An international research team have traced the Mpox virus outbreak in monkeys to a species of wild rodent, identified as a new intermediate host.

Mpox – formerly known as monkeypox – is a virus endemic to Central and West Africa, where sporadic outbreaks occur in both humans and non-human primates. Despite rodents being long suspected as the natural reservoirs of the virus, direct evidence remained elusive.

The research team led by Helmholtz Institute for One Health, Austria, with the Tai Chimpanzee Project, Côte d’Ivoire, traced an mpox outbreak in sooty mangabeys displaying signs of mpox in Taï National Park.

Over the course of several weeks, at least 26 of the 80 mangabeys in the habituated group showed lesions and swollen lymph nodes, and four infant monkeys died.

Researchers analysed 170 monkey samples and found the mpox virus strain was nearly identical to one found in a rope squirrel three kilometers away and several weeks earlier. DNA traces of the same squirrel species were found in the monkeys’ faeces, and field observations documented a mangabey consuming those species of squirrels. This suggests the rope squirrel as the likely outbreak source.

Fabian Leendertz from the Helmholtz Institute, first author of the study in pre-print, told Science: “This is one of the first direct, genetically confirmed cases of mpox spilling over from a wild animal to a non-human primate.”

A new mRNA vaccine for mpox, developed by Moderna, is showing promise in monkey studies, but this study enhances the understanding how these cross-species outbreaks actually start.