A new strain of mpox, formerly known as monkeypox, has been detected in the United Kingdom, according to health officials.

The UK Health Security Agency (UKHSA) said in a Dec. 8 statement that a new recombinant mpox virus had been identified in England in a person who recently visited Asia. The new strain is a mix of clade (or type) II and clade I, the latter of which is associated with more severe illness.

"This is not unexpected as both clades are circulating, but highlights the continued potential for mpox virus to evolve and the importance of continued genomic surveillance," said the UKHSA statement.

The agency was already monitoring the presence of the illness after reports of locally-acquired cases came out of the U.S., Spain, Italy, the Netherlands and Portugal in October. Locally acquired cases, or those that were contracted by people who have not recently traveled to a country where the virus is commonly found, are less prevalent in some areas, including the U.S.

Earlier this year, California health officials announced the first known case of clade I mpox to spread locally in the U.S.

What is mpox?

Mpox, also known as monkeypox, is an infectious viral illness caused by the monkeypox 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).

Mpox spreads from animals to people, and it received its former name, monkeypox, because that is the animal it was first discovered in. It comes from the same virus family as smallpox, and it spreads through physical contact, according to the Centers for Disease Control and Prevention (CDC).

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 also been identified in recent outbreaks in Africa.

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 CDC. Risk from infected animals is highest in Western and Central Africa, where cases are more common.

Infections are often associated with sexual or intimate contact. Direct contact with mpox rash or scabs, as well as contaminated bodily fluids — like mucus, saliva 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 bodily 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, which means sharing the same air in a room is unlikely to lead to infection.

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.

Those who are infected usually experience a telltale rash that appears in stages and is located on the hands, feet, chest, face, 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:

• Fever

• Chills

• Swollen lymph nodes

• Exhaustion

• Muscle aches and backache

• Headache

• 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, the CDC suggests. 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.

This article originally appeared on USA TODAY: New hybrid mpox strain discovered in UK