Officials with San Mateo County Health confirmed with ABC7 the patient lives in the county.
The person diagnosed with clade I mpox had recently traveled from Eastern Africa, where an mpox outbreak is ongoing. The person was treated at a local medical facility and released, the CDC said. The person has been isolating at home and their symptoms are improving. The state and CDC are working to identify and follow up with potential contacts.
Clade II mpox has been circulating in the United States since a large outbreak in 2022 and 2023. A more recent outbreak in Africa is driven by clade I, which is known to cause more severe disease.
"Although clade II mpox has been circulating in the United States since 2022, clade I mpox has never been reported in the United States until now. Historically, clade I mpox has caused more severe illness and deaths than clade II mpox," San Mateo County Health said on its website.
San Mateo County Health said it is assisting the California Department of Public Health and the Centers for Disease Control on tracing and notifying close contacts of potential exposure.
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Currently, there have been 108 cases of clade II mpox, the less severe strain, reported in San Mateo County, the county health department said.
Officials say the risk to the public remains very low, writing, "there is no concern or evidence that mpox clade I is currently spreading among individuals in San Mateo County or elsewhere in the United States."
Persons who have certain risk factors should contact their primary health care provider about vaccination and take steps to reduce exposure.
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Mpox, formerly known as monkeypox, is a viral disease related to the now-eradicated smallpox virus. It can spread through close contact such as touching, kissing or sex, as well as through contaminated materials like sheets, clothing and needles, according to the WHO.
Initial symptoms are typically flu-like - including fever, chills, exhaustion, headache and muscle weakness - often followed by a painful or itchy rash with raised lesions that scab over and resolve over a period of weeks.
The CDC says people can protect themselves from mpox by avoiding contact with people with symptoms and contaminated materials such as clothing, bedding, sex toys and toothbrushes. People who are eligible are recommended to get two doses of mpox vaccine.
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More information about mpox is available from the California Department of Public Health and the Centers for Disease Control and Prevention.
The subtype that's responsible for most of the ongoing spread, clade Ib, is relatively new. Travel-related cases of mpox clade Ib have been reported in Germany, India, Kenya, Sweden, Thailand, Zimbabwe, and the United Kingdom. Some countries outside of Africa have also reported locally transmitted cases.
"The recent travel-associated clade I mpox cases outside of Africa have all been attributed to subclade Ib; there have been no deaths associated with these cases and available data for a subset has detailed relatively mild disease courses," the CDC said Saturday.
CNN's Jamie Gumbrecht contributed to this report