Health workers are rushing to contain an Ebola outbreak in the Democratic Republic of Congo that was detected late and appears to be spreading rapidly.

The World Health Organization (WHO) on Saturday night declared the outbreak a global health emergency after two cases were reported in neighbouring Uganda.

The agency has also warned the outbreak could be “much larger” than the case numbers and death toll suggest.

Here’s what you should know about the latest Ebola outbreak.

What is it and how does it spread?

Ebola is a rare but deadly disease caused by a haemorrhagic fever virus that is found in parts of sub-Saharan Africa. This is the 17th outbreak in Congo since the virus was first identified near the Ebola River, from which it gets its name, in 1976.

An outbreak from 2018 to 2020 ​in North Kivu and Ituri provinces was the second deadliest on record, killing nearly 2,300 people.

The main natural hosts of Ebola are fruit bats, which can carry the virus without becoming ill.

After jumping to people – bushmeat consumption and contact with saliva or other fluids are the most common routes of zoonotic infection – it spreads through contact with the bodily fluids, such as blood and vomit, of an infected sick or dead person.

The virus can survive in the body for a long time, posing a threat even after symptoms have subsided.

For example, Ebola can survive in the semen of male survivors for months after they have recovered.

An outbreak of Ebola in Guinea in 2016 was linked to a survivor who had recovered almost a year and a half earlier, but transmitted the virus sexually, causing a cluster of cases.

What is the Bundibugyo variant and what are its symptoms?

The ​current outbreak involves the rare Bundibugyo variant of the virus. Unlike the more common Zaire strain, there are no approved vaccines or treatments for it.

The Bundibugyo strain has caused only two known outbreaks before, one in Uganda in 2007 and another in the DRC in 2012.

It has a case fatality rate (CFR) ranging from 25 to 50 per cent, according to the US Centres for Disease Control and Prevention (CDC).

This is already the largest known outbreak of the Bundibugyo variant.

The symptoms are the same across all Ebola variants, and begin with a flu-like fever, fatigue, headache, muscle pain, sore throat, vomiting and diarrhoea, before progressing to internal and external bleeding, organ failure and death in severe cases.

On average, symptoms appear between eight to 10 days after exposure, but they can arise as soon as two days and as many as three weeks after infection.

How many cases are there and how many deaths?

At least 139 people are thought to have died and almost 600 suspected cases have been reported in the DRC’s conflict-hit, eastern Ituri province, according to the WHO. Eight cases have been confirmed by laboratory tests.

The first known case was a nurse who developed symptoms on April 24, but there are fears the virus has been spreading unchecked for weeks.

Most cases have been in people aged between 20 and 39 years old, and two-thirds have been in female patients, according to the CDC.

Where is it spreading?

The virus is spreading in a “vulnerable and fragile region,” according to the CDC.

Home to more than four million people, Ituri is a vast area of savanna, fertile highlands, and dense tropical rainforest which borders Uganda and South Sudan.

The region serves as a gateway for East African trade, connecting the resource-rich but landlocked DRC to the Indian Ocean ports of Mombasa in Kenya and Dar es Salaam in Tanzania.

Suspected cases have been identified in three districts: Bunia, Rwampara and Monbwaluk.

But there are concerns the outbreak could be considerably more widespread than the data suggests.

Dr Ngashi Ngongo, professor of epidemiology and principal advisor of programmes management at Africa CDC, said it was too early to discern how fast the disease was spreading.

“Once we get good surveillance data, we can then see the rate at which cases are increasing,” he said. “The surveillance system is still weak, the contact tracing is still very weak”.

Two cases have also been reported in Uganda’s capital, Kampala, with one death. Both had travelled from the DRC. There has not yet been any suspected onward spread in the country.

Uganda on Sunday postponed next month’s celebrations for Martyrs’ Day, ⁠a national ​holiday that usually draws thousands of pilgrims from eastern DRC, because of the outbreak.

Neighbouring countries have expressed concern about the possibility of further cross-border spread.

Dr Menelas Nkeshimana, Head of the Health Workforce Development Department at Rwanda’s Ministry of Health, who was on the frontlines of the country’s Marburg outbreak, said haemorrhagic fever epidemics should be detected and contained much earlier.

“Strong early detection benefits not only the affected country, but also protects neighbouring nations and the broader region,” he told The Telegraph.

When did the outbreak start?

It is not clear exactly when the current outbreak began – a fact that poses challenges for the response.

Health officials in the DRC are working on the assumption that the virus began spreading in mid-April, but the first confirmed case was not reported until May 14.

