“If human-to-animal transmission occurs, and the virus spreads in an animal population, there is a risk that the disease could become endemic in Europe,” the agency warned.
Some experts have feared for years that there might be a resurgence of monkeypox.
The last known cases of smallpox occurred in 1977, and it was declared eradicated in 1980. With the end of immunization programs for smallpox, the number of people who are susceptible to this family of viruses has risen each year.
Scientists in Portugal sequenced the monkeypox virus in skin lesion samples collected from a male patient on May 4, but that sequence is incomplete. So far, the current strain appears to be closely related to versions exported from Nigeria to Britain, Israel and Singapore in 2018 and 2019, researchers have concluded.
The United States sequenced a sample from the first confirmed case, in Massachusetts, within 48 hours and has shared the sequence publicly. The samples from Massachusetts and Portugal seem closely related, Dr. McQuiston said.
“All signs point to this being an outbreak associated with person-to-person spread,” she added.
The West African version of the virus is thought to cause milder symptoms. It does not spread easily among people and has a fatality rate of about 4 percent, compared with a mortality rate of about 11 percent with the Central African version and 30 percent for smallpox. No deaths have been reported in the current outbreaks.
There is no indication that the virus has changed significantly in virulence or mode of transmission. Unlike the rapidly mutating RNA-based coronavirus, monkeypox is a large DNA virus that is capable of correcting its genetic errors.
“These are particularly stable viruses, so monkeypox is less prone to mutations,” said Dr. Raina MacIntyre, an epidemiologist at the University of New South Wales in Sydney, Australia, and a member of the W.H.O.’s working group on monkeypox.