As mpox continues to generate localized outbreaks in Africa and occasionally elsewhere, researchers are working hard to understand how the virus spread globally in 2022 and how it might do so again.
A study released in December indicated that the strain responsible for the 2022 outbreak lingered in the testes of mice for weeks and caused tissue damage, raising concerns about its potential impact on male fertility—though these effects have yet to be explored in humans.
This study, available on the preprint server bioRxiv, awaits peer review. Meanwhile, the virus’s evolution continues. In December, health officials announced a new strain of the mpox virus that combines genetic components from two existing types for the first time. While it’s typical for viruses like mpox to evolve, increased spread could lead to them evading vaccine and treatment protections.
According to Boghuma Titanji, an infectious-disease physician at Emory University, the information we currently have suggests there’s still much to be learned about both existing and emerging strains. Given that mpox belongs to the poxvirus family, which includes smallpox, it poses a risk if it becomes entrenched in human populations and adapts further.
Mpox is evolving
Mpox infections can lead to painful, fluid-filled skin lesions, fever, and severe cases can even result in death. There are four identified clades of the mpox virus: clades Ia, Ib, IIa, and IIb.
This virus has been infecting humans since the 1970s, but it rarely spread widely until the late 2010s when a clade II strain triggered a significant outbreak in Nigeria. A similar clade IIb strain caused the 2022 global outbreak, infecting over 100,000 people and is still ongoing.
In 2025, a significant increase in clade I mpox infections occurred. Historically, this caused sporadic severe outbreaks in rural Central Africa, but a new subtype, clade Ib, began spreading in urban areas in late 2023, possibly via sexual interactions, raising concerns for scientists. The emergence of clade Ib closely resembles the trajectory observed with clade II before its global spread.
Over the past two years, researchers have been eager to discover how the new clades, Ib and IIb, differ from earlier strains. Evidence from rodent studies suggests these newer clades may be less deadly but are better at spreading among people, as they tend to result in milder symptoms.
Infected rats showed higher survival rates with clade Ib than with clade Ia, yet both transmitted similar levels of the virus. Moreover, clade Ib infections resulted in a delayed onset of visible skin lesions, which might explain more efficient transmission through sexual means, as individuals could unknowingly spread the virus before showing symptoms.
Fertility issues?
Another team of researchers looked into how clade IIb mpox infects mice. They discovered that infectious virus levels were high in the rodents’ testes for over three weeks post-infection, indicating that the male reproductive tract might act as a virus reservoir, thus explaining its effective transmission through sexual contact.
The researchers noted tissue damage that resulted in reduced sperm production. “We anticipated some inflammation, but finding this potential impact on male fertility was surprising,” remarked Alyson Kelvin, a co-author of the study and emerging-virus specialist at the University of Calgary.
