Since a deadly Ebola outbreak struck Zaire in 1995, central Africa has suffered nearly a dozen outbreaks of the deadly hemorrhagic fever. The most recent bout killed at least 29 people in the Republic of Congo. The human toll is tragic enough, but the disease has also killed thousands of great apes in the past 5 years. Now, a study suggests that the outbreaks spring from multiple introductions of the virus--and possibly from multiple hosts.
Public health authorities and conservationists are urgently trying to pinpoint the source of the continuing--and apparently increasing--outbreaks. They suspect the virus lurks in an animal that is somehow impervious to its deadly effects but can infect other vulnerable animals. One of the main puzzles is whether the recent flare-ups are all part of one larger outbreak that is spreading through the forest from animal to animal, or whether each cluster stems from an independent introduction of the virus from its elusive host. The answer has implications for strategies to stop the virus.
Eric Leroy of the Institute of Research for Development in Franceville, Gabon, and his colleagues sequenced virus samples from human and animal victims of five outbreaks in Gabon and the Republic of Congo between 2001 and 2003. Surprisingly, they found eight strains of the virus. Previous studies have suggested that the Ebola virus is relatively stable; isolates from nine human patients infected during an outbreak in 1996 and 1997 were identical, and sequences from a 1976 outbreak in Zaire and a 1996 flare-up 3000 kilometers away in Gabon differed by less than 2%. Therefore, the eight strains from Gabon probably diverged over decades or even centuries, suggesting that they came from different sources, the team reports in the 16 January issue of Science.
That raises a disturbing possibility, says co-author Pierre Formenty of the World Health Organization in Geneva: The various strains suggest that there may be more than one host species, perhaps insects as well as bats, rats, or shrews, he says, which could make controlling the ongoing outbreaks even trickier.
However, not everyone is convinced that the virus has emerged from the jungle more than once. Peter Walsh, an ecologist at Princeton University in New Jersey, who has argued that the human and ape outbreaks are part of a single epidemic, says the apparently different strains do not rule out a single spreading wave of Ebola. If the virus passes through multiple animal hosts as it moves through the forest, he says, it could quickly mutate.