Portugal and Spain reported over 40 suspected cases of the rare monkeypox infection on Wednesday as the outbreak spread in Europe after cases were initially reported in the UK.

Across the Atlantic, the U.S. also reported its first case of the year in a Massachusetts resident.

Read more: What is monkeypox?

Health authorities in Spain's Madrid region said in statement on Wednesday that they had detected "23 possible cases of monkeypox" and The Guardian newspaper reported that the country's health ministry had issued a nationwide alert to guarantee a "swift, coordinated and timely response."

In neighboring Portugal, health authorities said they had confirmed five cases of monkeypox and were investigating another 15 suspected cases, all in the Lisbon region.

In both countries, the patients were all male and for the most part young.

Madrid health authorities on Wednesday said they believed all 23 cases had been transmitted through sexual activity.

"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," The Guardian quoted them as saying in a statement.

Monkeypox is endemic in parts of Central and Western Africa and is usually transmitted via close contact with infected animals such as rodents and monkeys, but human-to-human transmission can also occur.

The latest infections come after the UK first reported three cases of monkeypox earlier this month, and another four on Monday.

In the U.S. meanwhile, health officials confirmed Wednesday that a Massachusetts resident had tested positive for monkeypox, the first case detected in the country this year.

The Massachusetts Department of Public Health completed initial testing on Tuesday and the diagnosis was confirmed by the Centers for Disease Control and Prevention. The patient was an adult male who recently traveled to Canada, the department said.

Symptoms of monkeypox in humans include a rash that often starts on the face then moves to other parts of the body, fever, muscle ache and chills. Most people recover from the illness within several weeks. It has only been fatal in rare cases.

(With input from AP, AFP)