Egyptian fruit bats in a cave in Uganda's Queen Elizabeth National Park are known to carry the deadly Marburg virus. (Bonnie Jo Mount/The Washington Post)

Scientists have discovered the deadly Marburg virus in fruit bats in Sierra Leone, the first time this cousin of Ebola has been found in West Africa. There have been no reported cases of people or animals with active infections. But the pathogen’s presence in the bats raises the potential for it to infect humans in a new region more than a thousand miles from previously known outbreaks.

There have been a dozen known Marburg virus outbreaks in other parts of Africa, most recently in Uganda in 2017. Like Ebola, Marburg virus initially infects people through contact with wild animals. It can then spread person to person through contact with bodily fluids. It kills up to 9 in 10 of its victims, sometimes within a week.

The discovery was made by two teams of U.S. researchers and their partners in Sierra Leone. One was led by the U.S. Centers for Disease Control and Prevention and Njala University; the other was led by the University of California at Davis and the University of Makeni.

The virus was found in Egyptian fruit bats, which are the natural reservoir for Marburg. That means the virus can live and grow inside the bats without harming the animals, and can be passed on to humans or other animals through bat saliva, urine or feces. The bats live in caves or underground mines throughout much of Africa and parts of the Middle East and southwest Asia.

Marburg virus has been detected in Egyptian fruit bats caught in sub-Saharan Africa, primarily in Uganda and Congo but also Kenya, South Africa and Gabon. But to find it in fruit bats in Sierra Leone — more than 1,600 miles from Gabon, the “closest dot on the map” of countries where the virus had previously been detected — provides scientists much more information about the potential viral landscape, said Brian Bird, a virologist and bat expert at UC Davis.

For a country like Sierra Leone that was devastated by Ebola, “this finding of a very dangerous pathogen on a par with Ebola can put them ahead of the game. They can take proactive steps” to warn communities about its hazards, said Jonathan Towner, a CDC ecologist who led the CDC team. Sierra Leone was among the three West African countries ravaged by the 2014-2016 Ebola epidemic, which killed more than 11,000 people.

Bats are commonly hunted for food across Africa. Public health messages can warn people not to eat bushmeat and to avoid direct contact with bats, as well as fruit that may have been nibbled on by the animals. Mining companies also need to be aware of potential risks if these fruit bats live in underground shafts, the scientists said.

U.S. officials are so concerned about Marburg becoming a global threat that Towner and experts from CDC and Uganda recently traveled to a bat cave deep inside a Ugandan forest to put GPS trackers on Egyptian fruit bats to see where they fly each night. They hope that information will help better predict areas most at risk and prevent future outbreaks.

CDC scientists attached GPS devices to Egyptian fruit bats captured in the Bat Cave in Uganda's Queen Elizabeth National Park. The bats were temporarily housed in a tent before being released. (Bonnie Jo Mount/The Washington Post)

The two teams in Sierra Leone have been working independently to find the natural reservoir for the Ebola virus. Unlike Marburg, researchers still don’t know what animal or animals carry Ebola, much less how it spreads to people. After testing thousands of bats, they found five bats from three underground caves in Sierra Leone tested positive for active Marburg virus infection. The bats were caught last year and testing took place this year.

In eastern and central Africa, the bats can roost in colonies of more than 100,000. But the colonies of Egyptian fruit bats identified in Sierra Leone so far have been much smaller, the scientists said, which may explain why there haven’t been any known Marburg virus outbreaks in that country.

Scientists don’t know whether Egyptian fruit bats in other parts of the world are also carrying the virus. They also don’t know how big a bat colony has to be to maintain the virus long-term, or whether other environmental factors play a role. For a virus to spill over into humans requires “a sequence of events that line up just right,” said Bird.

The largest and deadliest Marburg virus outbreak occurred in Angola in 2005. It killed 90 percent of the 252 people who were infected. Two of the four strains identified among the five Marburg-positive bats in Sierra Leone are genetically similar to the strain that caused the Angola outbreak.

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