The Marburg virus, a deadly cousin of Ebola, has been isolated in fruit bats in Sierra Leone, marking the first time it has been found in West Africa.

Five bats caught in three health districts tested positive for the virus, according to two teams of scientists. One was led by the Centers for Disease Control and Prevention and Sierra Leone’s Njala University, and the second by the University of California, Davis, and Sierra Leone’s University of Makeni.

Egyptian rousette fruit bats are known to harbor the virus elsewhere in Africa, “so it’s not surprising that we’d find the virus in bats there” in West Africa, said Jonathan S. Towner , an ecological diseases specialist who led the C.D.C. team.

The testing was done as part of a relatively new American-led effort to spot dangerous pathogens lurking in animals and take steps to prevent them from crossing over into humans and triggering lethal outbreaks.