United States’ first known case of more severe strain of mpox confirmed in California

The United States’ first known case of a more severe strain of mpox was confirmed in California, the US Centers for Disease Control and Prevention said Saturday. The risk of clade I mpox to the public remains low, the agency said.

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.

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.

“Historically, clade I mpox has caused more severe illness and deaths than clade II mpox; however, recent data demonstrate that infections from clade I mpox in the current outbreak may not be as clinically severe as in previous outbreaks. While outbreaks of clade I mpox used to have death rates around 3%-11%, more recent outbreaks have had death rates as low as approximately 1% when patients received good medical oversight and supportive clinical care. Death rates are expected to be much lower in countries with stronger healthcare systems and treatment options, including the United States.”

The World Health Organization says the number of mpox cases in Africa has been rising, driven mostly by cases in the Democratic Republic of Congo, Burundi and Uganda. On November 22, the WHO is set to convene a meeting of its emergency committee on mpox to advise whether the current outbreak still constitutes a public health emergency of international concern.

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.