A rare strain of Ebola has prompted the World Health Organisation to declare a public health emergency of international concern. Most of the cases have occurred in the Democratic Republic of Congo, with more than 100 suspected deaths and nearly 400 suspected infections as of Monday.

Here is what we know about ​this strain of the Ebola virus, known as Bundibugyo.

What is Bundibugyo Ebola?

The current Ebola outbreak - so far limited to the Democratic Republic of ‌the Congo and Uganda - is due to a rare strain of the virus known as Bundibugyo, named after Uganda’s Bundibugyo province, where it was first identified during an outbreak in 2007-2008. A second Bundibugyo outbreak occurred in 2012 in the DRC.

Bundibugyo kills 30% to 40% of infected people, making it less lethal than the more common Zaire strain, which causes death in up to 90%, according ​to a global study published in 2024.

Bundibugyo is one of the four species in the genus Ebolavirus that cause life-threatening illness in humans. All Ebola viruses are ​transmitted through direct contact with the bodily fluids of infected animals or humans or objects contaminated with such fluids.

Body fluid transmission ⁠is a particular risk for hospital workers. A US doctor working in the DRC has been infected in the current outbreak.

According to the World Health Organisation, ebolaviruses initially cause flu-like symptoms, including fever, fatigue, malaise, muscle pain, headache, and sore throat, that can start suddenly, followed by vomiting and diarrhoea, and eventually by internal and external bleeding and multi-organ failure.

Are there treatments for Bundibugyo?

There are no approved vaccines or drugs for Bundibugyo ebolavirus. Emergency use authorisation would be necessary for the deployment of any experimental treatments or existing treatments that have been effective against other strains.

Potential candidates that have helped to control Bundibugyo in trials in non-human primates include Merck’s Ervebo, Mapp Biopharmaceutical’s MBP 134, and Auro Vaccines’ VesiculoVax.

NanoViricides said its experimental antiviral drug ​NV-387, currently in clinical trials against mpox, could be effective against the Bundibugyo strain. It mimics the immune cell surface proteins to which all ebolaviruses attach themselves ​and could thereby act as a decoy to “soak up” the virus and prevent it from attaching to healthy cells.

Earlier in the pipeline, an mRNA vaccine being developed in China has shown promise ‌against Bundibugyo ⁠in mice but has not yet been tested in primates.

For now, response efforts will rely on public health measures such as rapid case detection, isolation, contact tracing, infection prevention and control, safe burials, and community engagement, said Dr Daniela Manno of the London School of Hygiene & Tropical Medicine in a statement.

“These measures were critical in eventually controlling the 2014–2016 West Africa Ebola epidemic, the largest Ebola outbreak ever recorded, and if implemented rapidly and effectively, they can also help control this outbreak,” Manno said.

Is there a test for Bundibugyo?

Tests for Bundibugyo exist but ​are not widely used. Initial analysis ⁠of samples in the current outbreak using standard tests did not detect the infections.

“Because early tests looked for the wrong strain of Ebola, we got false negatives and lost weeks of response time,” Dr Matthew Kavanagh, director of the Georgetown University Centre for ​Global Health Policy & Politics in Washington, DC, said in a statement.

“By the time the alarm was raised, the virus had ​already moved along major ⁠transport routes and crossed borders,” Kavanagh said.

What makes BundiBugyo different from other strains?

Differences in genetic makeup between Bundibugyo and other ebolaviruses impact its virulence, or infectiousness, its diagnosis, and the availability of medical treatments.

Compared to the Zaire strain, which quickly reproduces itself to reach high levels in the patient’s body, Bundibugyo replicates more slowly.

Bundibugyo is also slower to invade, disable ⁠and kill ​immune cells, eventually crippling the patient’s immune defences.

The incubation periods for the Bundibugyo virus and the Zaire ​virus are nearly identical, averaging 8 to 10 days but sometimes lasting up to three weeks.

A recent study of survivors of the 2007 Bundibugyo outbreak that found persistent symptoms and immune and metabolic alterations nevertheless ​concluded that overall, Bundibugyo may have less severe long-term effects on the liver and kidneys than the Zaire strain.