Transmission electron micrograph of mpox virus particles (teal) within an infected cellNIAID
Urgent action is needed to try to contain the spread of a new strain of mpox transmitted mainly by heterosexual sex that has caused more than 1000 cases in the Democratic Republic of the Congo, say health experts dealing with the outbreak. They fear the condition is poised to spread to neighbouring countries and possibly further afield.
“It’s undoubtedly the most dangerous so far of all the known strains of mpox considering how it is transmitted, how it is spread and also the symptoms,” says John Claude Udahemuka at the University of Rwanda.
The new strain was first identified in the small mining town of Kamituga in South Kivu province in the east of the DRC in September. In recent weeks it has spread to cities in the area. It may already have reached neighbouring countries such as Rwanda, Burundi and Uganda, says Leandre Murhula Masirika at the South Kivu health department.

Yesterday, the city of Goma, which is on the border with Rwanda, confirmed its first cases, says Masirika.
The experts hope a vaccination campaign targeting sex workers and health workers can halt the spread of the new strain. This could use existing stocks of smallpox vaccines, which are known to protect against other mpox strains. However, it isn’t yet known whether any existing vaccine will work against this new strain, says Udahemuka. “We hope it will.”
So far, no vaccines used for mpox have reached the affected region, and when they will arrive is unclear.
For decades, there have been occasional outbreaks of mpox – formerly known as monkeypox – in Central Africa caused by clade 1 mpox, which circulates in non-human animals and can infect people who handle bushmeat. Clade 1 can spread by close contact with an infected individual or contaminated items, but doesn’t usually spread beyond households.
In September 2023, health workers in Kamituga noticed a number of cases that appeared to be spread via sex, as well as other close contact. It appeared different in other ways as well, causing more serious symptoms that lasted longer than usual, and with many cases among children. “They are seeing horrendous whole body rashes,” says Trudie Lang at the University of Oxford.
The sexual transmission suggested the cases might be due to the clade 2 mpox strain that spread to many countries around the world in 2022. In the West, vaccination was targeted at those most at risk, including men who have sex with men.
However, when Udahemuka and his colleagues sequenced the genome of samples from Kamituga in February, they discovered the cases were caused by a mutated strain of clade 1, now known as clade 1B. Unlike clade 2, clade IB is spread primarily by heterosexual sex and non-sexual contact.
“It’s incredibly worrying,” says Lang. There is high transmission between mothers and other carers and children, and also non-sexual person-to-person transmission outside of households, she says.
The mortality rate of clade 1B is around 5 per cent in adults and 10 per cent in children, says Lang. The virus can also cause miscarriages and stillbirths.
These figures are based on people who have been treated in hospitals, says Lang. It is possible that a larger number of people are becoming infected but developing milder symptoms or none at all.
If so, the true mortality rate will be lower. But if people can be infectious without symptoms then preventing the spread of the virus will be more difficult. “We’re only seeing the really severe cases and we don’t know how many non-severe cases are hidden,” says Lang.

The outbreak in Kamituga began in the rainy season, says Udahemuka. At this time the roads are poor and it can take days to reach the town, which may have limited the spread of the condition. Now the dry season has arrived, it has spread to many other towns.
“The cases are still going higher and higher every day,” says Masirika, who thinks it is only a matter of time before the new strain spreads.
Other countries should prepare for the potential arrival of the virus, says Udahemuka. Even more importantly, they should support the local health response in the region so it doesn’t spread, he says.

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