All of the Portuguese cases involve men, most of them young, authorities said. They have skin lesions and are said to be in stable condition. Authorities did not say whether the men had a history of travel to Africa or links to recent cases in Britain or elsewhere.
British health authorities said earlier this week they had identified four cases of monkeypox spread in London among gay and bisexual men. The Spanish Ministry of Health said it had detected eight suspected cases of monkeypox that had yet to be confirmed.
The disease belongs to a family of viruses that includes smallpox. Most people recover from monkeypox within a few weeks. Sporadic cases of monkeypox have previously been seen in countries such as Britain and the United States, but almost all involved people who may have been infected while traveling in Africa.
Dr Ibrahim Soce Fall, the World Health Organization’s deputy director general for emergency response, said the spread of monkeypox in the UK needed to be studied to understand how the disease was transmitted among men with sex with other men.
Fall said health officials still need to better understand how monkeypox spreads in general, even in countries where it’s endemic.
He noted that although there were more than 6,000 cases reported in Congo and around 3,000 cases in Nigeria last year, there are still “so many unknowns in terms of transmission dynamics”.
Britain had previously reported three previous cases of monkeypox, two involving people who lived in the same household and the third someone who had traveled to Nigeria, where the disease commonly occurs in animals.
The virus has generally spread to people from infected animals such as rodents, although human-to-human transmission has been known. Among people, the disease spreads when there is very close contact with lesions, bodily fluids, respiratory droplets, or contaminated materials, such as bedding.
Some British experts have said they will soon conclude monkeypox was spread through sexual contact, although the outbreak there has suggested that possibility.
“Recent cases suggest a potentially new way of spread,” said Neil Mabbott, a disease expert at the University of Edinburgh, adding that related viruses were known to be spread sexually.
Keith Neal, an infectious disease expert at the University of Nottingham, said transmission may not have occurred through sexual activity, but simply “close contact associated with sexual intercourse”.
Monkeypox usually causes fever, chills, a rash, and lesions on the face or genitals resembling those caused by smallpox. A vaccine developed against smallpox has been approved for monkeypox, and several antivirals appear to be effective as well.
Cheng contributed from London. Joe Wilson contributed from Barcelona, Spain.