Health officials have confirmed the first detected case of a new strain of monkeypox (mpox) in the United States.

The clade I strain, which was first reported in the eastern Congo earlier this year, was found in a traveler who had returned from eastern Africa to northern California, the California Department of Public Health (CDPH) said on Nov. 16.

There is currently an ongoing outbreak of the new mpox strain in central and eastern Africa, spreading mostly through sexual transmission and through close contact among children, pregnant women, and other vulnerable groups.

Other countries to report cases of the clade I strain include Germany, Sweden, the UK, India, Thailand, Kenya, and Zimbabwe.

The clade I strain is distinct from the mpox clade II strain that spread earlier in 2022 and has become a global outbreak. Gay men made up the vast majority of clade II cases.

Mpox is a rare disease caused by a virus from the same family as smallpox. Endemic in parts of Africa, people there have been infected through bites from rodents or small animals.

Milder symptoms of the disease can include fever, chills, and body aches. More serious symptoms include lesions on the face, hands, chest, and genitals.

‘Very Low’ Risk

State health officials say the risk to U.S. public health from the returned traveler with the clade I strain is “very low.”

“There is no concern or evidence that mpox clade I is currently spreading between individuals in California or the United States,” the CDPH stated.

The U.S. Centers for Disease Control and Prevention (CDC) stated, “The individual was treated shortly after returning to the United States at a local medical facility and released.”

The patient is now isolating at home in San Mateo County, according to the agency.

The person’s symptoms are improving, health officials said, and health workers haven’t detected other cases within the person’s close contacts.

“Casual contact, like you might have during travel, is unlikely to pose significant risks for transmission of mpox,” the CDC stated, citing an apparent lack of transmission of the strain outside of household close contacts in cases reported in the UK, and no transmission for cases in Sweden, Thailand, Germany, and India.

To date, there have been 3,100 confirmed cases of the strain, mostly in the three central African countries of the Democratic Republic of the Congo, Burundi, and Uganda, according to the World Health Organization. Reported cases spiked in late September.

Health officials said earlier this month that the situation in the Congo appears to be stabilizing.

Disease Severity

African outbreaks of clade I mpox were initially reported to be more severe than those of clade II, with death rates of about 3 percent to 11 percent. However, recent data indicate that when patients have access to good medical oversight and supportive clinical care, as they do in the United States, death rates are much lower at about 1 percent.

Current data support that subclade Ib has a death rate of less than 1 percent “both in and outside of Africa,” according to the CDC. The California case has been identified as clade I, subclade Ib.

The virus has an incubation period of 3 to 17 days, during which a person does not have symptoms and feels fine.

Information about the CDC’s recommendations for individuals at risk of mpox is available online.

The Associated Press contributed to this report.