The California Department of Public Health confirmed, through laboratory testing, the first known case of clade I mpox in the United States. This case is related to an ongoing outbreak of clade I mpox in Central and Eastern Africa. The risk of clade I mpox to the public remains low, and there continue to be sporadic clade II mpox cases in the United States.
The case was diagnosed in a person who recently traveled from Eastern Africa. The individual was treated shortly after returning to the United States at a local medical facility and released. Since then, the person has isolated at home, is not on treatment specific for mpox, and symptoms are improving. Based on their travel history and symptoms, patient specimens were tested and confirmed for the presence of clade I monkeypox virus. Specimens are being sent to CDC for additional viral characterization. Additionally, CDC is working with the state to identify and follow up with potential contacts.
Casual contact, like you might have during travel, is unlikely to pose significant risks for transmission of mpox. While investigations continue into this case, CDC guidance has not changed. Protect yourself from mpox by:
- Avoiding close contact with people who are sick with symptoms of mpox, including those with skin or genital lesions
- Avoiding contact with contaminated materials used by people who are sick (such as clothing, bedding, toothbrushes, sex toys, or materials used in healthcare settings)
There are two types of mpox, clade I (with subclades Ia and Ib) and clade II (with subclades IIa and IIb; IIb caused the ongoing global outbreak). You can’t tell which type of mpox someone has by looking at them. Outbreaks from the different subclades can have different characteristics, like who they affect, how they’re spread, or how many deaths they cause. Although clade II mpox has been circulating in the United States since 2022, clade I mpox has never been reported in the United States before now. Travel-associated cases of subclade Ib have been reported in Germany, India, Kenya, Sweden, Thailand, Zimbabwe, and the United Kingdom. Historically, clade I mpox has caused more severe illness and deaths than clade II mpox; however, recent data demonstrate that infections from clade I mpox in the current outbreak may not be as clinically severe as in previous outbreaks. While outbreaks of clade I mpox used to have death rates around 3%-11%, more recent outbreaks have had death rates as low as approximately 1% when patients received good medical oversight and supportive clinical care. Death rates are expected to be much lower in countries with stronger healthcare systems and treatment options, including the United States. Current data supports that subclade Ib has a lower death rate of < 1% both in and outside of Africa. The recent travel-associated clade I mpox cases outside of Africa have all been attributed to subclade Ib; there have been no deaths associated with these cases and available data for a subset has detailed relatively mild disease courses.
People with mpox often get a rash that may be located on hands, feet, chest, face, mouth and/or near the genitals, including penis, testicles, labia, vagina, and anus. The incubation period is 3–17 days. During this time, a person does not have symptoms and may feel fine.
The anticipated overall risk of clade I mpox to the general population in the United States from the outbreak in Central and Eastern Africa is low. Earlier this year CDC conducted a risk assessment which included epidemiologic data from Central and Eastern Africa, data from the ongoing mpox outbreak in the United States caused by clade IIb, and historical data on clade I mpox outbreaks in DRC and other affected countries. In addition, CDC has simulated clade I mpox outbreaks. These simulations indicate that close-contact transmissions within and between households are unlikely to result in a large number of mpox clade I cases in the United States. Additionally, in Sweden, Thailand, Germany, and India there was no apparent onward spread of the virus and the onward spread in the United Kingdom has been limited to close, household contacts so far.
CDC continues to work in Central and Eastern Africa to help stop mpox transmission at the source. This ongoing work includes laboratory training, supplies for diagnostic testing (including genetic sequencing), training of frontline health and epidemiologic workers, support for surveillance in people and animals, support for infection prevention and control, risk communication and community engagement, and direct technical assistance in outbreaks, as well as research collaborations.
CDC has more than two years of experience responding to mpox in the United States due to the ongoing 2022 global clade II mpox outbreak and has adjusted existing domestic public health systems and structures to respond to any outbreak of clade I mpox in the United States. CDC issued guidance for travelers to countries in Central and Eastern Africa experiencing outbreaks earlier this year. CDC continues to recommend that clinicians request expedited clade specific testing for suspect clade I mpox cases with travel history to Central and Eastern Africa. CDC is also helping communities monitor the presence of both clades of mpox virus in wastewater samples. Data from samples can provide an early warning of mpox activity and spread in communities. CDC combines wastewater data with other data to decide if there is a need for further testing or other actions in collaboration with state and local public health partners.
More information on mpox is available online on the CDC website.
