Portuguese authorities say they have identified five cases of the rare monkeypox infection and Spain’s health services are testing 23 potential cases after Britain put Europe on alert for the virus.

The five Portuguese patients, out of 20 suspected cases, are all stable.

They are all men and they all lived in the region of Lisbon and the Tagus Valley, the Portuguese health authorities said.

European health authorities are monitoring an outbreak of the disease since Britain reported its first case of monkeypox on May 7. It has found six more infections since then.

Spain issued an alert on Tuesday saying it had eight suspected cases under testing.

The figure rose to 23 cases by later in the day, Madrid region health authorities said in a statement.

All cases remain unconfirmed.

In the US on Wednesday, state health officials in Massachusetts reported a case of monkeypox in a resident who had travelled to Canada. Investigators are looking into whether it is connected to the European outbreaks.

Monkeypox is a rare viral infection similar to human smallpox, although milder. It was recorded in the Democratic Republic of Congo in the 1970s.

The number of cases in west Africa has increased in the past decade.

Symptoms include fever, headaches and skin rashes starting on the face and spreading to the rest of the body.

It is not particularly infectious between people, Spanish health authorities said, and most people infected recover within a few weeks although severe cases have been reported.

Four of the cases detected in Britain self-identified as gay, bi-sexual or other men who have sex with men, the country’s Health Security Agency said, adding evidence suggested there may be a transmission in the community.

The British agency urged men who were gay and bisexual to be aware of any unusual rashes or lesions and to contact a sexual health service without delay.

The newspaper El Pais quoted the head of public health in the Madrid region, Elena Andradas, as saying, “22 of the 23 suspected cases have reported having had sex with other men in recent weeks”.

Portugal’s DGS health authority did not release any information on the sexual orientation of the monkeypox patients or suspected patients.

The two countries sent out alerts to health professionals to identify more possible cases.