Bundibugyo virus fuels Congo and Uganda Ebola emergency

The World Health Organization has declared the Ebola outbreak in the Democratic Republic of the Congo and Uganda a “public health emergency of international concern.” The epidemic is caused by the rare Bundibugyo virus—an uncommon Ebola-causing species with no

By May 5, health officials had already detected an Ebola outbreak in the Democratic Republic of the Congo. When the World Health Organization later escalated it to a “public health emergency of international concern. ” the stakes became unmistakable: the death toll and confirmed cases kept rising. and more infections were expected.

The virus behind the outbreak is Bundibugyo virus, a rare species of orthoebolavirus that has been linked to only two other known outbreaks. So far, Bundibugyo has killed more than 130 people and infected more than 500 others, according to the current tracking by global health officials.

Bundibugyo belongs to the group of viruses known as Orthoebolaviruses, which can cause Ebola disease. These viruses are part of the filovirus family, which also includes the Marburg virus. Scientists recognize four species of orthoebolavirus that cause disease in humans: the Ebola virus—formerly called the Zaire virus—the Sudan virus. the Taï Forest virus. and the Bundibugyo virus.

Elke Mühlberger, a professor of virology, immunology and microbiology at Boston University, described Bundibugyo as a relatively rare species. Before this outbreak. it had been identified in only two other known outbreaks: one in Uganda in 2007. when the species was first identified. and another in the DRC in 2012.

The symptoms can be hard to distinguish early from other orthoebolaviruses. Early signs include an intense headache, high fever, body aches and fatigue. As the illness progresses, people can develop intense vomiting and diarrhea, which can become life-threatening. Orthoebolaviruses also cause hemorrhagic fever by infecting specific immune cells. triggering a massive inflammatory response that can lead to internal bleeding and organ failure.

Severity is part of the worry—but so is uncertainty. Bundibugyo infections have a mortality rate of up to 50 percent, and the WHO reports fatality rates ranging from 30 to 50 percent. Mühlberger cautioned that calling it “mild” misses the human reality: “It kind of sounds cynical to say that it is mild because if a third of patients die. it means it is still very dangerous.”.

Some virologists also warn that those fatality numbers come from only two outbreaks, both with far fewer infections than the Ebola virus has produced. That makes it difficult to know whether the current epidemic will follow similar rates.

In general. filoviruses are skilled at defeating immune defenses—especially the innate immune system. which normally provides a rapid. nonspecific response and signals other immune cells to respond. Steven Bradfute. an immunologist at the University of New Mexico Health Sciences Center. said some research suggests Ebola virus is more effective at blocking this early innate immune response than Bundibugyo virus. He pointed to a mechanism researchers associate with differences in viral proteins: some orthoebolaviruses have proteins that block early innate responses. and different changes to those proteins can alter how well that blocking works.

“The [orthoebolaviruses] have a couple of proteins that are good at blocking that early innate response. And different changes to those proteins that are found in different [orthoebolaviruses] may not be as good at blocking that, and that could lead to less pathogenic infection,” Bradfute said.

That immune battle matters in part because there are no treatments for the Bundibugyo virus right now.

For Ebola disease more broadly, the two main tools communities rely on are vaccines and monoclonal antibody treatments. Vaccines work by preventing infection in the first place. while antibody treatments boost the immune response by neutralizing and blocking viruses from entering cells. But there are no approved vaccines specifically for Bundibugyo. and the antibody options designed for Ebola virus are not expected to meaningfully protect against it.

Erica Ollmann Saphire. an immunologist at La Jolla Institute for Immunology in California. said two antibody treatments and a vaccine exist for the Ebola virus—but they are unlikely to have any significant effect on Bundibugyo. When Bundibugyo’s genetic sequence was first published in 2008. shortly after the initial 2007 outbreak in Uganda. it showed the virus was more than 30 percent genetically distinct from all other known orthoebolaviruses. That level of difference, Ollmann Saphire said, makes it less likely that vaccines designed for other orthoebolaviruses will work well.

“Bundibugyo is the closest to [Ebola virus] compared to all other [orthoebolaviruses], but it’s different enough that [the vaccine for Ebola virus] may not induce sufficient cross-reactive protection,” Saphire said.

Researchers are developing vaccines specifically for Bundibugyo, and early candidates have shown high effectiveness in animal trials. Yet even as that work progresses. Saphire said large drug companies have shown limited interest in conducting further human testing—largely because such efforts may not be profitable. Scientists are also working on vaccines intended to protect against multiple orthoebolaviruses at once. a strategy that could better address the reality that outbreaks are not predictable.

“These outbreaks are very difficult to predict, and you never know what the next virus is,” Mühlberger said. “Everybody in the field is well aware that we need antivirals and vaccines that protect you against a range of these viruses.”

The WHO’s emergency declaration places the focus where science and policy meet: on a rare virus that has historically appeared in only a handful of outbreaks. is currently spreading in two countries. and still has no approved vaccines or established treatments. With more cases expected and mortality remaining substantial. the question facing public health teams is brutally practical—how to respond effectively when the next threat may not match the tools built for the last one.

Bundibugyo virus Ebola Democratic Republic of the Congo Uganda public health emergency of international concern filoviruses Orthoebolaviruses WHO mortality rate vaccines monoclonal antibodies