Mpox, formerly known as monkeypox, has been a public health concern in parts of Africa since 1970, but received little global attention until it surged internationally in 2022, prompting the World Health Organization to declare a global health emergency. That declaration ended 10 months later.

A new strain of the virus, known as clade Ib, has the world's attention again after the WHO declared a new health emergency in August after the worst outbreak on record in the Democratic Republic of Congo spread to neighbouring countries.

There have now been cases reported in Burundi, Rwanda, Uganda, and Kenya, and further afield in countries including Sweden, India, Germany, the United States, and the United Kingdom stemming from travellers who visited the affected regions.

The viral infection typically causes flu-like symptoms and pus-filled lesions, and while usually mild, it can kill. Children, pregnant women and people with weakened immune systems, such as those with HIV, are all at higher risk of complications.

Mutating fast

The latest strain is a mutated version of clade I, a form of mpox spread by contact with infected animals that has been endemic in Congo for decades.

A mutation is a change in an organism’s genetic material. A virus survives by finding a host and using its living cells to replicate, or to have itself reproduced. When a virus invades a host, it enters its cells and hijacks the cell’s natural production systems to make new copies of itself.

When it makes millions of copies of itself and moves from host to host, not every copy is identical. These small mutations accumulate as the virus is passed on – and are copied again and again. Many are harmless, but some can affect the severity or transmissibility of the virus.

Genetic sequencing of clade Ib infections, which the WHO estimates emerged in mid-September 2023, show they carry a mutation known as APOBEC3, a signature of adaptation in humans.

The virus that causes mpox has typically been fairly stable and slow to mutate, but APOBEC-driven mutations can accelerate viral evolution, said Dr. Miguel Paredes in August, who is studying the evolution of mpox and other viruses at Fred Hutchison Cancer Center in Seattle.

"All the human-to-human cases of mpox have this APOBEC signature of mutations, which means that it's mutating a little bit more rapidly than we would expect," he said earlier this year.

The mpox family tree

Each virus has a unique genetic material, or DNA. Scientists can unravel and document this information as samples are taken.

Scientists can upload the sequences of the viruses to databases like GISAID, where the global research community can then track these changes and determine when or where major new strains are forming.

The mpox virus has two main clades, or subtypes, and several subclades, such as Ib, which is driving the latest outbreak.

Clade I is typically a more severe form of the disease, with fatality rates of 4%-11%, compared to around 1% for clade II – but scientists have cautioned that these differences could be linked to the quality of healthcare in the areas where the disease has spread. Testing has also been a challenge, particularly in isolated rural areas, so the full picture of disease severity is not clear.

Transmission

Mpox is primarily transmitted by close physical contact.

In the past, the disease was predominantly acquired through human contact with infected animals. That is still driving a rise in Congo in clade I cases – also known as clade Ia - likely due in part to deforestation and increased consumption of bushmeat, scientists said. It often spread to children in this way in the past, who then passed it on to family members, but that was usually where outbreaks ended.

Things have changed in recent years: the outbreak of clade II mpox spread globally mainly through sexual contact among men who have sex with men, showing that the virus could transmit more efficiently among humans than was previously thought.

Most mpox cases this year are in Africa, where numbers could be underreported due to patchy testing. There have been more than 46,000 suspected cases and more than 1,000 suspected deaths, but only 11,000 confirmed by labs, including 53 deaths. These have mainly been in Congo and caused by clade Ia or b.

In total there have been more than 115,000 confirmed mpox cases in more than 120 countries since 2022, mostly in Europe and the Americas. That includes all forms of the disease.

Clade Ib, the new form of mpox, spreads through close contact. This includes sexual contact, as in an outbreak among sex workers in a mining town in Congo. But it has also spread in refugee camps among children in close contact.

Factors such as crowded households, lack of access to disinfection and hygiene supplies, medical care, and malnutrition, may add to the risk, according to the U.S. Centers for Disease Control and Prevention (CDC).

Mpox outbreaks can be stopped with changes in behaviour, including isolating patients if they have symptoms. Basic hygiene measures like frequent handwashing, as well as other public health tools like contact tracing and vaccination, are all important too, although vaccines were unavailable in Africa until October this year. They were widely used globally in the 2022 outbreak.

Vaccinations began in Congo last month.