The monkeypox virus outbreak, which has seen more than 780 laboratory-confirmed cases in 27 countries, may be circulating outside Africa since 2017, according to scientists who analysed its DNA.

The virus is known to be endemic in western and central Africa, and this is the first time that the outbreak is most geographically widespread outside Africa. "We, therefore, suggest that the pattern we see... means that there has been sustained human to human transmission since at least 2017," wrote Aine O'Toole and the team at the University of Edinburgh in the UK, in a report.

The World Health Organisation has also maintained the monkeypox virus may be spreading "undetected". "The sudden appearance of monkeypox in many countries at the same time suggests there may have been undetected transmission for some time," said WHO Director-General Tedros Adhanom Ghebreyesus, in a statement.

Further, the UK team conducted genome sequencing of the monkeypox virus, and found that the viruses responsible for the new cases are closely related to ones detected in a small number of cases in Israel, Nigeria, Singapore and the UK between 2017 and 2019, New Scientist reported.

There are up to 47 DNA-letter changes in the latest viruses compared with these earlier cases. That is an unexpectedly high number given that monkeypox is thought to evolve slowly, by around one mutation per year.

About 42 of these 47 changes involve the DNA letters TT changing to TA, or GA to AA. There is a group of human enzymes called APOBEC3 that help defend against viruses by inducing mutations in their DNA, and these are the kinds of changes they produce, the report said.

"If these APOBEC3 edits are specifically indicative of replication in humans as opposed to another host species then this would confirm this entire clade to be representative of the emergence of a human epidemic by 2017," O'Toole stated in a recent update to their report.

Three of the 10 monkeypox viruses sequenced in the US also showed some differences, while still being related to the 2017 virus, the US Centers for Disease Control and Prevention said. The three cases were found in people who had travelled to different countries in Africa and the Middle East in 2021 or 2022.

The infections could be due to independent instances of the virus jumping from some animal reservoir into people. But since they were found to have lots of APOBEC3-like mutations, it could be said that monkeypox has been spreading quite widely in people in Africa since 2017, the report said.

However, the researchers were surprised to find that the existing viruses may be less fit than the 2017 ones because they are accumulating lots of mutations that are probably detrimental.

"The mutations we see in the virus today are certainly not ones that kill the virus or we wouldn't be seeing them, the virus would be dead. But there may still be some that are dragging it down a little bit, that is adding to the mutational burden," Emma Hodcroft from the University of Bern in Switzerland was quoted as saying.

While this is reassuring, we shouldn't assume that monkeypox won't evolve to be better at spreading in people if we give it a chance to do so, she says. And while the monkeypox cases so far may have been mild, this might not be true if the monkeypox virus starts infecting children or people who are immunocompromised, Hodcroft said.