The United States reported its first case of a more aggressive strain of mpox this Saturday. It was detected in an individual in California who had recently traveled from East Africa.
The case was confirmed by the state Department of Public Health and reported to the Centers for Disease Control and Prevention (CDC).
The person was treated in San Mateo County based on their travel history and symptoms, and is now isolated at home and recovering, the state health department said in a news release. Public health workers are contacting people who had close contact with the patient, “but there is no concern or evidence” that the strain is currently spreading in California or the United States, according to the statement.
The United States is the sixth country outside Africa to have detected the strain, according to the CDC, which can cause more severe illness than the version of the virus that spread in 2022.
The spread of this strain in several African countries led the World Health Organization to declare a global public health emergency in August, the second time for mpox in two years.
Mpox, formerly known as monkeypox, is a viral infection that causes painful lesions, often accompanied by flu-like symptoms. It is classified into two different groups: the most aggressive strain, clade I, and a somewhat milder strain, clade II.
Clade II was responsible for the 2022 global outbreak, which caused at least 32,000 infections and 58 deaths in the United States.
Until recently, clade I had not spread beyond central Africa, where it is considered endemic, and eastern Africa. But an outbreak in the Democratic Republic of the Congo has led to cases in eight other African countries this year. In August, the first known case outside Africa was confirmed in Sweden. Several days later, Thailand confirmed a case. Both patients had spent time in Africa.
Travel-related cases have also been detected in Germany, India and the United Kingdom, according to the CDC.
Mpox is spread through direct contact with an infected person, an animal, or contaminated items such as clothing or bedding. The clade II version that spread in 2022 was primarily transmitted through sexual contact, particularly between men who have sex with men.
In the current outbreak, clade I has largely spread through contact with infected animals and transmission within households, according to disease experts. Two-thirds of suspected cases from January 2023 to April 2024 were in children 15 years old or younger, according to the Centers for Disease Control and Prevention. But a version of the strain, clade Ib, has also spread through sexual contact between sex workers and men who have sex with men in the Congo.
It is not clear what the transmission routes could be like in the United States, but some experts predict that clade I could spread among the same sexual networks as in 2022.
“I think we are going to see some cases,” said Dr. Marc Siegel, an associate professor of medicine at the George Washington School of Medicine and Health Sciences. “I think it will be more likely that this is the same population that was most at risk in the 2022-2023 outbreak. For the general population, I think this represents a very small risk.”
Previous clade I mpox outbreaks in Africa have killed up to 10% of people who fell ill, but the current outbreak in the Democratic Republic of the Congo has a mortality rate of around 5%. That compares with a global mortality rate of 0.2% for the 2022 outbreak.
However, health officials have said there is reason to believe that clade I could pose a milder threat in the United States than in African countries this year, given lower malnutrition rates and the health care system. It is more solid.
The United States is also better prepared for an outbreak than in 2022. The CDC has ordered doctors to test for mpox and is scanning for both clades at wastewater sites across the country, including some airports.
Additionally, the country has an adequate supply of the two-dose Jynneos vaccine, which is available for high-risk groups. The Department of Health and Human Services says two doses of the vaccine or a prior infection should protect against severe disease caused by clade I.
The United States administered about 1.2 million vaccine doses between May 2022 and January 2023. Health officials hope to expand that coverage, as only 23% of the population at significant risk has been fully vaccinated.
Mpox usually begins with a rash that progresses to small bumps on the skin, followed by fluid-filled blisters that eventually form scabs. People may also experience fever, headache, muscle aches, back pain, low energy, or swollen lymph nodes.
Some people infected with clade II in the United States have described the lesions as extremely painful. They have historically appeared on the face, chest, palms of the hands, and soles of the feet, but during the 2022 outbreak, people developed lesions around the genital and anal region or inside the mouth and throat.
