The first known case of clade I mpox, a potentially more severe form of the disease formerly known as monkeypox, has been identified in the United States. California health officials reported the case, involving an individual who recently traveled from Africa, where this clade is endemic. The California Department of Public Health (CDPH), in collaboration with the Centers for Disease Control and Prevention (CDC), made the announcement, raising concerns about the potential spread of this variant beyond Africa.
Mpox typically presents with flu-like symptoms, headaches, and a characteristic bumpy rash. Symptoms usually appear within three weeks of exposure. Transmission primarily occurs through direct contact, and individuals can be contagious several days before experiencing symptoms, remaining so until the rash fully heals, a process that can take two to four weeks. While historically a zoonotic disease, primarily spreading from animals to humans, mpox caused significant human-to-human outbreaks in 2022. These outbreaks were driven by clade II mpox variants, which have a fatality rate below 1%.
This year, a new clade I variant, clade Ib, has emerged, spreading amongst people in parts of Africa and appearing in isolated cases outside the continent. These developments, along with the ongoing spread of other variants, led the World Health Organization (WHO) to declare a public health emergency of international concern in April 2024, the second such declaration related to mpox. Historically, clade I mpox has demonstrated higher fatality rates, reaching up to 10%. However, many of these cases occurred in regions with limited healthcare access, suggesting that fatality rates could be significantly lower in areas with better medical resources. According to the WHO’s latest situation report, only two deaths have been documented among over 2,000 reported cases of clade Ib this year.
Health officials have assured the public that the immediate risk posed by clade I in the U.S. is minimal. The infected individual is isolating and recovering, and contact tracing is underway. The CDPH has stated there is currently no evidence of clade I spreading between individuals in California or the United States.
Despite the current low risk, the potential for clade I mpox to trigger wider outbreaks, similar to other variants, remains a concern. Ongoing vigilance and proactive measures are crucial for tracking and preventing further spread. While transmission can occur through any close direct contact, the predominant mode of spread between humans has been sexual contact. The 2022 outbreaks largely affected men who have sex with men, while recent clade Ib outbreaks have also involved women, particularly sex workers.
The CDPH has emphasized the importance of continued monitoring and preventative measures. Further information about the California case can be found in their official statement. The WHO’s latest situation report on mpox, including data on clade Ib cases and fatalities, is available here.
