The question is who benefits?”
Some experts have warned for years that the eradication of smallpox in 1980 left the world vulnerable to the broader family of smallpoxviruses and increased the chances of monkeypox developing into a successful human pathogen.
In West Africa, the incidence of monkeypox has increased at least twenty-fold since 1986. In African countries in general, said Dr. Adetifa, “we suspect some under-reporting because there is relatively little awareness and perhaps low perceived risk of monkeypox.” He added that Nigeria is stepping up its surveillance of monkeypox and the number of cases could rise as more people become aware of the virus will.
Although monkeypox has a distinctive skin rash that appears on the palms and soles of the feet, it is often confused with chickenpox. Many men in the current epidemic have genital sores, but they can be confused with sexually transmitted infections such as syphilis, gonorrhea, and chlamydia.
Researchers in Italy and Germany have reported finding monkeypox DNA in semen, but it’s unclear whether the virus is transmitted this way or is only present in semen and vaginal secretions.
The prevalence has been observed at least once before in young men with genital ulcers. In 2017, Nigeria recorded 228 suspected cases of monkeypox and 60 confirmed cases. The virus mainly spread to young men with genital ulcers.
The UK experience shows how complex tracing contacts of a sexually transmitted virus can be, particularly in cases where those infected have had many anonymous partners. In a preliminary analysis of a subset of cases, officials said they got the names of fewer than a third of the 78 reported sexual contacts.
Many cases in Africa have been traced to contact with wildlife or the use of animal products in medicinal or cultural practices.
As deforestation and urbanization bring humans and animals closer together, more viruses could make the leap to human hosts. Monkeypox is most likely to spread from rodents to humans. There are about 2,000 species of rodents worldwide, accounting for 40 percent of all mammal species. The African rope squirrel is a prime candidate as the main reservoir for monkeypox, but there are other contenders, including striped rats and dormouse, giant opossums, rustling rats, and porcupines.
In a 2003 outbreak in the United States, a shipment of Gambian opossums imported from Africa transmitted monkeypox to prairie dogs, which then infected 71 Americans. But after the spate of cases across the United States ended, officials found no symptoms of the virus in animals.
There is no guarantee that luck will assist you this time. “These are spreading from other species, and what that means and what the trajectory is — it’s very unpredictable,” said Dr. Malim. “And it’s happening more and more.”