Scientists have warned that they expect monkeypox cases to continue to rise this week as more infected people are traced by health authorities.
More than 90 cases have already been reported in Europe, the US and Australia, including 20 in Britain.
The World Health Organization said there were a further 28 suspected cases, in addition to the 92 confirmed in 12 member states which were not endemic for the virus. The global figure is unprecedented for a disease that is normally confined to central and west Africa.
Monkeypox does not spread easily between people, so doctors have been puzzled by the outbreak and by the appearance of cases on different continents at the same time. The virus is only transmitted from person to person through close physical contact – including sexual intercourse.
Symptoms are usually mild – headaches, aching muscles and exhaustion – but monkeypox also causes skin lesions which can become infected, resulting in secondary infections.
“I am certain that we are going to see more cases,” said Charlotte Hammer, a Cambridge University expert on emerging diseases. “First, health authorities are now – very actively – looking for cases, so we are more likely to spot people with mild versions which we might previously have missed or misdiagnosed.
“In addition, monkeypox has an incubation period of between one and three weeks so it is likely we will see new infections among those who were in early contact with the outbreak’s first cases.”
Monkeypox has been found outside Africa in the past, but the number and range of cases in the present outbreak is unusual.
“Essentially, we face two options,” Hammer said. “Either the virus is inherently different now or perhaps our susceptibility to it has changed. Alternatively, it could just be that we have an encountered a perfect storm of conditions that have allowed the virus to spread this way. I think the latter scenario is the more likely one.”
One possibility is that the effects of past mass smallpox vaccinations is fading, leaving fewer people with protection against the closely related monkeypox.
However, Prof Keith Neal, of Nottingham University, added: “Has the virus changed? Well it does not actually appear to be any more lethal, though something may have affected its transmissibility. And don’t forget this is a DNA virus and is unlikely to mutate at the rates that RNA viruses do, including those that cause Covid or HIV. Overall, I am not too worried.”