Until this year's epidemic, Ebola did not exist in West Africa. Now with nearly 2,300 people dead from the virus, mostly in Liberia, Guinea and Sierra Leone, scientists still don't fully understand how Ebola arrived from Central Africa, where outbreaks of this strain of the virus had occurred in the past.
A new model by Oxford University, published in the journal eLife, takes a look at the most likely explanation -- that Ebola's animal reservoir, fruit bats, could spread the disease in the animal kingdom and to humans through the dense forest that spans 22 countries.
Several species of fruit bats are suspected -- though not confirmed -- to carry Ebola without showing symptoms. Unlike humans and other animals who are likely to die from an Ebola infection, bats can carry the disease and infect other bats and animals, such as monkeys and rodents through migratory activities.
Bats along with other animals, such as monkeys, are also one form of "bush meat" consumed in some African countries where there have been reports of Ebola outbreaks. And though consuming cooked bush meat is unlikely to spread the virus, hunting or preparing raw meat for consumption increases the likelihood that an infection might occur.
RELATED READING: 20,000 cases or 100,000? How researchers predict Ebola's spread.
According to the Oxford model, in addition to the seven countries who have reported Ebola outbreaks in this epidemic and in past outbreaks since the disease was identified 1976, 15 other countries are at risk. There are five known strains of Ebola, and the one currently causing the West African outbreak, Zaire, is the most virulent. The other strains, Sudan, Taï Forest and Bundibugyo, have caused contained outbreaks in Ivory Coast, Sudan, and Uganda in the past. And the Reston species has not caused any known outbreaks, according to the World Health Organization.
According to the Oxford prediction, these countries are at risk of animal-to-human transmission of Ebola by virtue of their geography: Nigeria, Cameroon, Central African Republic, Ghana, Liberia, Sierra Leone, Angola, Togo, United Republic of Tanzania, Ethiopia, Mozambique, Burundi, Equatorial Guinea, Madagascar and Malawi.
"Our map shows the likely ‘reservoir’ of Ebola virus in animal populations, and this is larger than has been previously appreciated," said the study's author Nick Golding, a researcher at Oxford University’s Department of Zoology. "This does not mean that transmission to humans is inevitable in these areas; only that all the environmental and epidemiological conditions suitable for an outbreak occur there.’"
MORE READING: In Liberia, Ebola strengthens its hold
[This post has been updated.]