Congo Ebola surge stalls as Bundibugyo unknowns persist

unknowns about – As the Ebola outbreak in the Democratic Republic of the Congo grows, researchers say key uncertainties about the Bundibugyo virus are limiting what clinicians can deploy immediately—especially because existing vaccines and treatments were built for Ebola Zaire

For the second morning in a row, responders in eastern Congo are trying to outpace a virus that has already slipped past the best defenses they had prepared.

The outbreak. centered in Ituri Province. has been described as fast-moving and hard to contain—an emergency unfolding in a region embroiled in decades-long conflict. where even routine disease surveillance can struggle. By May 31, there were 330 confirmed cases, including 49 deaths, and another 116 suspected cases were being investigated. Most cases are in Congo, with a few in neighboring Uganda.

Under that pressure, the scientific question that keeps coming up is painfully simple: what, exactly, is Bundibugyo doing in people—and what can clinicians do with what they already know?

Bundibugyo virus has been behind just two previous outbreaks: one in Uganda that began in late 2007 and another in Congo in 2012.

Most Ebola outbreaks in Africa have been caused by Ebola virus, one of four Orthoebolavirus species that sporadically sicken people. After the 2014–2016 West Africa outbreak—more than 28. 600 cases and more than 11. 000 deaths—countries including Congo stockpiled vaccines and drugs for the next time. But this time the virus is different.

“Similar, but they’re different,” says Thomas Geisbert, a virologist at the University of Texas Medical Branch at Galveston. Each Ebola-causing virus is genetically distinct in ways that can make vaccines or treatments designed for one largely ineffective against another.

Those differences show up in more than lab notebooks. Three of the four Orthoebolavirus species that infect people cause the same severe illness. including fever. vomiting. bleeding and. sometimes. death. The fourth has infected only one person. Geisbert says the grim ending can look alike: “The disease course may be different. and the mortality rates may be different. ” but “in the cases that are lethal. the outcome. the disease towards the end. is pretty much the same.”.

Right now, officials and clinicians are working under uncertainty on three fronts—how Bundibugyo’s traits will change the outbreak’s shape, where the virus hides between episodes, and whether tools built for Ebola Zaire can work.

First, the virus’s traits in the real world—its timing and its lethality—are still difficult to pin down. Geisbert points to nonhuman primate studies, the closest animal model that mimics what happens in infected people. In a high. “worst case scenario” dose. the Ebola virus is “uniformly lethal” without treatment. with all infected macaques dying roughly five to nine days after exposure.

Bundibugyo is not as uniformly deadly in those experiments. Around a quarter of animals infected with Bundibugyo typically survive, but the ones that die succumb around 14 to 15 days after infection—and they tend to be sick for longer.

What that means for Congo is unknown. “We can’t predict that; it’s way too early,” Geisbert says. Even if a relatively low case fatality rate would be welcome. a longer infectious period is “not a good thing. ” because Ebola infections are contagious not only while patients have symptoms. but also after death.

Even this uncertainty has a human edge. Krutika Kuppalli. an infectious diseases physician at UT Southwestern Medical Center in Dallas who led an Ebola treatment unit in Sierra Leone during the 2014 West Africa outbreak. stresses that case fatality rates are shaped by more than the virus alone. How quickly cases are detected. how strong health care resources are. and whether patients can access them can all influence survival.

That matters in Congo now, where the outbreak caught officials off guard in part because cuts in international aid slowed disease surveillance and response.

And then there is the simplest, most limiting fact of all: because vaccines and treatments were designed for the Ebola Zaire virus, there are no available vaccines or treatments to protect people from Bundibugyo.

For the immediate present, that pushes the focus toward managing whatever cases arrive—ensuring people get the care they need, including fluid management to help with blood loss, in a region with ongoing conflict. Kuppalli’s message is blunt: “High-quality supportive care saves lives.”

Second, even finding likely sources of infection between outbreaks remains unsettled. “One of the problems with Ebola is that we still are trying to figure out what the reservoirs are,” says Kartik Chandran, a virologist at Albert Einstein College of Medicine in New York City.

When Bundibugyo outbreaks first became visible in records. it took until 2007 for the first outbreak tied to Bundibugyo virus to be detected. even though the first known cases of Ebola disease were documented in 1976. Kuppalli suggests Bundibugyo may cause outbreaks infrequently because the virus is rare in nature. But she adds other possibilities: officials may have misclassified prior outbreaks, or may have missed them due to limited surveillance. And people may not often encounter the animals that carry Bundibugyo virus.

Chandran says previous studies largely pointed toward fruit bats as the source, though insect-eating bats may also be involved in spreading viruses to people.

