The new Ebola sub-strain spreading through central Africa may have jumped from fruit bats to humans in the first incident of its kind.

Researchers are now racing to contain the disease in the Democratic Republic of Congo (DRC), where at least 204 people have died and an estimated 870 have been infected. 

The World Health Organisation (WHO) on Friday escalated its warning, upgrading the threat level from 'high' to 'very high'.

Health officials believe the outbreak may have been silently spreading for months before it was identified.

The earliest known victim was a nurse in the eastern city of Bunia who fell ill on April 27, though experts suspect the virus had already been circulating undetected long before then.

The scale of the crisis has grown dramatically in just days. Since the WHO declared a public health emergency last Sunday, suspected cases have surged from 246 to around 750, while deaths have climbed from 65 to 177. 

The epidemic is already the third-largest Ebola outbreak ever recorded, behind only the devastating 2014-16 outbreak that killed 11,300 people and the 2018 to 2020 epidemic that claimed 2,300 lives.

Scientists say the current outbreak is being driven by Bundibugyo Ebola - a rare strain previously seen only in two relatively small outbreaks in Uganda in 2007 and the DRC in 2012. 

However, genetic sequencing has shown this version differs significantly from earlier outbreaks, suggesting a fresh transmission from animals into humans.

'The current outbreak is almost certainly a new transfer from wildlife to humans,' Aris Katzourakis, a professor of evolution and genomics at the University of Oxford, said.

Experts believe fruit bats may be responsible, although no definitive source has yet been identified.   

David Matthews, a professor of virology at the University of Bristol, said bats were the most likely culprit because of their known links to earlier Ebola outbreaks. 

'But that is a guess and a starting point to look for better evidence,' he added. Primates are also known to carry the virus in the wild.

The emergence of a different Ebola species has created major problems for health workers. 

Existing vaccines and treatments were developed against the Zaire strain responsible for previous major epidemics - not Bundibugyo.

There are currently no approved vaccines, treatments or rapid diagnostic tests specifically designed for the strain now spreading through central Africa, severely limiting efforts to halt transmission.

Trudie Lang, professor of global health research at Oxford University, warned that the disease can initially resemble malaria, making it harder to identify in its early stages.

Although experts stress the chances of the virus spreading widely in the West remain low because Ebola spreads through bodily fluids rather than through the air, Lang warned countries could not afford complacency.

She said: 'It may seem like a distant problem but the world needs to step up. Although this is not Covid, people do get on planes. 

'Unless we have systems in place to respond globally in a co-ordinated way, it leaves us at risk of new diseases when they emerge in these settings.'

Fears are also mounting that the outbreak could spready into neighbouring South Sudan, a country already grappling with violence, severe flooding and food shortages.

'The potential of this virus spreading rapidly is very high — that has changed the whole dynamic,' said Dr Abdirahman Mahamud, the WHO's director of response operations, admitting officials had been surprised by the pace of transmission. 

'Linked to that is the capacity to respond to this outbreak, and [we have] insufficient control.'

The response effort inside the DRC has faced major obstacles. 

Last week, tents at treatment centres were reportedly set ablaze after grieving relatives were stopped from removing the body of an Ebola victim for burial. 

Some infected patients then fled the makeshift facilities, raising fears the virus could spread even further.

Alasdair Munro, a paediatric immunologist and infectious diseases specialist at the University of Southampton, said Bundibugyo Ebola has a mortality rate of roughly 32 per cent, compared with around 79 per cent for untreated Zaire Ebola. 

'The fact this makes [Bundibugyo] the 'mild' ebola is a depressing thought,' he added.

Doctors believe survival rates improve significantly when patients receive rapid treatment and access to modern hospital care.

Uganda has already reported cases linked to the outbreak.

On Saturday, officials confirmed three more infections, bringing the country's total to five cases and one death.

Those infected include a driver who transported Uganda's first patient and a health worker exposed during treatment.

An American surgeon infected with the virus has been evacuated to Germany, while another US doctor exposed to Ebola - though not currently showing symptoms - has been taken to the Czech Republic for isolation. 

The Washington Post reported claims that the Trump administration resisted allowing the two men to return to the United States.

International scientists are urgently attempting to adapt existing drugs and vaccines to fight the new strain. 

Officials are preparing trials involving four experimental treatments, including antibody therapies developed by Regeneron and Mapp Biopharmaceutical, alongside antiviral drugs produced by Gilead.

The WHO is also drawing up a shortlist of vaccine candidates that could be tested in the DRC, though officials warned any rollout remains months away.

Researchers at Oxford, who helped develop the Covid vaccine during the pandemic, have already begun work on a vaccine specifically targeting Bundibugyo Ebola using the same technology platform deployed in 2020.

Professor Teresa Lambe, head of vaccine immunology at the Oxford Vaccine Group, said: 'My hope is that this outbreak can be brought under control quickly and that vaccines are ultimately not needed. 

'Nevertheless, our team and partners will continue working to ensure that potential vaccine options are available if they are needed.'

However, the WHO warned the Oxford vaccine must still undergo animal testing before it can be trialled in Africa, slowing the process further.

Scientists are simultaneously working on broader vaccines capable of protecting against entire families of deadly viruses, including Ebola, Marburg and Sudan virus strains, though those projects remain at a much earlier stage.

Richard Hatchett, chief executive of the Coalition for Epidemic Preparedness Innovations (CEPI), said the fastest route may involve modifying the existing Ervebo vaccine made by US pharmaceutical giant MSD, which already works against the Zaire Ebola strain.

'People are confident that if we can develop that it has a very high probability of success. It's got a good safety track record.'

But Hatchett cautioned that altering the vaccine would not be straightforward. 'It is a tricky platform to work with - modifying it is not a trivial undertaking. It will be months before we have vaccines that are ready for clinical trials.'

Experts also warned the outbreak may have expanded unchecked because of the severe challenges facing healthcare systems in eastern Congo. 

Professor Helen Rees, a global health expert at the University of the Witwatersrand in Johannesburg, said the disease was not recognised until samples were eventually analysed in Kinshasa - around 1,800 miles from the outbreak zone.

'Surveillance is really difficult there,' she said. 'It mushroomed before it was recognised.'

The affected region is already facing a humanitarian catastrophe, with the WHO estimating there are two million displaced people and ten million facing acute hunger.

Lang described the area as particularly vulnerable because of its transient populations and overstretched healthcare infrastructure.

'These are mining areas with highly transient populations. You have sex workers and miners and truck drivers, you have really remote, very underfunded healthcare services, and there is a lot of malaria, TB and HIV.'

CEPI - founded after the 2014 Ebola epidemic to accelerate global vaccine responses - says the current crisis will serve as a major test of its ambition to develop vaccines within 100 days of new threats emerging.

'This outbreak is exactly the kind of scenario that we envisioned when we articulated the ambition of the 100-days mission,' Hatchett said. 

'Are we ready to do a 100-day mission? No. But this outbreak will be a good litmus test of where we are with respect to that mission. It will give us an update on how much progress we've made.'