Health authorities in Spain have issued an alert over a possible outbreak of monkeypox after 23 people showed symptoms compatible with the viral infection, which has already been detected in the UK and Portugal.

The health ministry cautioned that the suspected cases – all in the Madrid region – had yet to be confirmed, but said a nationwide alert had been issued "to guarantee a swift, coordinated and timely response".

Madrid's regional health department said the suspected cases were being analysed by the National Microbiology Centre to obtain a definitive diagnosis.
"Generally speaking, monkeypox is spread by respiratory transmission, but the characteristics of the 23 suspected cases point towards transmission through mucus during sexual relations," it said in a statement.

"The people involved are doing well and are isolating at home, but a close eye is being kept on them in case they need hospital treatment."

Fernando Simón, an epidemiologist who heads Spain's health emergencies centre, said while it was unlikely that monkeypox would spread significantly, "that can't be ruled out".

Portuguese authorities have confirmed five cases and are investigating another 15 suspected cases. In a statement on Wednesday, Portugal's health ministry said the cases it had detected – all in the Lisbon and Tagus Valley region – had all involved men whose symptoms included ulcerative lesions.

"The UK has reported similar cases of ulcerative lesions, with confirmed monkeypox virus infection," it said. "We are monitoring the situation at national level and in conjunction with European institutions."

The first of the seven cases logged in the UK involved a person who had recently been to Nigeria, where they are believed to have contracted the infection. According to the UK Health Security Agency, all four of the most recent cases are men who identify as gay or bisexual or other men who have sex with men.

The health body is asking these groups "to be alert" to possible symptoms, which include rashes or lesions on any part of their body, especially their genitalia, and to contact a sexual health service if they have concerns.

Early symptoms of monkeypox include fever, headache, muscle aches, swollen lymph nodes and chills. A rash that can look like chickenpox or syphilis can also develop and spread from the face to other parts of the body, including the genitals. Most people recover within a few weeks.

A handful of cases of monkeypox, which has two forms and is typically found in central and western Africa, have been diagnosed in the UK in the past. It is thought the seven UK cases involve the west African strain, which is milder than the more severe central African, or Congo, strain.