There is also a genetic clue emerging from the current outbreak. Preliminary genetic analyses suggest the ongoing outbreak is the result of a single new spillover from an unknown reservoir into people.

Some bat species are starting to look like contenders. The hammer-headed bat is one potential reservoir. Researchers have found genetic fragments of the Ebola virus and antibodies that recognize the virus in some bats, but not live virus.

Chandran and colleagues reported in 2025 in Cell Host and Microbe that Orthoebolaviruses including Bundibugyo may be capable of slipping inside the cells of a variety of African bats. Species including the hammer-headed bat (Hypsignathus monstrosus) and the hairy slit-faced bat (Nycteris hispida) appeared as possible reservoirs that harbor the viruses without getting sick themselves. Another global study posted May 19 on bioRxiv.org hints that the Angolan free-tailed bat (Mops condylurus) is worth keeping an eye on.

Chandran says researchers are getting closer, but “there’s still a lot of mud in the field.”

Third, researchers are looking hard at the most practical hope—whether the vaccines and treatments built for Ebola Zaire can be repurposed.

Experts say the answer is grim. Chandran doubts the tools will work well against Bundibugyo because the virus is different enough. Antibody treatments target glycoprotein, a protein on the virus surface. But glycoproteins across Orthoebolaviruses are constructed with different protein building blocks.

In practical terms. Chandran says approved antibody treatments effective against the Zaire virus “are going to be DOA just pretty much off the bat” in the Bundibugyo outbreak—because the antibodies can’t attach to parts of the glycoprotein that differ between the two viruses. He describes Zaire virus treatments as essentially blind to Bundibugyo as it replicates in the body.

Still, one pathway gives researchers a reason to keep pushing. In 2017. Chandran and colleagues discovered antibodies from a survivor of the West African Ebola outbreak capable of attacking not only Zaire virus but also its close cousins. Two of those antibodies were developed into a cocktail called MBP134.

Geisbert. who was also involved in testing. says the treatment has proved effective at protecting ferrets and macaques from both Zaire and Bundibugyo. as well as Sudan virus. the third Orthoebolavirus that is lethal to people. Chandran calls it “a genuine therapeutic. ” not just a prophylactic. adding that it can be given once the animal is already sick and it reverses disease and recovers the animals almost completely.

The problem is delivery. The treatment worked in animals in advanced stages of disease. Geisbert says. which could be helpful in an outbreak setting—but MBP134 is administered intravenously. which can be difficult in resource-challenged areas like Congo. “An oral antiviral would be fantastic in this situation,” Geisbert says.

Public officials in Congo do plan to test MBP134 in clinical trials during the ongoing outbreak. along with an antibody treatment from Regeneron. Regeneron is a major financial supporter of the Society for Science, which publishes Science News. The Regeneron antibody treatment is a cocktail of three antibodies. one of which might work against Bundibugyo. and it is approved by the U.S. Food and Drug Administration for use against Zaire virus.

There is also an oral drug on the table. The antiviral, called obeldesivir, can protect macaques from developing disease after exposure to Zaire virus, but it hasn’t yet been tested in people.

Vaccines are also under consideration. Several vaccine candidates attacking the Bundibugyo glycoprotein exist, though none are ready to be deployed, Geisbert says. One candidate is based on Ervebo, a vaccine approved for Zaire outbreaks. Studies in macaques show the Bundibugyo version of the shot protects macaques from disease not only before exposure. but also after.

“It’s really encouraging. ” Geisbert says—though he warns that definitive conclusions are hard because the study included only a handful of animals. There is also the timeline risk. It could take months to make a vaccine for use in human clinical trials. The World Health Organization estimates that the tweaked Ervebo shot could be available in seven to nine months.

The Coalition for Epidemic Preparedness Innovations, which funds vaccine development, is accelerating development of that vaccine and two others in hopes of getting clinical trials started as quickly as possible.

But months-long wait times do not fit the pace of an outbreak like this. “The hope,” Chandran says, “is that we come out of this with some kind of weaponry for next time.”

Back in Congo. the practical reality is that surveillance delays and limited tools are meeting a virus that can spread while patients are ill and even after death. Researchers can see the outlines—Bundibugyo’s different lethality in animal studies. its distinct glycoprotein. possible bat reservoirs—but they can’t yet turn those clues into a fast. reliable set of protections.

The unknowns are not academic. They decide how quickly doctors can prepare beds, how long infections linger, what gets tested first, and whether the next wave of cases will meet a response designed for the right virus.

Democratic Republic of the Congo Congo Ebola outbreak Bundibugyo virus Orthoebolavirus Ebola Zaire vaccines therapeutics MBP134 obeldesivir Ervebo reservoirs bat reservoirs Ituri Province surveillance delays