A type of mpox that causes severe illness has been identified in three California residents who had not traveled abroad, the first time the more virulent form has spread within the United States, health officials said on Friday.
The type, known as Clade 1, has been widely circulating in central and eastern Africa, causing tens of thousands of infections and hundreds of deaths. Clade 2, the form that swept the United States in 2022 and sickened 30,000 people, causes less severe illness.
Clade 2 continues to circulate in the United States at low levels, but infections have been rising in a number of cities since the summer. Los Angeles health officials have reported 118 cases of mpox, formerly monkeypox, so far this year.
California health officials say the three patients — one in Long Beach and two in Los Angeles — were hospitalized and are now recovering at home in isolation. They have not found a link among the three cases.
Officials in Long Beach announced the first case on Tuesday.
Dr. Kelly Johnson, an infectious disease specialist at the University of California, San Francisco, said that although the risk of infection was low for most people, she urged vigilance, especially among people with weakened immune systems and men who have sex with men.
“My concern is that person-to-person, community spread could be ongoing,” she said.
Sonali Kulkarni, medical director of the Division of H.I.V. and S.T.D. Programs at the Los Angeles County Department of Public Health, said that officials were increasing surveillance efforts and testing existing lab specimens to gain a better understanding of the spread. She said all three patients are in a high-risk group of gay and bisexual men.
That all three cases resulted in hospitalization, Dr. Kulkarni said, was worrisome. “It’s still too early to tell, but we’re concerned there will be more severe disease,” she said.
Both mpox clades can be spread through intimate contact, within households or by sharing personal items. Signs of illness often begin with flulike symptoms followed by a rash and lesions that can be extremely painful.
Because the virus can incubate for up to three weeks without causing symptoms, those infected can unknowingly spread the virus to others. Men who have sex with men have a higher risk of infection, and for those with untreated H.I.V., mpox can be deadly.
The Centers for Disease Control and Prevention referred a request for comment to health officials in California. The C.D.C. website recommends that those at risk receive two doses of a vaccine manufactured by the pharmaceutical company Jynneos. The vaccine is highly effective at preventing severe illness.
The only previous case of a Clade 1 infection in the United States was reported last November in California, when a resident of San Mateo County who had traveled to East Africa was hospitalized. Health officials at the time said that case did not lead to additional infections.
Although locally transmitted cases of Clade 1 mpox in the United States were not entirely unexpected, health experts said the Trump administration’s funding cuts to the nation’s public health and disease surveillance systems could hamstring efforts to contain any outbreak.
Joseph Osmundson, a clinical associate professor of biology at New York University, said the continuing cutbacks in the nation’s public health system combined with drastic funding reductions to research, surveillance and lifesaving health initiatives abroad could make it more difficult to contain a mpox outbreak.
“The infrastructure we built during the 2022 outbreak has just been eviscerated,” Dr. Osmundson said. “The very things we need to understand if we have a problem now, and if we will have a problem in the future, are being systematically dismantled.”
Andrew Jacobs is a Times reporter focused on how healthcare policy, politics and corporate interests affect people’s lives.
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