Two days later, there were eight confirmed cases, 246 suspected cases and 80 suspected deaths, according to the CDC.

Tedros Adhanom Ghebreyesus, the WHO director general, on Friday said the global health body first learned of suspected cases on May 5 and dispatched ​a team to Ituri, but samples ​collected in the field initially tested ⁠negative.

Jean Pierre Badombo, the former mayor of Mongbwalu, a mining town in Ituri, said people started falling ill in mid-April after a large open-casket funeral procession arrived from Bunia, the regional capital.

“After that, we experienced a cascade of deaths,” he said.

Is there a vaccine?

While a vaccine for the Zaire strain has been in use since 2019, there is currently no approved vaccine for the Bundibugyo variant, nor are there any vaccine candidates in late-stage clinical development that could be readily deployed to fight the outbreak.

As a result, the response effort is largely relying on classical public health control measures and supportive clinical care, said Dr Daniela Manno, Clinical Assistant Professor at the London School of Hygiene and Tropical Medicine.

These measures include isolating the infected, contact tracing, and enforcing infection prevention and control for health workers.

Four experimental treatments are under consideration for use against the outbreak, according to the Africa CDC. These include remdesivir, the antiviral drug widely used during the Covid-19 pandemic, and a monoclonal antibody therapy called DP134.

Officials said discussions were underway with the authorities in Uganda and the DRC to deploy the therapeutics under randomised controlled trial protocols, although none of the studies had yet begun.

What threat does it pose to the rest of the world?

The WHO has already declared the outbreak a public health emergency of international concern (PHEIC) – a step only taken eight times before now – after cases were reported in Uganda.

Experts say the outbreak poses a threat to the African continent, especially if cases are confirmed in neighbouring countries in the following days.

Jeremy Konyndyk, president of Refugees International and a former senior adviser to USAID, said the outbreak was “already dramatically bigger” than the 2014 to 2016 West African Ebola epidemic at a comparable stage.

“That is just astonishing and makes me quite nervous,” he said.

The 2014-16 outbreak in Guinea, Liberia and Sierra Leone was the deadliest Ebola epidemic ever recorded, killing more than 11,000 people and exposing major weaknesses in global outbreak response systems.

Mr Konyndyk warned that the world may now be less prepared to respond than it was a decade ago, largely due to reductions in international aid funding.

“Global aid cuts have made the international infrastructure to deal with such outbreaks vastly weaker than it was a couple years ago,” he said.

Lievin Bangali, International Rescue Committee’s senior health coordinator in DRC, said declining funding from international donors had weakened disease detection.

“When surveillance networks break down, dangerous diseases ​like Ebola are able to spread further and faster before communities and health workers can respond,” he said.

How are countries responding?

Governments, aid agencies and health authorities are now scrambling to contain the outbreak.

The DRC has extensive experience responding to Ebola outbreaks. It has established laboratory networks, trained outbreak response teams, vaccination strategies, and international partnerships that can be rapidly mobilised.

On Sunday, the government sent a delegation led by Samuel Roger Kamba, the health minister, to Bunia, Ituri’s capital, with tents to set up treatment centres to ​support strained local hospitals.

In Uganda, President Yoweri Museveni has postponed the Martyrs’ Day pilgrimage, an annual Christian holiday held in early June which usually draws thousands of people from the DRC.

In Britain, the UK Health Security Agency has activated its ‘Returning Workers Scheme’ for Ituri, under which UK aid workers, government staff, journalists and others going to affected areas must register before travel.

And on Monday the Trump administration imposed a travel ban preventing travellers who have been in the DRC, Uganda or South Sudan in the last 21 days from entering the US.

International aid organisations including the International Rescue Committee and Doctors Without Borders (MSF) say they have teams responding to the outbreak.

How have conflict and aid cuts complicated the outbreak response?

The security situation in Ituri, the region at the epicentre of the outbreak, remains volatile.

In recent weeks, clashes between rival armed groups have killed scores of civilians, exacerbating a humanitarian crisis marked by displacement, food shortages, and limited access to healthcare.

“This outbreak is hitting a country already stretched to breaking point,” said Dr Manenji Mangudu, Oxfam Country Director in the DRC. “Ongoing conflict and years of aid cuts have deepened a humanitarian crisis of staggering scale: one in four people are going hungry”.

He added that global aid cuts had left the DRC “effectively blind to Ebola”, weakening the surveillance systems that should have detected the outbreak weeks earlier.

“This crisis is arriving at a moment of critically depleted humanitarian funding. Without urgent financial assistance, efforts to save lives risk being fatally undermined,” he said.

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