California has reported the first US case of mpox – formerly known as monkeypox – that is part of a new outbreak.

The state’s department of health, the CDPH, said the new case was from the Clade I strain – different from the Clade II strain that has been in circulation in the US since 2022.

The person in question, who is now isolating at home, had recently travelled to Africa and the CDPH said their case was “related to the ongoing outbreak of Clade I mpox in Central and Eastern Africa”.

Mpox was declared a global health emergency by the World Health Organization earlier this year.

The CDPH said that while Clade I cases had tended to cause more severe illness than Clade II in the past, “recent infections from Clade I mpox may not be as clinically severe as in previous outbreaks”.

Mpox is caused by a virus in the same family as smallpox but is usually much less harmful.

It was originally transmitted from animals to humans but now also passes between humans.

Initial symptoms include fever, headaches, swellings, back pain and aching muscles. A rash can then develop, which can be extremely itchy or painful.

The infection can clear up on its own and lasts between 14 and 21 days, but in some cases has been fatal, particularly for vulnerable groups including small children.

Mpox is most common in remote villages in the tropical rainforests of West and Central Africa, in countries such as the Democratic Republic of Congo (DR Congo), where it has been seen for many years.

Hundreds of people died during an initial outbreak in DR Congo earlier this year, and the disease has since spread to areas of Central and East Africa.

Outbreaks can be controlled by preventing infections with vaccines, though these are usually only available for people at risk or those who have been in close contact with an infected person.