Animal Origins of the Ebola Epidemic
Ebola seemed to come out of nowhere.
On March 25, 2014, the World Health Organization first confirmed an outbreak in four districts of Guinea. There was a total of 86 suspected cases.
By August, there had been over 3,000 cases. By November? 16,000.
The media exploded with reports of the virus’ spread, sparking international fear and speculation. It seemed as though Ebola had jumped out of the jungle poised for pandemic success.
But, fortunately, science tells us otherwise. While the future of Ebola’s spread remains uncertain, we do know about its past. Most serious human viruses originate in animal populations. The virus multiplies and mutates in its animal “reservoir” until it can break into the human population.
So, what is Ebola? Where did it come from? And how did the epidemic begin?
“Ebola virus” is actually a genus name referring to five separate species of the virus, each of which has been historically associated with different, mostly small, outbreaks. Scientists also track the Ebola virus in our primate cousins; there are frequent outbreaks in wild chimpanzee and gorilla groups.
The current human epidemic is caused by the Zaire virus species. This virus was first observed in 1976, when it caused a relatively small outbreak in Sub-Saharan Africa. But what is a virus from Zaire (officially known today as the Democratic Republic of the Congo) doing on Africa’s Western coast, in Guinea?
The answer, according to scientists, probably has to do with bats.
For a virus to jump from an animal host to a human one, two things must occur: first, humans must come into contact with infected animals – which can occur on farms or through hunting and eating wild game. Second, the virus must mutate to infect humans. This is why most human viruses come from other mammals, such as bats (the source of SARS), primates (HIV), and pigs (H1N1, “swine flu”): it doesn’t take much for a mammalian virus to adapt to infect humans.
Despite the presence of Ebola in chimp and gorilla populations, scientists think these primates are secondary hosts, like humans, and not the virus’ primary reservoir. The top suspects for this role are colonies of fruit bats, which can transmit the virus long distances and then ultimately convey it to human hosts.
Some analysts suspect an unusually dry season may have contributed to the outbreak, perhaps by increasing the number of animals infected and the likelihood of their contact with humans.
Yet these linkages, and the precise origin of the virus’ transmission from animals to humans, remain mysterious. Was the infection transmitted through urine from bats nesting near a Guinea village? Was it a slightly undercooked piece of wild meat? Blood from a wounded animal? Continuing research, primarily genetic analysis of the virus in animals and humans, may yield answers – and, most likely, further questions.