The first known case of a more severe strain of mpox, known as clade I, has been confirmed in the United States, health officials reported over the weekend. The U.S. Centers for Disease Control and Prevention (CDC) announced that a traveler returning from Eastern Africa, where an active outbreak of this strain is ongoing, tested positive in California. The individual, now isolating at home, is reported to be improving, according to the CDC.
The agency emphasized that the public risk from this new mpox strain remains low. The individual was treated at a local facility before being released, and health authorities are tracing and monitoring possible contacts. "The state and CDC are collaborating to ensure appropriate follow-up," the CDC stated.
Clade I mpox, historically known to cause more severe illness compared to clade II-which circulated widely during the 2022-2023 U.S. outbreak-has been reported in recent outbreaks in Africa, including the Democratic Republic of Congo, Burundi, and Uganda. This strain has caused rising concerns in the region but has shown comparatively milder clinical presentations during its recent global spread. The CDC highlighted that cases of clade I mpox detected in countries such as Germany, the United Kingdom, and Thailand were travel-related and resulted in generally mild symptoms.
"Historically, clade I mpox has led to more severe illness and fatalities than clade II," the CDC stated. However, recent trends indicate that effective medical care has significantly reduced mortality rates. "While outbreaks of clade I mpox once had death rates as high as 3%-11%, current data shows that these rates have dropped to around 1% when proper clinical care is available," the agency noted.
The World Health Organization (WHO) is closely monitoring the situation, with a meeting of its emergency committee on mpox scheduled for November 22 to assess whether the ongoing outbreak still constitutes a public health emergency of international concern.
Mpox, previously known as monkeypox, is a viral disease related to smallpox, which was eradicated in the late 20th century. Transmission occurs through close contact-such as touching, kissing, or sexual activity-or through contact with contaminated materials like bedding and clothing. The illness often begins with flu-like symptoms, including fever, muscle aches, and fatigue, followed by a rash that progresses into painful or itchy lesions before scabbing over and healing within weeks.
To mitigate mpox risks, the CDC advises the public to avoid contact with symptomatic individuals and potentially contaminated items. Vaccines are available and recommended for those at higher risk of exposure, with two doses providing the best protection.
In California, the state's Department of Public Health (CDPH) confirmed that the case stemmed from exposure in Eastern Africa, where clade I mpox has been active. "While historically more severe, recent clade I infections have not been as clinically concerning as previous outbreaks," a CDPH spokesperson stated.
Mpox outbreaks have primarily occurred in remote areas of West and Central Africa for decades, with the Democratic Republic of Congo bearing much of the burden. Transmission has expanded to human-to-human spread in recent years, exacerbating public health challenges. Efforts to curb transmission focus on vaccination, targeted public health interventions, and close monitoring of contacts.
The mpox case in California underscores the global interconnectedness of emerging infectious diseases and the importance of vigilant monitoring and response. With the potential for severe outcomes mitigated by strong healthcare infrastructure, officials remain cautiously optimistic. However, as mpox continues to evolve, global health agencies stress the need for international cooperation and community awareness to prevent further spread.
