More than two years after the mpox outbreak in the United States was officially declared over, a fresh cluster of cases in California has caught the attention of infectious disease experts.The previous U.S. outbreak, which spread primarily among men who have sex with men, was declared over in early 2023, though sporadic cases have continued to appear. Now, health officials are worried after identifying three unrelated mpox cases in California linked to a more infectious and potentially more severe strain of the virus known as clade I mpox.What Is Mpox?Mpox, previously called monkeypox, is a viral infection caused by the monkeypox virus, a member of the Orthopoxvirus genus. According to the World Health Organization, the virus is divided into two main groups—clade I (which includes Ia and Ib) and clade II (which includes IIa and IIb). The 2022–2023 global outbreak was linked to the clade IIb strain. The disease continues to pose a global health concern, with a sharp rise in cases reported in the Democratic Republic of the Congo and other regions due to clades Ia and Ib, and now in the United States.How This Strain Differs From the 2022 OutbreakThe mpox variant responsible for the 2022 outbreak belonged to clade II, a strain with a relatively low fatality rate. During that outbreak, over 32,000 infections were reported nationwide, resulting in 58 confirmed deaths—a mortality rate of about 0.2 percent.Clade I mpox, by comparison, has been found to cause more serious illness and spread more easily. Both strains exist in parts of Central Africa, but clade I has historically shown higher transmissibility and mortality than clade II.California Cases Raise Alarm Over Possible Community SpreadAll three of the California patients infected with the clade I strain required hospitalization. According to the California Department of Public Health (CDPH), the pattern of infection suggests that, much like the 2022 outbreak, transmission is again occurring primarily within networks of men who have sex with men.Dr. Joseph Cherabie, assistant professor of infectious diseases at Washington University in St. Louis and a board member of the HIV Medicine Association, said it was “only a matter of time” before clade I reached the U.S.While a previous California case of clade I mpox was linked to travel in Africa, these recent infections have no known travel connections, a detail that worries experts. “That makes us a little bit uncomfortable as epidemiologists and as public health folks,” Cherabie said, adding that it signals local transmission that health authorities may not yet be fully detecting.Transmission and Concerns About Undetected CasesMpox spreads mainly through direct skin-to-skin contact with someone who is infected, often through the painful rashes characteristic of the disease. Since the source of infection for these cases remains unclear, Cherabie believes there are likely more undetected cases circulating in the community.When mpox first appeared in nonendemic countries in 2022, health agencies faced the challenge of having no treatments or vaccines made specifically for it. However, existing smallpox treatments were found to be effective against mpox, given their viral similarities.Treatment and Vaccine OptionsThe smallpox vaccine Jynneos and the antiviral drug tecovirimat (Tpoxx) continue to be the leading tools for protection and treatment. Experts expect them to work against both clade I and clade II mpox.Jynneos requires two doses for complete protection, while Tpoxx can be given either intravenously or orally, depending on the severity of symptoms.
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