CDC concerned about possible undetected monkeypox spread in U.K.

The Centers for Disease Control and Prevention expressed concern Tuesday about an unusual outbreak of monkeypox in the United Kingdom, suggesting there appears to be at least some undetected transmission of the virus there and warning of the possibility that the outbreak could spread beyond U.K. borders.

“We do have a level of concern that this is very different than what we typically think of from monkeypox. And I think we have some concern that there could be spread outside the U.K associated with this,” Jennifer McQuiston, a senior CDC official, told STAT in an interview.

Seven confirmed and one probable case of monkeypox have been discovered in the U.K. since early May — an unusually large number given that human monkeypox cases are uncommon, and are especially rare outside West and Central Africa. While one of the cases had recently traveled to Nigeria, where monkeypox is endemic, the others appear to have contracted the virus in the U.K. None of the people infected domestically has any known connections with the traveler and the timing of the onset of the cases suggest he was not the source of those infections.

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Complicating the situation — and amplifying the concern — is the fact that the other cases comprise two distinct groups that have no discernible links to one other. Two of the confirmed cases and the single probable case are a family unit, Maria Van Kerkhove, a World Health Organization expert, said at a press conference on Tuesday. The other four confirmed cases, disclosed most recently, were identified by the U.K. Health Security Agency as gay, bisexual, or other men who have sex with men — three from London and a fourth from Newcastle, nearly 300 miles to the north. All four were likely infected in London, the agency said.

It is not known how any of these people contracted the virus. Transmission is thought to occur mainly through virus-laced droplets, but direct contact with lesions or bodily fluids from an infected person, or indirect contact via contaminated clothing or linens, can also result in transmission.

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The fact that there are two apparently unconnected clusters suggests there may be more than one chain of transmission in the country, each of which could contain additional cases that haven’t yet been detected. Van Kerkhove, who leads the emerging diseases and zoonoses unit in the WHO’s Health Emergencies Program, said virus sequencing is underway in the U.K. to try to determine if the cases are genetically linked.

There is no suggestion that monkeypox is spreading to large numbers of people in the U.K. Still, the evidence that there are likely some undetected cases is fueling worry about how much transmission there may be there, and whether the virus has traveled further afield.

“You have two clusters that have no link to travel or to other people who are known to be associated with a recognized outbreak. It suggests that there are unknown chains of transmission happening,” McQuiston said. “If there appears to be unknown chains of transmission, it just puts us on alert to be thinking: Could this be spreading outside the U.K.?”

Although human monkeypox cases outside Africa are rare, in recent years there has been a spate of exported cases to the United States (two in 2021), the U.K., Israel, and Singapore. The four most recent cases were infected with the West African clade of monkeypox, which causes milder disease than the Central African clade, the UKHSA said.

Monkeypox has symptoms similar to but milder than smallpox, which was declared eradicated in 1980. In addition to flu-like symptoms, infection triggers a distinctive rash. Many conditions can cause rashes but the monkeypox rash has some unusual features, notably the fact that vesicles can form on the palms of the hands.

There are no monkeypox-specific drugs or vaccines, but smallpox vaccine has been used in the past to help to stop monkeypox transmission, such as during a 2003 outbreak in the United States traced back to imported exotic animals. In that outbreak, 71 people in six states contracted monkeypox.

In Africa, monkeypox has been fatal in about 1 in 10 cases, with severe disease and death more likely among children.

Because of the discovery of cases among people who are gay, bisexual, or other men who have sex with men, public health authorities in the U.K. have alerted sexual health clinics to be on the lookout for people with unexplained rashes. McQuiston said the CDC is considering sending out a health alert to medical professions and sexually transmitted infection clinics in this country.

“There’s a lot of travel between the U.K. and the United States and other global areas. So I think our concern is that given that you do have four cases among men who have sex with men, that we probably need to be thinking about…