First U.S. Case of Severe Form of Mpox Reported in California

The unidentified patient had recently returned from Africa, where the virus has caused a deadly epidemic, health officials said.

A person in California has tested positive for a form of mpox causing a widespread epidemic in Africa, the state’s Department of Public Health reported on Saturday. It is the first known case in the United States.

The individual, who was not identified, had recently returned from East Africa. The patient was diagnosed in San Mateo County, just south of San Francisco, and was isolating at home.

Officials at the California Department of Public Health and at the Centers for Disease Control and Prevention are reaching out to potential contacts of the patient for further testing.

There is no evidence that this version of the mpox virus, called Clade Ib, is circulating in communities in the United States, C.D.C. officials said.

Infections in people returning from Africa, however, have been found in Germany, Sweden, Thailand and the United Kingdom, among other countries. A case in India was reported in a person returning from the United Arab Emirates.

In Germany, Sweden, Thailand and India, the virus was transmitted no farther. In the United Kingdom, the infected individual passed mpox to three household contacts.