Fruit bats may be acting as reservoirs of the killer Ebola virus, responsible for several deadly outbreaks in central Africa, research suggests.
Bats are thought to harbour several deadly viruses
Three bat species captured during outbreaks between 2001 and 2003 in Gabon and the Republic of Congo show evidence of symptomless infection.
Writing in Nature, researchers in Gabon say this means the animals may play a key role in spreading the virus.
They say local residents should be encouraged to refrain from eating bats.
The first human outbreak of Ebola was recorded in 1976, but scientists have still to pin down which species harbour the virus.
If bats are among the culprits, they are more likely to pass the virus on to great apes such as gorillas and chimpanzees, which have been badly affected.
One of the most virulent viral diseases
Damages blood vessels and can cause extensive bleeding, diarrhoea and shock
Killed more than 240 people in the Democratic Republic of Congo in 1995
Transmitted by infected body fluids
Kills up to 90% of victims, depending on the strain
There is no cure
However, it is also possible that bats could infect humans directly.
Researchers from the Centre International de Recherches Medicales de Franceville trapped and tested more than 1,000 small animals in Ebola-affected areas.
They found fruit bats of three species - Hypsignathus monstrosus, Epomops franqueti and Myonycteris torquata - had either genetic sequences from the virus or evidence of an immune response to it.
Traces of the virus were found in the animals' liver and spleen - two organs specifically targeted by Ebola.
Each of the three species has a broad geographical range that includes regions of Africa where human Ebola outbreaks occur.
Previous research has suggested that bats may also harbour the deadly Marburg and Sars viruses.
Death rates from Ebola among the great apes tends to increase during dry seasons, when food is scare in the forest, and animals are more likely to come into contact with other species as they compete with food.
Immune function in bats is also known to change during these periods, providing the virus with more favourable conditions in which to reproduce.
Professor Tony Hart, of the University of Liverpool, told the BBC News website bats had long been suspected of harbouring Ebola.
One of the earliest outbreaks of the virus in Sudan was linked to a cotton factory filled with the animals.
Professor Hart said: "This is another piece in the jigsaw. It is good to know where this virus comes from, and it might help us to get some idea about the diversity of different strains.
"But whether it will enable us to do anything about the virus is another matter.
"Ebola tends to amplify itself through the great apes, so the best way to avoid infection is to avoid contact with them."
Dr Dilys Morgan, of the Health Protection Agency, said bats appeared to harbour many viruses that were posing a growing threat to man.
For instance, they have been implicated as the natural reservoir for the recently discovered nipah virus, which also produces deadly fever.
"Bats are long lived, highly gregarious animals, and there is a suspicion that they may have modified immune systems which we don't fully understand that can harbour these viruses," she said.
Dr Morgan said humans were coming into increasing contact with bats because agriculture was encroaching into territories where the creatures traditionally thrived.