In a letter published in Nature Medicine, researchers highlight how mpox — traditionally spread from animals to humans — is now showing clear signs of sustained human-to-human transmission.

Mpox is a viral infection caused by a virus that belongs to the same family as smallpox. The virus can cause a painful rash, fever, and swollen glands and, in some cases, lead to more serious illness. Mpox usually spreads through close contact with an infected person or animal.

Carlos Maluquer de Motes, Reader in Molecular Virology at the University of Surrey, said:

“The most recent outbreaks show that intimate contact is now a significant way the virus spreads. That shift in how it’s transmitted is leading to longer transmission chains and lasting outbreaks.”

The article notes that this change coincided with the rapid spread of clade IIb (a clade is a group of viruses that share a common ancestor) mpox viruses, but different clade I variants are now on the rise too. Researchers are also concerned because clade I viruses are thought to be more aggressive. These viruses appear to be accumulating specific genetic mutations — driven by enzymes in the human body — that may be changing viral properties, so the longer these viruses circulate amongst us, the higher the chances these mutations help mpox adapt to humans.

Although mpox was once mainly seen in Central Africa, the virus caused an outbreak worldwide in 2022 and is now causing outbreaks in multiple sub-Saharan countries. While it currently affects adults the most, the researchers stress that it has the potential to spread among other groups, including children, a group at greater risk of serious illness — although sustained transmission in children has not yet been reported.

Dr Maluquer de Motes added:

“Mpox control has to climb up the global health agenda. We have limited diagnostic tools and even fewer antiviral treatments. We urgently need better surveillance and local or regional capacity to produce what we need — otherwise, we are at risk of future epidemics.”

Unlike smallpox, mpox has an animal reservoir, meaning it can’t be fully eradicated. The authors warn that unless international action is taken now — including investment in point-of-care testing and new treatments — mpox will continue to re-emerge and threaten global health.