Western Lowland Gorillas in Danger

Over the last 15 years the Zaire strain of Ebola virus has emerged repeatedly in gorilla and chimpanzee populations in Gabon and Republic of Congo (Congo Brazzaville) causing massive die-offs. Here I briefly review the impact of Ebola on gorilla populations, discuss the potential for future population impact, and describe ongoing efforts to protect remaining gorillas through vaccination.
The impact of Ebola on gorillas was first recognized after the 1994 and 1996 Ebola outbreaks in humans in villages on the fringes of the Minkebe forest block in north-central Gabon. The first human outbreak was caused by contact with the carcass of an infected chimpanzee and, in many independent reports, local villagers described finding carcasses of both gorillas and chimpanzees in the forest. Subsequent surveys conducted jointly by the World Wildlife Fund (WWF) and Gabonese Water and Forests Ministry suggested gorilla mortality rates on the order of 95% over an area covering 20,000-30,000 km².
There was then a lull until 2001, when human outbreaks flared up in the Mekambo region of northeast Gabon, again prompting reports of dead ape carcasses in the forest. The outbreaks then spread eastward into Congo, ultimately causing massive ape die-offs at the Lossi Sanctuary and Odzala National Park: probably the largest protected area population of great apes in the world. Surveys by the Wildlife Conservation Society, WWF, the Congolese and Gabonese Forest Ministries, the European Union's ECOFAC program, and the University of Barcelona confirmed not only the extent of impact on ape populations in Congo, but also the massive impact at two Gabonese parks (Mwagne and Ivindo) situated between Mekambo and the 1996 outbreak site in central Gabon. Together these Ebola outbreaks appear to have killed about one third of the world's protected area population of gorillas.
The underlying cause of the outbreaks appears to be a spreading epidemic in the reservoir host for Ebola, which genetic research by the Centre International de Recherches Médicales de Franceville in Gabon suggests is bats. As the bat epidemic has moved across Gabon and Congo, the virus has "spilled over" from bats to apes, with subsequent chains of secondary transmission amongst apes. Several large protected areas in Congo, Cameroon, and the Central African Republic lie in the putative future spread path.
In parallel to the Ebola crisis, poaching continues to cause serious declines in western gorillas. Most of the impact is not caused by subsistence hunting but rather by commercial hunting in which gorillas and other large mammals are hunted in remote areas and then transported large distances to urban markets. The emergence of commercial hunting as a major threat to gorillas has been fostered by an explosion of mechanized logging, which has created road access to once inaccessible areas and built towns full of bushmeat customers (salaried logging employees) in these areas. Although gorilla hunting is illegal in all range countries, protection efforts are chronically understaffed, underfunded, and confined largely to a few protected areas.
Both Ebola and poaching have already had massive impacts on gorilla populations and both represent serious threats to gorillas in the future. Consequently, the World Conservation Union IUCN recently upgraded western gorillas to the highest threat status (Critically Endangered) on its Red List of Threatened Species. Unlike most such cases, the change in status was not due to the dwindling of western gorillas to very low numbers. Accurate abundance estimates are not available but western gorilla numbers are probably in the tens of thousands. Rather, western gorillas fit a second criterion of risk, rapid decline: in particular, a decline of 80% in three generations or less. Gorilla generation time is about 22 years, survey data suggests a decline of at least 60% in the last 25 years, and the causes of decline are continuing. Therefore, western gorillas easily met the rapid decline criterion
Although many at first felt that the Ebola situation was hopeless, there are growing signs that controlling Ebola impact on wild gorillas and chimpanzees is feasible. Six vaccines have successfully protected laboratory monkeys against Ebola and would likely work on gorillas and chimps in the wild. Coordinated by the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, a consortium of research institutions, government laboratories, non-governmental organizations, and private biotech is now working to adapt these human vaccines for use on gorillas and chimpanzees in the wild. The major stumbling block now is money to fund the laboratory and fieldwork necessary to implement a wild ape vaccination program. More information on this program

Peter D. Walsh

In October 2007, a report was published in the Proceedings of the National Academy of Sciences by researchers of the CIRMF (Centre International de Recherches Médicales de Franceville) who found a new lineage of the Ebola virus isolated from wild apes in the Gabon/Congo region. It is capable of genetically merging with other strains to create new variants. This ability has important implications for vaccine development; a vaccine that is made up of weakened viruses could merge with the wild virus to form new strains, making the spread of the virus in humans and apes harder to predict and control (PNAS 104: 17123-17127).

Dr. Peter D. Walsh is a quantitative ecologist at the Max Planck Institute for Evolutionary Anthropology. He does both applied work on the distribution, abundance, and threats affecting apes and other large mammals, and basic research on the dynamics of disease in wildlife populations.

Western gorilla overview

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