The highly contagious disease – formerly known as monkeypox – has killed at least 450 people during an initial outbreak in the Democratic Republic of Congo.
It has now spread across parts of central and east Africa, and scientists are concerned about how fast a new variant of the disease is spreading and its high fatality rate.
WHO chief Tedros Adhanom Ghebreyesus said the potential for further spread within Africa and beyond “is very worrying”.
“A co-ordinated international response is essential to stop this outbreak and save lives,” he said.
Mpox is transmitted through close contact, such as sex, skin-to-skin contact and talking or breathing close to another person.
It causes flu-like symptoms, skin lesions and can be fatal, with four in 100 cases leading to death.
Outbreaks can be controlled by preventing infections with vaccines, though these are usually only available for people at risk or those who have been in close contact with an infected person.
There are two main types of mpox – Clade 1 and Clade 2.
A previous mpox public health emergency, declared in 2022, was caused by the relatively mild Clade 2. However, this time it is the far more deadly Clade 1 – which has killed up to 10% of those getting sick in previous outbreaks – that is surging.
There was a change in the virus around September last year. Mutations led to an offshoot – called Clade 1b – that has since spread rapidly. This new variant has been labelled “the most dangerous yet” by one scientist.
Since the start of the year, there have been more than 13,700 cases of mpox in the DR Congo, with at least 450 deaths.
It has since been detected in other African countries – including Burundi, the Central African Republic, Kenya and Rwanda.
It is hoped the declaration of mpox as a public health emergency will lead to research, funding, and the introduction of other international public health measures being accelerated.
Dr Josie Golding, from the Wellcome Trust, said it was a “strong signal”, while Emory University’s Dr Boghuma Titanji said the move “underscores the gravity of the crisis”.
Prof Trudie Lang, the director of the Global Health Network at the University of Oxford, said it was “important and timely”, but added that the emergence of a new strain meant there were “many unknowns that need to be addressed”.
In July 2022 the milder Clade 2 strain of mpox spread to nearly 100 countries, including some in Europe and Asia.
It spread rapidly, and there were more than 87,000 cases and 140 deaths reported during that outbreak, according to a WHO count.
Although anyone can catch monkeypox, the outbreak was largely concentrated among men who had sex with men.
That outbreak was brought under control by vaccinating vulnerable groups.
On Tuesday, scientists from the Africa Centres for Disease Control and Prevention declared a public health emergency.
The head of the organisation, Jean Kaseya, warned that this current outbreak could spiral out of control if immediate steps were not taken to contain it.
“We must be proactive and aggressive in our efforts to contain and eliminate this threat,” he said.
