The ranks of the ebolavirus genus have grown for the first time in a decade, with the identification of a new strain in free-tailed bats in Sierra Leone. It is not yet known if it is harmful to humans, but its discovery will assist scientists trying to better understand how the virus hides between outbreaks, and by extension help our efforts to better contain them.

The Bundibugyo ebolavirus described by scientists in 2008 was the last time a new ebolavirus strain was discovered, though the virus behind the devastating 2013 outbreak that killed more than 11,000 in West Africa was another by the name of Zaire ebolavirus, discovered in 1976.

The new virus was discovered by scientists working on the PREDICT project, a program led by the University of California, Davis that focuses on the spillover of emerging diseases from animal hosts to humans. Dubbed the Bombali virus for now, the new strain was identified in insectivorous bats roosting inside people's homes.

After running experiments in the lab, the scientists have concluded that although the virus could potentially infect human cells, there is no evidence yet of whether it has done so, or if it is harmful if it does. Further work is needed to assess the threat it poses to humans.

"Identifying viruses like Bombali virus and testing their capacity for human infection can enhance our understanding of the pre-emergent viral diversity circulating in animals," says Simon Anthony, a virologist at Columbia University and collaborator on the PREDICT project. "We want to discover viruses that have the genetic prerequisites for human infection and prioritize them for further study and intervention."

Bats are believed to be natural hosts of the ebolavirus, along with more than 100 other viruses, due to their superior immune systems. This allows them to maintain a 24/7 defence against some deadly diseases, but then transmit them through saliva and feces that humans or other animals come into contact with.

"The Ebola virus outbreak in 2013-2016 devastated local communities here in Sierra Leone," says the Honorable Minister of Technical and Higher Education in Sierra Leone, Professor Aiah Gbakima. "It is critically important to understand more about where these viruses come from and what causes them to spill over into humans. There is still much to do to understand the transmission dynamics of Ebola virus, but the discovery of Bombali virus in bats is an important step in the right direction."