Ebola: Outbreaks Cause Crisis for Great Apes and Humans
Ebola, a virus that needs no introduction, has been wreaking havoc in West Africa. Although reports of death by hemorrhagic fever began only three weeks ago (the epidemic itself may have started as early as January), there are already over 100 deaths. The majority of these have occurred in the epidemic’s source country, Guinea, but new confirmed cases are also emerging in Liberia. Officials of the World Health Organization (WHO) are calling this epidemic the most challenging to date, at least in part because of widespread transmission, as well as misinformation and stigma surrounding the disease.
The abject fear associated with Ebola is not unwarranted. Having many characteristics of lethal diseases typified by blockbuster movies, Ebola hemorrhagic fever results in severe illness whose symptoms include diarrhea, vomiting, fever, and hemorrhaging from the eyes, ears, mouth, and rectum. Of the five known Ebola strains, Zaire Ebola virus causes the most serious disease – the result of infection is death in 9 out of 10 cases. Unfortunately, this is the virus responsible for the West African epidemic. Unlike the air-borne movie viruses, however, Ebola is transmitted through exposure to virus-laden fluid from the infected. So, as is the case with the current outbreak, the virus is transmitted in predictable “chains”, often to caretakers of the sick that are in direct contact with vomit, saliva, and blood.
While the pathology of Ebola is largely understood, its natural history and associations with wildlife are not. For example, the question of where Ebola “hides” in between human outbreaks remains a mystery. However, there is some evidence that the natural host of the virus may be fruit bats because they do not show symptoms of infection, but do shed the deadly virus particles.
Since the discovery of Ebola in 1976, human outbreaks of the disease have been well documented. Surprisingly, though, some of these outbreaks have not occurred in isolation; they are partnered with coincident epidemics occurring in nearby wildlife populations, especially great apes (gorillas and chimpanzees). Sharing 98% of our DNA, great apes are also victim to Ebola and show the same pathology as we do. Unlike human epidemics, however, wild ape epidemics tend to go unnoticed for months or even years. Between 1994 and 1997, three Ebola outbreaks occurred in human populations of northeastern Gabon. Each was linked to hunting and killing wild chimpanzees. Some of these chimpanzees were noted to demonstrate abnormal behaviours prior to their death, and it was known that uncharacteristic numbers of dead apes were being found in nearby forests. Retrospective analysis revealed that the gorillas and chimpanzees had suffered a prolonged outbreak of Ebola that resulted in thousands of deaths, and contact with these infected animals had likely led to the three secondary epidemics in humans.
In the last two decades, Ebola has been responsible for several catastrophic great ape population declines. For example, the Tai Forest (Ivory Coast) chimpanzee community was reduced by 50% in an outbreak of Tai Forest Ebola virus that lasted two years. When all Ebola mortality is summed together, an estimated one third of the world’s gorillas and chimpanzees have been killed by this disease. Outbreaks of Ebola are infrequent, but many wildlife populations are unmonitored. Therefore, infection of a single member of a highly affiliative animal species like chimpanzees can lead to population-level spread, especially if carcasses are left uncollected to be handled or scavenged by other animals. Indeed, research has predicted that a single outbreak will take several lifetimes for a given great ape population to recover from – perhaps even longer when other imminent threats (poaching, encroachment, pollution, habitat degradation) are included.
When all Ebola mortality is summed together, an estimated one third of the world’s gorillas and chimpanzees have been killed by this disease. Image Credit, Andy Nelson. Copyright JGI Canada
Despite the severity of the current West African outbreak, its origin remains unknown. However, all Ebola outbreaks share a common feature: initial contact with an infected animal. In this respect, Ebola is unique among the world’s deadly diseases. Unlike HIV, malaria, or tuberculosis, which are sustainable in the long term among human populations, Ebola outbreaks are (so far) unsustainable without recurring wildlife contact. Therefore, Ebola control is directly linked to conservation – understanding and mitigating the effects of wildlife diseases, as well as minimizing human encroachment on wild places can go a long way in preventing a future outbreak.
To find out how Jane Goodall Canada monitors chimpanzee populations, click here