California health officials are investigating cases of a more severe form of mpox after confirming it had, for first time known to authorities, spread locally in the United States.
Three cases of clade I mpox were confirmed in Los Angeles County this week in people who had not recently traveled to a country where the virus is commonly found, the LA health department said in an Oct. 17 press release. While a different clade, or group, of mpox has been present in the United States for years, these new cases represent the first of this kind that appear to have been spread locally rather than via exposure during travel abroad.
This type of mpox is associated with more severe illness, said public health officials. While there have been nine reported cases of clade I mpox in the United States this year, the rest were associated with travel outside the country.
The first local case was reported in Long Beach on Oct. 14, while two more were reported in other areas of LA County on the 16th and 17th. All three people were hospitalized before being sent home to quarantine and recover, officials said. No further details about the patients were released, and no clear link between the cases has been identified.
The risk to the public remains low, the Los Angeles County Department of Public Health said in a statement. It is recommended, however, that people at higher risk get vaccinated against the illness if they have not already.
“The confirmation of a third case with no travel history raises concerns about possible local spread in Los Angeles County,” said Los Angeles County Health Officer Muntu Davis in a press release. “We’re working closely with our partners to identify potential sources and understand how this potentially more serious type of the mpox virus may be spreading.”
What is mpox?
Mpox, also known as monkeypox, is an infectious viral illness caused by a virus. It is most commonly associated with a painful rash, enlarged lymph nodes, fever, headache, muscle ache, back pain and low energy, according to the World Health Organization (WHO).
There are two clades, or groups, of the virus: clade I and clade II, each of which has two subclades. Clades refer to biological groups that all descend from one common ancestor.
Clade II is usually associated with mild to moderate illness, while clade I may be more severe. Clade II has been present in the United States since 2022, accounting for 118 infections in 2025, according to the Los Angeles County Department of Public Health. While there have been some cases of clade 1 brought back to the United States by people traveling abroad, the new cases are the first associated with local spread. Clade I has been identified in recent outbreaks in Africa, as well.
A woman infected with mpox poses for a photograph at the Kamenge University Hospital’s Mpox treatment center in Bujumbura, Burundi in central Africa on August 22, 2024. While cases of mpox in the U.S. are usually associated with travel to other countries, including Africa and the Dominican Republic, recent cases in California are believed to have spread locally.
How does mpox spread?
Both clades of the virus can be spread via close contact with a person with mpox, contact with contaminated materials, or contact with live or dead infected wild animals, according to the federal Centers for Disease Control and Prevention (CDC). Risk from infected animals is highest in Western and Central Africa, where cases are more common.
Infections are often associated with intimate contact such as kissing, sex or skin-to-skin touch. Direct contact with mpox rash or scabs as well as contaminated bodily fluids — like mucus, salivia or blood — are the most common ways people pass the virus to each other. Mothers can pass the infection to babies during pregnancy, during or after birth.
Animals transmit the virus to humans with bites and scratches, or when humans come in contact with infected body fluids while hunting, preparing, cooking or eating animals, according to the WHO. It can also spread through contact with objects, fabrics and surfaces that have not been disinfected.
The likelihood of casual transmission from respiratory exposure is low, meaning sharing the same air in a room is unlikely to lead to infection.
“Anyone with an unexplained rash or lesions should seek care promptly. Speak openly with intimate partners and your healthcare provider. Early action protects you and others,” said Dr. Cliff Okada, Long Beach Public Health Physician, in an Oct. 14 statement.
An example of mpox rash. The rash, especially in initial stages, can appear as raised bumps.
Mpox symptoms
Mpox symptoms start within 21 days of exposure to the virus with a typical incubation, or symptom-free, period of three to 17 days, according to the CDC and WHO.
Infected people usually experience a telltale rash that appears in stages and is located on the hands, feet, chest, face, or mouth, or near the genitals. The rash can appear as raised dots, almost like pimples. Symptoms typically last two to four weeks.
Most people can recover without serious medical intervention, though it is possible to become severely ill from the virus.
Other common symptoms include:
Muscle aches and backache
Respiratory symptoms (e.g., sore throat, nasal congestion, or cough)
If you may have been exposed to mpox and experience any symptoms, contact your doctor. They may recommend testing or supportive care. If you suspect you may be infected, avoid close contact with others and wear a mask in public, suggests the CDC.
This article originally appeared on USA TODAY: Cases of severe mpox strain confirmed in California, officials say
