Health officials are investigating the links between four new cases that have been identified in London and the northeast of England, none of which have known connections to the previous three infections detected by the UK Health Security Agency.
As a typically mild, self-limiting viral disease which does not easily spread between people, the risk posed to the general UK population is low, UKHSA said.
But the sudden emergence of cases, and uncertainties around how and where the individuals contracted the virus, has left scientists puzzled, with sexual contact now seen as a likely route of transmission.
“This outbreak of monkeypox is unprecedented in the UK and has provoked urgent public health action,” said Jimmy Whitworth, a professor of international public health at the London School of Hygiene & Tropical Medicine.
Dr Michael Head, a senior research fellow in global health at the University of Southampton, said: “There’s currently gaps in our knowledge, and the contact tracing and public health investigation being carried out by UKHSA will no doubt reveal more in due course, for example about how pattern of transmission.”
He added: “However, it would be very unusual to see anything more than a handful of cases in any outbreak, and we won’t be seeing Covid-style levels of transmission.”
Four out of seven cases in the current outbreak are gay or bisexual, which is “highly suggestive of spread in sexual networks,” according to Mateo Prochazka, an infectious disease epidemiologist at UKHSA who is leading the agency’s investigation.
“This is further suggested by the fact that common contacts have been identified for only 2 of the 4 latest cases,” he said on Twitter, adding it was “bizarre” that people appear to have acquired the infection via sexual contact, as “this is a novel route of transmission” that has not previously been reported in the UK.
In a statement released by UKHSA on Monday, Dr Susan Hopkins, chief medical adviser, said: “We are particularly urging men who are gay and bisexual to be aware of any unusual rashes or lesions and to contact a sexual health service without delay.”
However, some scientists have questioned the theory that monkeypox has developed the ability to transmit sexually.
“It may not be actual transmitted by sexual intercourse rather than the close contact associated with sexual intercourse,” said Professor Keith Neal, an epidemiologist at the University of Nottingham.
“Further work looking at whether the virus is found in semen is required to say truly sexually transmitted.”
Professor Francois Balloux, director of UCL Genetics Institute, said: “I would urge some caution at this stage before concluding that monkeypox has morphed into a Sexually Transmitted Infection (STI).
“Monkeypox is not particularly transmissible and the number of cases to date where the route of transmission is known remains relatively small.”
A number of the patients ill with monkeypox, which is caused by a pox virus, are receiving medical care at specialist infectious disease units at the Royal Victoria Infirmary in Newcastle upon Tyne, and the Royal Free Hospital and Guy’s and St Thomas’s, in London.
None of the newly infected individuals have travelled to a country where monkeypox, which can kill up to 10 per cent of people it infects, is endemic, UKHSA said.
Seven cases have now been identified between 6 and 15 May, the first of whom had recently returned from Nigeria, which is where they are believed to have contracted the infection.
Given the lack of certainty around the recent spread of monkepox, UKHSA is working with the NHS to identify if any infections have been missed.
Prof Whitworth said there was “a need to engage with the at-risk community of gay and bisexual men to ensure they know about the presence of this infection and report any sign and symptoms to health facilities.
“Cases need to be identified, isolated and treated, either in hospital or at home, depending on severity and circumstances.”
The seven people diagnosed with monkeypox in the UK have been infected with the west African variant of the virus, which is mild compared to the central African strain, health officials said on Monday.
The UK and other countries have reported “small numbers of sporadic travel-related cases in recent years,” said Dr Michael Skinner, a reader in virology at Imperial College London.
The UK reported its first cases of monkeypox in 2018, when three people contracted the virus after a man travelled back from Nigeria.
Dr Skinner said there were likely to be two factors behind the international cases that have been detected in recent years.
“One is that the cessation of smallpox vaccination after the global eradication of variola virus in 1980 means that levels of cross-reactive immunity to monkeypox have now waned or disappeared, so that people can now be infected with monkeypox virus,” he explained.
“The second is the possibility that the distribution of the virus in West African wildlife has altered in some way, possibly increasing or broadening, so that humans are more likely to be infected. Investigating this possibility would require extensive testing of wildlife in the field.”