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The recent human cases of Ebola identified in Uganda have brought back into the

spotlight the epizootics raging among lowland gorillas (Gorilla gorilla) and common

chimpanzees (Pan troglodytes).

Ebola takes its name from the Ebola River in the Democratic Republic of the

Congo (formerly Zaire), where one of the first outbreaks of the disease was

observed.

The virus belongs to the Ebolavirus genus of the Filoviridae family. There are

three different ebolavirus subtypes in Africa: the Ivory Coast ebolavirus, the Sudan

ebolavirus and the Zaïre ebolavirus. Filoviridae are endemic in central Africa in the

area between the 10th parallel north and the 10th parallel south of the Equator.

The reservoir of infection has not yet been fully identified but certain species of

fruit-eating bats are thought to play a role. The Ivory Coast ebolavirus was isolated

for the first time in West Africa, during an episode of haemorrhagic fever among

wild common chimpanzees in the Tai Forest in Côte d'Ivoire.

African Ebolaviruses cause haemorrhagic fever in humans; the incubation

period is around one week and the disease breaks out suddenly with fever and

headaches. Following this, many patients suffer chest pains, vomiting, diarrhoea

and a dry throat, followed by maculopapular rashes on the trunk, which spread

rapidly to other parts of the body and tend to converge. The disease is often fatal;

for example, during the Kikwit episode in 1995 in the Democratic Republic of the

Congo, 245 of the 316 cases resulted in death.

In most instances the disease starts as an outbreak that spreads to a large

number of contact persons, who are usually either family members or medical

personnel. The disease is transmitted from one person to another by direct

contact with the blood or other secretions of infected persons.

zoonosesEbola among lowland gorillas and common chimpanzees

Lowland gorillas (Gorilla gorilla)

Ebolavirus

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The cases in lowland gorillas and common chimpanzees have occurred in the

border zone between Gabon and the Democratic Republic of the Congo. The

relatively untouched forests of equatorial West Africa represent the last major

bastion of these two primate species. It is estimated that Gabon and the Democratic

Republic of the Congo alone harbour 80% of the world’s lowland gorillas and most

of the common chimpanzee populations.

Unfortunately, the populations of both species declined by more than half

between 1983 and 2000. The chief cause of decline is the bushmeat trade,

facilitated by the rapid expansion and mechanisation of logging and compounded

by the current Ebola epizootics.

The mortality rate in gorillas is between around 90% and 95%. In the Lossi

Sanctuary in the north-west of the Democratic Republic of the Congo, an estimated

5,000 gorillas have died from the disease in an area of over a 5,000 km2.

Hunting, combined with this epizootic, ultimately threatens the survival of both

these species. In addition, ten or so outbreaks of the disease have occurred in

humans in the same areas since 1994.

Our thanks to Dr Pierre Formenty for his critical review of the manuscript.

Prof. P.-P. Pastoret

Common chimpanzees (Pan troglodytes)

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