Clade I mpox strain identified for the first time in the US, CDC says

Health officials said Saturday they have identified the first person in the US infected with a mpox strain This has recently prompted global health warnings over concerns about its spread, severity and transmissibility.

The person had recently returned to California after a trip to East Africa, where the strain (clade I mpox) has hit several countries, the Centers for Disease Control and Prevention said in a news release. State and federal health officials said the risk to the public from this first U.S. case is low.

The person is isolating at home and there is no evidence that the mpox clade I strain is spreading between people in California or elsewhere in the U.S., state health officials said. People who had close contact with the person are also being contacted by public health workers.

Clade I has historically caused more severe disease than clade II. Public health officials in California said recent infections indicate the clade I strain may not be as clinically severe compared to previous outbreaks, especially when people have access to quality medical care.

Mpox can cause a painful rash with ulcers, enlarged lymph nodes, fever, headache, muscle aches, back pain and low energy. World Health Organisation said. Most people recover completely after a few weeks.

In severe cases, some people become very ill from bacterial infections of the lesions, pneumonia, vision loss, dehydration, or malnutrition due to vomiting or diarrhea. People can also have infections of their blood, brain, heart, or genitals. Deaths occur in rare cases.

Human cases of Mpox date back to the 1970s in the Democratic Republic of Congo. according to the National Institutes of Health. But in recent years the virus has spread worldwide, and the CDC says more than 100,000 people have been infected with the clade II strain.

How does Mpox spread?

Mpox spreads from animals to humans and was given the former name monkeypox because that is the animal in which it was first discovered. It comes from the same virus family as smallpox and spreads through physical contact, the CDC said.

The new strain of MPOX is believed to spread through close, intimate and sexual contact that occurs skin-to-skin, and by touching contaminated materials such as clothing or bedding. The CDC said incidental contact, such as during a flight, is unlikely to pose a risk of spread.

People at greater risk of contracting MPOX include: Gay men; bisexual men; men who have sex with men; and transgender and non-binary people, as well as others who have been exposed to an infected person. Health officials recommend that these groups receive the mpox vaccine to protect against both strains.

How did officials identify the case?

The California Department of Public Health identified the strain through confirmatory testing press release said. The infected person received health care in San Mateo County, near San Francisco, based on his travel history and symptoms. State health officials reported the case to the CDC, which plans to conduct further laboratory testing.

Health officials are monitoring the spread of mpox

In August the The World Health Organization has declared a global public health emergency for the species, which is different from the clade II species that has been circulating in the US since 2022. African Centers for Disease Control and Prevention said there have been more than 51,000 cases and more than 1,000 people died on the continent during the last MPox outbreak in 2024. The cases have mainly occurred in Congo.

Outside Africa, a handful of cases have emerged in Europe, India and Southeast Asia.

The US also issued travel advisories to central and eastern Africa earlier this year due to outbreaks.

The incubation period is between 3 and 17 days. During this time, a person has no symptoms and may feel well.

(This story has been updated to add new information.)