Even without lab confirmation on the human deaths, scientists are treating this epidemic as an Ebola outbreak. "The clinical presentation of the cases, the high death rates and confirmed reports of primate deaths which tested positive for Ebola all point in that direction," said the WHO's Thompson.
The first victim, or "index case," is believed to have eaten or come in contact with an infected animal. An Ebola epidemic in the same region in 2001, which killed 73 people, was also linked to people eating infected primates. After the initial infection, the virus is usually transmitted from human to human.
Containing the epidemic is particularly difficult because the affected villages are tucked into impenetrable forests about 440 miles (710 kilometers) from Brazzaville, the capital of the Republic of Congo. Cordoning off the area is impossible. Cultural practices also complicate efforts. Religious rites, for example, dictate that family members wash the body of a dead person before burial.
While past outbreaks have been concentrated in, say, a village, the current epidemic is "multi-centric," or spread over a vast area. This suggests that the natural host for Ebola could also be active over a large area. Scientists have speculated that insects, or maybe birds, could carry the virus.
In December, a Purdue University science team presented new research that links Ebola with birds. According to the study, the outer protein shell of filoviruses, such as Ebola, have a biochemical structure similar to retroviruses carried by birds, making a common evolutionary origin more likely.
"There can be no doubt now that an ancestral virus had a shell that evolved to become the shells of the Ebola virus and bird retroviruses," said David Sanders, the professor who headed the research team.
Sanders stresses that his discovery does not prove that birds are the natural reservoir for Ebola. But it makes them more plausible hosts. The prospect of migratory birds carrying Ebola has obvious health implications.
Some scientists already worry that Ebola could mutate and become airborne. Recent outbreaks have suggested it can evolve on its own. All the Ebola subtypes have shown the ability to be spread through airborne particles under research conditions. One strand, Ebola-Reston, may have been transmitted from monkey to monkey through the air in a Virginia science lab. So far there have been no similar transmissions involving humans.
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