By monitoring a large population of gorillas during an Ebola outbreak in the rain forest of the Republic of the Congo, researchers have found that in a few months the virus exhibited dramatic--but disproportionate--impacts on group-dwelling and solitary gorillas. The findings offer a unique glimpse into the factors affecting the threat the deadly virus poses to great apes.

The work is reported in the July 12th issue of the journal Current Biology by a team of researchers including Damien Caillaud and colleagues from the University of Montpellier and the University of Rennes, France.

Ebola virus is extremely lethal for humans and other great apes. Since 1994, the Zaïre subtype of the Ebola filovirus has been responsible for nine human outbreaks in Gabon and the Republic of the Congo; a majority of these outbreaks have originated from the handling of infected great-ape carcasses. In fact, Ebola virus has become one of the major threats to the survival of western lowland gorillas (Gorilla gorilla gorilla) and chimpanzees (Pan troglodytes troglodytes) in this region. However, the causes of outbreak among wild animals, and the way the virus spreads, remain unclear. It has been argued that the infection of apes only occurs by way of the so-called "reservoir" species (unknown, but possibly fruit bats), with ape-to-ape transmission playing only a minor role due to an insufficient rate of encounters between groups.

In the new work, the researchers studied the spread of Ebola virus in a gorilla population of Odzala-Kokoua National Park, Republic of the Congo. Social units--defined as groups and solitary males--composing this population regularly visited a forest clearing where they were studied from 2001. In all, around 400 gorillas were identified from their individual morphological characteristics. Ebola virus affected this population in 2004. During and after the outbreak, the researchers used the data on the identity of animals previously observed to estimate Ebola-induced mortality and to test the validity of different possible contamination scenarios.

The authors of the study showed that transmission between gorillas was particularly important within groups and led to a very high mortality rate--97% of group-dwelling individuals. In contrast to the hypothesis expressed above, evidence suggested that transmission between social units is also likely to occur, either exclusively or in addition to reservoir-to-gorilla transmission, and the result was a mortality rate of 77% of solitary males. Overall, 95% of the population disappeared in approximately one year. From a conservation point of view, this finding is worrying because all adult female and young gorillas--on whom post-epidemic population recovery relies--live in groups.

These findings provide new insights into what is still a poorly understood disease and could shed light on the evolutionary costs experienced by primates living in groups. The authors note that thousands of gorillas have probably disappeared as a result of the Ebola outbreak studied here, and they point out that because the impact of Ebola on apes is still difficult to control, strengthened protection of gorillas and chimpanzees is needed throughout their range, especially against the other primary threats to their survival: poaching and logging.