When the first cases of smallpox in monkeys were identified in early May, European health officials were perplexed. The virus was not known to spread easily among people, let alone infect dozens, and soon hundreds, of young men.
The origins of the outbreak are becoming increasingly clear. Genetic analysis suggests that, although the monkeypox virus is spreading rapidly in the open air, it has been circulating silently among people for years.
Health officials have already identified two versions of the monkey’s smallpox among American patients, suggesting at least two separate transmission chains. Researchers from several countries have found cases with no known source of infection, indicating an undetected community spread. And a research team argued last month that monkeypox had already crossed the threshold of sustainable person-to-person transmission.
The genetic information available so far indicated that at some point in recent years, the virus has spread better among people, said Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle.
“Genomic patterns would suggest that this happened around 2018,” said Dr. Bedford.
If the virus has adapted to include people as hosts, monkeypox outbreaks could be more frequent and more difficult to contain. This carries the risk that monkeypox can be shed from infected animals to animals, probably rodents, in countries outside Africa, which have struggled with this problem for decades. The virus can persist in infected animals, sporadically causing new infections in people.
“We can also transmit this to animals that can spread the disease to wildlife and to humans,” said Sagan Friant, an anthropologist at Pennsylvania State University who has studied human-animal interactions in Nigeria for about 15 years.
The longer it takes to contain the virus, the greater the chances of finding a new permanent home in people or animals, Dr. Friant said.
As of Wednesday, the United States had identified 156 cases in 23 states and the District of Columbia. The overall number has exceeded 3,400 confirmed cases and another 3,500 cases are being evaluated, tripling 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 Congo.
Monkeypox is a large double-stranded DNA virus, about seven times larger than the coronavirus. DNA-based viruses can correct their own mistakes when replicating their genetic material. They can pick up only one or two mutations per year compared to 20 to 30 mutations for an RNA virus like coronavirus.
But the monkeypox virus appears to have accumulated an unexpectedly high number of mutations – about 50 compared to a version that circulated in 2018, according to preliminary analyzes.
Of the 47 mutations identified in one analysis, 42 bear the different signature of an enzyme called Apobec3. This enzyme, first discovered by researchers studying HIV, is the so-called host defense factor, a weapon in the immune system that animals and people use to disarm viruses such as monkeypox.
The enzyme essentially forces viruses to make mistakes when they try to replicate, causing them to self-destruct. Mice carry only one version of this enzyme, while humans have seven. The rapid accumulation of mutations, characteristic of the enzyme since 2018, suggests that the monkey’s smallpox may have passed to people as hosts at the time, Dr. Bedford said.
It is unclear how the mutations could change the virus. Of the 48 mutations identified in Britain, 21 may affect how the disease spreads, its severity and how it responds to a treatment called tecovirimat, according to the UK Health Safety Agency.
But because the mutations introduced by the enzyme Apobec3 are intended to damage the virus, its amount alone is not worrisome, said Michael Malim, a virologist at King’s College London who discovered Apobec3 in 2002. mutations are “more likely to be debilitating.” “He said. Comparing the current version of the virus with samples from recent years may help to understand how it has evolved, but this information is scarce. Nigeria did not have the capacity to sequence genetic material until 2017.
Since then, scientists have been analyzing the sequences of about 50 cases of smallpox in the monkey, according to Dr. Ifedayo Adetifa, director of the Center for Disease Control of Nigeria. But without the specialized equipment or experience needed for a quick analysis, scientists have not yet finished their work, he said.
Although researchers have sent several requests for data from outside Nigeria, Dr Adetifa said they would wait to publish their work to prevent teams with more resources from surpassing them and getting credit.
What you need to know about the monkeypox virus
Card 1 of 5
What is monkeypox? Smallpox is a virus endemic to parts of central and western Africa. It is similar to smallpox, but less severe. It was discovered in 1958, after outbreaks occurred in monkeys kept for research, according to the Centers for Disease Control and Prevention.
What are the symptoms? Monkeypox creates a rash that begins with flat red marks that rise and fill with pus. Infected people can also have fever and body aches. Symptoms usually appear between six and 13 days, but can take up to three weeks after exposure and can last between two and four weeks. Health officials say smallpox vaccines and other treatments can be used to control an outbreak.
How infectious is it? The virus spreads primarily through body fluids, skin contact, and respiratory droplets, although some experts suggest that it could occasionally pass through the air. It does not usually cause major outbreaks, although this year it has spread in unusual ways and among populations that have not been vulnerable in the past.
Do I have to worry? The likelihood of the virus spreading during sexual contact is high, but the risk of transmission in other ways is low. Most people have mild symptoms and recover within a few weeks, but the virus can be fatal in a small percentage of cases. Studies also suggest that older adults may have some protection against smallpox vaccines from decades ago.
“I’m in favor of open data sharing and all that,” he said. “The question is, who benefits?”
Some experts have warned for years that smallpox eradication in 1980 left the world vulnerable to the wider family of poxviruses and increased the chances of monkeypox evolving into a successful human pathogen.
In West Africa, the incidence of monkeypox has increased at least twenty times since 1986. In general, in African countries, said Dr. Adetifa, “we suspect there have been few reports because there has been a relatively low awareness and perhaps a perceived low risk of smallpox.” Nigeria is stepping up its surveillance for monkeypox and the number of cases may increase as more people become aware of the virus, he added.
Although chickenpox has a distinctive rash that appears on the palms of the hands and soles of the feet, it is often confused with chickenpox. Many men in the current outbreak have genital lesions, but these can be confused with sexually transmitted infections such as syphilis, gonorrhea and chlamydia.
Researchers in Italy and Germany have reported that they have found monkeypox DNA in semen, but it is unclear whether the virus spreads in this way or is only present in semen and vaginal secretions.
The spread among young men with genital ulcers was observed at least once before. In 2017, Nigeria recorded 228 suspected cases of monkeypox and confirmed 60. The virus was spread mainly among young men who had genital ulcers.
Britain’s experience indicates how complicated it can be to track contacts of a sexually transmitted virus, especially in cases where infected people have had multiple anonymous partners. In an initial analysis of a subset of cases, officials said they were able to obtain names for less than a third of the 78 reported sexual contacts.
Many cases in Africa go back to contact with wild animals or the use of animal products for medicinal or cultural practices.
As deforestation and urbanization drive people and animals to closer places, more viruses can make the leap to human hosts. Smallpox is more likely to jump on rodents. There are about 2,000 species of rodents worldwide, which make up 40 percent of all mammal species. The African rope squirrel is one of the main candidates as the primary reservoir of monkeypox, but there are other contenders, such as striped mice and dormice, giant rats, rusty-nosed rats and brush-tailed porcupines.
In a 2003 outbreak in the United States, a shipment of shrimp rats imported from Africa transmitted smallpox to monkeys in prairie dogs, which later infected 71 Americans. But officials found no signs of the virus in U.S. animals after the series of cases ended.
There is no guarantee that luck will endure this time. “These overflows from other species, and what that means and what the trajectory is, is very unpredictable,” Dr. Malim said. “And that’s happening more and more.”