Global health officials are tracking an outbreak of Ebola caused by a rare species of orthoebolavirus called the Bundibugyo virus that has so far killed more than 130 people and infected more than 500 others, with more cases likely.

First alerted to the outbreak in the Democratic Republic of the Congo (DRC) on May 5, the World Health Organization (WHO) has since declared it to be “a public health emergency of international concern,” the organization’s highest formal alert. 

Infection with Bundibugyo has a mortality rate of up to 50 percent. Here’s what scientists do and don’t know about this virus and why it’s so concerning to experts.

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What is the Bundibugyo virus, and how does it differ from other forms of Ebola-causing viruses?

Orthoebolaviruses are Ebola-causing members of a family of viruses called filoviruses, which also include the Marburg virus. Scientists are currently aware of four species of orthoebolavirus that cause disease in humans. These include the Ebola virus (formerly called the Zaire virus), the species responsible for the biggest and worst Ebola outbreaks, as well as the Sudan virus, the Taï Forest virus and the Bundibugyo virus. 

Compared with the Ebola virus, Bundibugyo is a relatively rare species of orthoebolavirus, says Elke Mühlberger, a professor of virology, immunology and microbiology at Boston University. Before the current outbreak, there have only been two other known outbreaks of the Bundibugyo virus: one in Uganda in 2007, when the species was first identified, and another in the DRC in 2012.

The symptoms of an infection with the Bundibugyo virus resemble those of other orthoebolaviruses. Early on, they include an intense headache, high fever, body aches and fatigue. As the infection progresses, people can develop intense vomiting and diarrhea, which can become life-threatening. Orthoebolaviruses are also known to cause hemorrhagic fever, in which the virus infects specific immune cells, triggering a massive inflammatory response that can lead to internal bleeding and organ failure.

Is the Bundibugyo virus more deadly than other Ebola-causing viruses?

Filoviruses in general have high fatality rates in humans. Data from past outbreaks show that the disease caused by the Ebola virus has a fatality rate of up to 90 percent if it is left untreated and between 50 and 60 percent with medical care; this includes both supportive treatment and vaccines and antibody therapies.

The Bundibugyo virus, by contrast, seems to cause milder but still severe disease. Its fatality rates range from 30 to 50 percent, according to the WHO. “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,” Mühlberger says. Some virologists also caution that the fatality rate is based on just two outbreaks with far fewer infections than the Ebola virus, making it hard to know whether the current outbreak will show similar rates.

One of the main reasons these viruses are so deadly is that they are incredibly skilled at evading the body’s immune defenses—particularly our innate immune system, says Steven Bradfute, an immunologist at the University of New Mexico Health Sciences Center. Normally, the innate immune system’s job is to provide a rapid, nonspecific response to an invading pathogen and to alert other, more specialized immune cells to prepare for battle. Bradfute says some research suggests the Ebola virus is more effective at blocking this innate immune response than the Bundibugyo virus, which could be one reason the latter has a lower fatality rate.

“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 says.

Do treatments exist?

Currently, there are no treatments for the Bundibugyo virus. The two main arms of defense that communities have against viruses, including highly pathogenic ones such as those that cause Ebola disease, are vaccines and monoclonal antibody treatments. Vaccines work to prevent the infection in the first place, and antibody treatments boost the immune response to an infection by neutralizing and blocking viruses from entering cells.

Two antibody treatments and a vaccine exist for the Ebola virus, says Erica Ollmann Saphire, an immunologist at La Jolla Institute for Immunology in California, but they are unlikely to have any significant effect on the Bundibugyo virus, she says. When Bundibugyo’s genetic sequence was first published in 2008, shortly after the initial 2007 outbreak in Uganda, it showed that the virus was more than 30 percent genetically distinct from all other known orthoebolaviruses, which made it different enough to be its own species and meant that vaccines designed to treat other orthoebolaviruses were likely to be less effective. “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 says.

Researchers are developing vaccines specifically for the Bundibugyo virus, and early candidates have been shown to be highly effective in animal trials. But so far there has been little interest from large drug companies to further test these vaccines in humans because they likely wouldn’t be profitable, Saphire says. Scientists are also working on creating vaccines that could protect against multiple orthoebolaviruses at once, which may be far more effective at preventing outbreaks.

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