National Geographic : 2015 Jul
Ebola 37 Ebola events, large and small, has been sporad- ic. During one stretch of 17 years (1977-1994) not a single confirmed human death from infec- tion with Ebola virus occurred. This is not a sub- tle bug that simmers delicately among people, causing nothing more than mild headaches and sniffles. If it had been circulating in human pop- ulations for those 17 years, we would have known. A virus can’t survive for long, or replicate at all, except within a living creature. That means it needs a host—at least one kind of animal, or plant, or fungus, or microbe, whose body serves as its primary environment and whose cell ma- chinery it can co-opt for reproducing. Some harmful viruses abide in nonhuman animals and only occasionally spill into people. They cause diseases that scientists label zoonoses. Ebola is a zoonosis, an especially nasty and perplexing one—killing many of its human victims in a mat- ter of days, pushing others to the brink of death, and then vanishing. Where does it hide, quiet and inconspicuous, between outbreaks? Not in chimpanzees or gorillas; field stud- ies have shown that Ebola often kills them too. Dramatic die-offs of chimps and gorillas have occurred around the same time and in the same area as Ebola virus disease outbreaks in hu- mans, and some carcasses have tested positive for signs of the virus. Scavenging ape carcasses for food, in fact, has been one of the routes by which humans have infected themselves with Ebola. So the African apes are highly unlikely to harbor Ebola. It hits them and explodes. It must lurk somewhere else. The creature in which a zoonotic virus ex- ists over the long term, usually without causing symptoms, is known as a reservoir host. Mon- keys serve as reservoir hosts for the yellow fe- ver virus. Asian fruit bats of the genus Pteropus are reservoirs of Nipah virus, which killed more than a hundred people during a 1998-99 out- break in Malaysia. Fruit bats also host Hendra Scientists wondered whether Angolan free- tailed bats might be Ebola reservoirs after they discovered that the first victim, a small boy named Emile Ouamouno, may have played in a tree (left) in Méliandou, Guinea, where the bats roosted. Above, Emile’s father holds snapshots of his family—all gone but him.