Where Ebola comes from - and why it matters
by Linda Weiford, WSU News
One year ago—on Dec. 6, 2013—an Ebola infection killed a 2-year-old boy in Guinea, West Africa, marking the start of the largest and longest known outbreak in the virus’s history.
The child is believed to be the first person to contract Ebola in the current outbreak, as documented by field researchers in the New England Journal of Medicine. (Click chart at right)
A year after the toddler’s death, the infectious agent has infected 16,900 people, killing 6,000 of them, according to the World Health Organization.
Where did it come from?
Ebola, like influenza and West Nile virus, is a zoonosis, meaning a pathogen that originates in animals and spills over into humans, said Guy Palmer, director of Washington State University’s Paul G. Allen School for Global Animal Health. While studies show that three species of fruit bats are natural reservoirs for the Ebola virus, meaning they are long-term hosts of the pathogen but aren’t sickened by it, it’s not clear how the toddler acquired the infection, Palmer said.
What is clear, according to researchers who used genetic analysis to trace the outbreak’s earliest known cases, is that after the child got infected, the virus spread among his family members, into their village and then beyond.
“If the boy was indeed the first case, how did he become exposed? asked Palmer, an infectious disease expert whose research on zoonotic diseases often takes him to Africa. “Did he pick up a piece of contaminated fruit dropped by a bat? Did he handle an injured bat that carried the virus? Scientists tracking its spread haven’t been able to pinpoint the source of transmission.”
Possibly, the virus came from another wildlife reservoir all together, where it had harbored inconspicuously in a rodent, monkey or bird, he said.
“Every new emerging infectious disease begins as a mystery,” he explained. “Researchers are still trying to connect the dots.”
Ebola is named after a river in West Africa near where the virus was first identified in 1976. Since then, sporadic and scattered outbreaks have made field research challenging, said Palmer. Unlike rabies, one of the oldest diseases known to human kind, “we don’t yet have a solid scientific understanding of which animals carry the Ebola virus, and of those animals, which ones can transmit the disease to humans.”
In past outbreaks, large numbers of gorillas and chimpanzees have been killed by Ebola, according to scientists monitoring the wild primate populations. When hunters butcher infected apes for bush meat, it’s suspected that the hunters, too, can contract the disease.
“That kind of information--knowing how the virus makes its leap from animals to humans--is critical if we are to prevent or contain future outbreaks,” said Palmer.
What happens after scientists definitely pin down Ebola’s reservoir host—or hosts?
“We will develop strategies to reduce human contact with those animals,” said disease ecologist Peter Daszak, president of the New York City-based EcoHealth Alliance, whose researchers examine how disruptions in ecosystems trigger the emergence of infectious diseases. Like Palmer at WSU’s Allen School, Daszak believes this Ebola outbreak highlights the need for more unified global surveillance strategies to better predict where diseases are likely to spill over into humans and how to spot them quickly when they do emerge.
Diseases such as Ebola, West Nile virus, SARS and Nipah “didn’t just happen,” said Daszak. They’re the result of human population growth and human activity in the natural environment.
“By knowing the animal origins of those diseases, we can modify human activities to prevent them from re-emerging in the future,” he said.