BRUSSELS — Belgium has become the first country to introduce a compulsory 21-day quarantine for monkeypox patients after reporting four cases of the disease in the last week.

Belgian health authorities took the decision on Friday, according to Belgian media. Monkeypox contact cases are not required to self-isolate but should remain vigilant particularly if they are in contact with vulnerable people.

Monkeypox is a disease in the same family as smallpox and symptoms include a distinct bumpy rash, a fever, sore muscles and a headache. Monkeypox is less deadly than smallpox, with a mortality rate below 4 percent, but experts are worried about the unusual spread of the disease beyond Africa where it usually circulates.

The Belgian Institute of Tropical Medicine has said the risk of a larger outbreak in the country was low, according to Belgian daily Le Soir.

On Saturday, the microbiologist Emmanuel André, who is in charge of the National Reference lab for COVID-19 in Belgium, tweeted that a fourth case of monkeypox had been confirmed in the country.

“This patient is being treated in Wallonia and is linked to the Antwerp event in which two other people were infected,” he wrote, referring to a festival in the port city held in May.

On Saturday, the World Health Organization reported that there were 92 confirmed cases in 12 different countries, with 28 suspected cases under investigation. Cases of monkeypox have been confirmed in the U.K., Portugal, Sweden, Italy, Spain, France, Belgium, Germany, the U.S., Canada and Australia.

In the U.K., Chief Medical Adviser at the UK Health Security Agency Susan Hopkins told the BBC on Sunday that they were “detecting more cases on a daily basis” of monkeypox. Education Secretary Nadhim Zahawi said the government was taking it “very, very seriously.” — Agencies