As the Ebola epidemic (hopefully) winds down, there is a hidden cost: Africa’s great apes are declining. With a physiology similar to humans, apes are susceptible to many of the same diseases.
Ebola has always been present in Africa’s equatorial forests but for unknown reasons it exploded amongst the ape population sometime in the 1990s. For obvious reasons, it is difficult to determine the exact toll among gorillas and chimpanzees living in the dense forests of the region but indications are that it is high.
It was long believed that territoriality and isolationist tendencies would limit the spread of communicable diseases among apes, but such assumptions might not be warranted, according to Peter Walsh and colleagues, writing in American Naturalist. For example, lowland gorillas were observed to closely approach the carcasses of gorillas from other groups.
In the case of Ebola, where large quantities of infected fluids are ejected from the body prior to death, merely approaching a carcass (e.g. walking over leaves soaked with infected waste) might be enough to cause illness. Western lowland gorillas, closer to the current outbreak, eat more fruit and have large territories, increasing the likelihood of encountering infected individuals.
An even bigger risk is communal feeding at fruit trees. Gorillas are very particular about the fruit they eat, so in any given area there will be a limited number of trees available that meet their criteria. Walsh realized that even if they visited at different times, multiple groups utilized the same trees. The picky gorillas even shared the same fruit— adults sometimes ate previous groups’ leftovers, a potential exposure to infected saliva.
Furthermore, chimpanzees and gorillas were observed sharing fruit trees, so transmission is possible between species as well as between groups. The trees may also be shared by other Ebola reservoir species, e.g. fruit bats, exposing apes to multiple Ebola sources at once. Given the close relationships within each group, it only takes one individual to introduce the infection to their own group—and possibly wipe them out.
It’s unknown why Ebola suddenly became a problem. The best guess is that deforestation and other stresses have increased apes’ vulnerability to epidemics. If apes are forced into a limited area such as a national park, there will be fewer fruit trees to share and greater contact between groups, increasing the risks of an epidemic. The hypothesis is unproven, although unquestionably the available range has shrunk dramatically in recent decades.
Some might ask why apes even matter while humans suffer, but if we are going to mess with our ape cousins in so many other ways we have an obligation to protect them from epidemics as well. From a purely practical standpoint, an infected ape population is a potential source of a new human outbreak, so controlling Ebola’s spread in apes is to our benefit. Gorillas can’t learn to wear protective gear when mourning their dead, so the only hope is vaccination, and the research required for an ape vaccine will ultimately benefit everyone. Let’s hope our nearest relatives hold out long enough to receive it.