The monkeypox virus (shown here in color transmission electron micrograph) is closely related to the smallpox virus. Credit: United Kingdom Health Security Agency / Scientific Photo Library
More than 120 confirmed or suspected cases of monkeypox, a rare viral disease rarely found outside Africa, have been reported in at least 11 non-African countries in the past week. The appearance of the virus in some populations around the world, where it does not usually occur, has alarmed scientists – and forced them to compete for answers.
“Eyes are open to see this kind of spread,” said Anne Remoin, an epidemiologist at the University of California, Los Angeles who has studied monkeypox in the Democratic Republic of the Congo for more than a decade.
Called monkeypox because researchers first discovered it in laboratory monkeys in 1958, the virus is thought to be transmitted from wild animals as rodents to humans – or from infected humans. On average, several thousand cases occur in Africa each year, usually in the western and central parts of the continent. But cases outside Africa are limited to a handful, which involve traveling to Africa or importing infected animals. The number of cases found outside Africa in the last week alone – which is almost certain to increase – has already surpassed the number found outside the continent since 1970, when the virus was first identified as causing disease in humans. This rapid spread is what keeps scientists on the alert.
But monkeypox is not SARS-CoV-2, the coronavirus responsible for the COVID-19 pandemic, said Jay Hooper, a virologist at the U.S. Army’s Institute for Infectious Diseases Medical Research in Fort Detrick, Maryland. It is not so easily transmitted from person to person, and because it is linked to the smallpox virus, treatments and vaccines are now available to limit its spread. So while scientists are concerned because any new viral behavior is alarming – they are not panicking.
Unlike SARS-CoV-2, which is spread by small airborne droplets called aerosols, monkeypox is thought to be spread by close contact with body fluids, such as cough saliva. This means that a person with monkeypox is likely to infect far fewer close contacts than someone with SARS-CoV-2, Hooper said. Both viruses can cause flu-like symptoms, but monkeypox also causes enlarged lymph nodes and ultimately distinctive fluid-filled lesions on the face, arms and legs. Most people recover from monkeypox in a few weeks without treatment.
On May 19, researchers in Portugal uploaded the first draft of the monkeypox virus genome, which was discovered there, but Gustavo Palacios, a virologist at Icahn Medical School in Mount Sinai in New York, stressed that this was still a very early option, and more work needs to be done before final conclusions can be drawn.
What researchers can learn from these preliminary genetic data is that the monkeypox virus is associated with a viral strain that is found mainly in West Africa. This strain causes a milder disease and has a lower mortality rate – about 1% in poor, rural populations – than that circulating in Central Africa. But how exactly the strain that causes the current outbreaks differs from that in West Africa – and whether the viruses that occur in different countries are related to each other – remains unknown.
The answers to these questions could help determine whether the sudden increase in cases is due to a mutation that allows this monkeypox virus to be transmitted more easily than in the past, and whether each outbreak leads to an origin, he said. Raina McIntyre, an epidemiologist on infectious diseases at the University of New South Wales in Sydney, Australia. Unlike SARS-CoV-2, a fast-growing RNA virus whose variants regularly evade immunity from vaccines and previous infection, monkeypox virus is a relatively large DNA virus. DNA viruses are better at detecting and repairing mutations than RNA viruses, which means it’s unlikely that the monkeypox virus suddenly mutated to become good at transmitting to humans, McIntyre said.
“Deeply concerned”
However, the discovery of monkeypox in people with no apparent link suggests that the virus may have spread silently, a fact that Andrea McCollum, an epidemiologist who leads the US Poxvirus Control and Prevention Center, calls “deeply concerned. “
Unlike SARS-CoV-2, which can spread asymptomatically, monkeypox usually does not go unnoticed when it infects humans, in part because of the skin lesions it causes. If monkeypox could spread asymptomatically, it would be particularly worrying because it would make the virus harder to track, McCallum said.
Another puzzle is why almost all groups of cases include men aged 20-50, many of whom are gay, bisexual and have sex with men (GBMSM). Although it is not known that monkeypox is sexually transmitted, sexual activity is certainly close contact, Rimoin said. The most likely explanation for this unexpected pattern of transmission, McIntyre says, is that the virus was accidentally introduced into the GBMSM community and the virus continued to circulate there. Scientists will have a better idea of the origins of outbreaks and risk factors for infection once the epidemiological investigation is completed, which can take weeks and includes strict follow-up of contacts.
Restriction strategies
Scientists have been keeping an eye on monkeypox since the campaign to eradicate smallpox, its cousin virus, ended in the 1970s. Since smallpox is no longer a threat thanks to global vaccinations, public health officials have stopped recommending smallpox vaccination – which also protected monkeypox. With each passing year since the eradication of smallpox, the population with weakened or no immunity to these viruses is growing, McIntyre said.
There have been several outbreaks since then. The Democratic Republic of the Congo, for example, has been battling monkeypox for decades, and Nigeria is experiencing a major outbreak, with about 500 suspects 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 epidemic in 2003 when a shipment of rodents from Ghana spread the virus to domestic prairie dogs in Illinois and infected more than 70 people.
Yet public health authorities are not powerless against monkeypox. As a precaution against bioterrorism, countries such as the United States maintain supplies of smallpox vaccines as well as antiviral treatments that are thought to be very effective against the virus. However, therapies are unlikely to be deployed on a large scale, McCollum said. Instead, health professionals are likely to use a method called ring vaccination to limit the spread of monkeypox: it will vaccinate close contacts of people who have been infected with monkeypox to cut off all routes of transmission.
Based on the data she has seen so far, McCollum believes the current outbreaks are unlikely to require containment strategies outside of ring vaccination. “Even in areas where smallpox occurs every day,” she says, “it’s still a relatively rare infection.”
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