As an Ebola outbreak grows in the Democratic Republic of the Congo, researchers are scrambling to learn more about the rare virus that has sickened hundreds. What they find may be crucial to help turn the tide on a virus that, for now, is outpacing efforts to contain it.
The Bundibugyo virus has been behind just two previous outbreaks: One in Uganda that began in late 2007 and another in Congo in 2012.
The majority of Ebola outbreaks in Africa have been caused by the Ebola virus, one of four Orthoebolavirus species that sporadically sicken people. The biggest, the 2014–2016 outbreak in West Africa that caused more than 28,600 cases and more than 11,000 deaths, fueled efforts to develop vaccines and treatments. As a result, countries including Congo stockpiled vaccines and drugs in preparation for future outbreaks.
But those efforts have been futile in the face of the Bundibugyo virus.
The viruses that cause Ebola disease are “similar, but they’re different,” says Thomas Geisbert, a virologist at the University of Texas Medical Branch at Galveston. Each is genetically distinct in ways that can make vaccines or treatments designed for one virus largely ineffective against another.
But the viruses share a grim narrative: Of the four Orthoebolavirus species that infect people, three cause the same severe illness including fever, vomiting, bleeding and, sometimes, death. (The fourth has infected only one person.) “The disease course may be different, and the mortality rates may be different,” Geisbert says. “But in the cases that are lethal, the outcome, the disease towards the end, is pretty much the same.”
The race is on to find answers that might help fight this outbreak and prevent future ones. Here are three big questions about the Bundibugyo virus.
How will Bundibugyo’s traits affect the trajectory of this outbreak?
Knowing could help public health officials and health care workers prepare and figure out the best ways to allocate resources.
Ebola virus is the deadliest Orthoebolavirus, with mortality rates of up to 90 percent without treatment. It’s also known as Zaire, Congo’s name back when the virus was first isolated there in 1976. In comparison, around 30 to 50 percent of patients died in the two Bundibugyo outbreaks, although it’s unclear why the virus seems to be less deadly.
In the current outbreak, there have been 330 confirmed cases as of May 31, including 49 deaths. Another 116 suspected cases are being investigated. Most are in Congo but a few are in neighboring Uganda.
Studies in nonhuman primates — the closest animal model that mimics what happens in infected people — show that at a high, “worst case scenario” dose, the Ebola virus is “uniformly lethal” without treatment, Geisbert says. All infected macaques die roughly five to nine days after exposure.
Infection with the Bundibugyo virus, while still bad, isn’t quite as lethal. Around a quarter of animals infected with Bundibugyo typically survive. But the animals tend to be sick for longer. The ones that die succumb around 14 to 15 days after infection.
What that means for the ongoing outbreak in Congo is unknown. “We can’t predict that; it’s way too early,” Geisbert says. While a relatively low case fatality rate would be good news, a longer infectious period is “not a good thing,” as it could extend the outbreak. Ebola infections are contagious not only while patients have symptoms, but also after death.
Krutika Kuppalli notes that “case fatality rates are influenced by many factors beyond the virus itself.” The infectious diseases physician at UT Southwestern Medical Center in Dallas led an Ebola treatment unit in Sierra Leone during the 2014 West Africa outbreak. How quickly cases are detected, the strength of health care resources and access to them can factor into survival rates.
This Bundibugyo outbreak caught officials off guard in part because cuts in international aid to the region slowed disease surveillance and response. What’s more, because vaccines and treatments were designed for the Ebola Zaire virus, there are no available vaccines or treatments to protect people from Bundibugyo.
For now, managing an outbreak that spread widely due to delayed detection is the immediate concern. It’s important to ensure that people get the care they need — including fluid management to help with blood loss — in a region with ongoing conflict, Kuppalli says. “High-quality supportive care saves lives.”
What animal hosts the Bundibugyo virus?
Knowing where Bundibugyo virus hides out between outbreaks could help experts mount guardrails around potential sources of infection. But for now, that remains a mystery.
While the first known cases of Ebola disease were documented in 1976, it wasn’t until 2007 that the first outbreak tied to Bundibugyo virus was detected. The reason why Bundibugyo infrequently causes outbreaks could be because the virus is rare in nature, Kuppalli says. It could also be that officials misclassified prior outbreaks, or missed them due to limited surveillance. Or people may not often encounter the animals that carry Bundibugyo virus.
“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. Previous studies largely pointed toward fruit bats as the source, although insect-eating bats may also be involved in spreading the viruses to people. Preliminary genetic analyses suggest that the ongoing outbreak is the result of a single new spillover from an unknown reservoir into people.
Orthoebolaviruses including Bundibugyo may be capable of slipping inside the cells of a variety of African bats, Chandran and colleagues reported in 2025 in Cell Host and Microbe. Species including the hammer-headed bat (Hypsignathus monstrosus) and the hairy slit-faced bat (Nycteris hispida) popped up 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.
Researchers are getting closer, Chandran says. But “there’s still a lot of mud in the field.”
Could the vaccine and treatments developed to fight Ebola Zaire virus work against Bundibugyo?
Knowing what therapeutics might be effective could give public health workers a few more tools to try to contain this outbreak. It could also buy a little time to see if any Bundibugyo-specific vaccine and treatments that have been tested in animals will work in people.
But it’s not likely. The Bundibugyo virus is different enough from the Ebola Zaire virus that experts doubt that the tools will be effective at curbing the outbreak.
Lab-made antibody treatments go after a wine glass–shaped protein called glycoprotein. Glycoproteins coat the surface of the virus, but those found on various Orthoebolaviruses are constructed with different protein building blocks.
Approved antibody treatments that are effective against the Zaire virus “are going to be DOA just pretty much off the bat” in the Bundibugyo outbreak, Chandran says. Those antibodies can’t attach to parts of the glycoprotein that differ between the two viruses. The Zaire virus treatments are essentially blind to Bundibugyo viruses replicating in the body.
But in 2017, Chandran and colleagues discovered some antibodies from a survivor of the West African Ebola outbreak that were capable of attacking not only Zaire virus but also its close cousins. Two of those antibodies were eventually developed into a cocktail called MBP134 that 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.
“It’s a genuine therapeutic,” Chandran says. “It’s not just a prophylactic. You can give it once the animal is already sick and it reverses disease and recovers the animals almost completely.”
The treatment worked on animals in the advanced stages of the disease, says Geisbert, who was also involved in the testing. That would be helpful in an outbreak setting. But the drug is administered intravenously, which can be difficult in resource-challenged areas like Congo. “An oral antiviral would be fantastic in this situation,” he says.
Public officials in Congo do plan to test MBP134 in clinical trials during the ongoing outbreak — as well as an antibody treatment from the drug developer Regeneron. (Regeneron is a major financial supporter of the Society for Science, which publishes Science News.) The antibody treatment — a cocktail of three antibodies, one of which might work against Bundibugyo — is approved by the U.S. Food and Drug Administration for use against Zaire virus.
There also is an oral drug on the table. The antiviral, called obeldesivir, can protect macaques from developing disease after exposure to Zaire virus, but hasn’t yet been tested in people.
Several vaccine candidates attacking the Bundibugyo glycoprotein also exist, though none are ready to be deployed, Geisbert says. One candidate is based off Ervebo, a vaccine approved for Zaire outbreaks. Studies in macaques show that the Bundibugyo version of the shot protects macaques from disease not only before exposure, but also after.
“It’s really encouraging,” Geisbert says. But it’s difficult to draw definitive conclusions because the study included only a handful of animals, he adds. What’s more, 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, an organization that funds vaccine development, is accelerating development of that vaccine and two others in hopes of getting clinical trials started as quickly as possible.
Still, a months-long wait is not fast enough to turn the tide on this outbreak any time soon. “The hope,” Chandran says, “is that we come out of this with some kind of weaponry for next time.”
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