Since 1976 the Ebola virus has caused some lethal human epidemics in Central Africa. Research now indicates that humans do not become directly contaminated from the animal reservoir, which is an any case still unknown, but from infected carcasses of chimpanzees, gorillas and certain forest antelopes. Results have come from work conducted over the past several years by IRD scientists and their partners (1) to shed light on the virus's propagation paths. The discovery of Ebola virus antibodies in several species of non-human primate suggests the existence within this fauna of different degrees of susceptibility to Ebola and, possibly, of strains of various levels of virulence. However, most large primates, once infected, soon die of the disease. Their bodies then become a potential source of contamination for humans, but also for certain domestic animals. Ebola virus antibodies were detected in dogs exposed to the virus during the latest epidemics, which suggests that these animals may well have been infected and can therefore be a new source of transmission to humans. Ebola virus infection in humans provokes a violent haemorrhagic fever. It usually flares up as intense epidemics. These kill 80 % of the people infected. Seven such outbreaks have hit Gabon and the Republic of Congo since 1994, leading to 445 cases resulting in 361 deaths. Ebola virus thus constitutes a grave public health problem in these countries. No medicine or vaccine is currently available, only prevention and rapid control of epidemics by isolation of disease victims can limit its spreading.
Since 2001, IRD research scientists and their partners (1) have been working to unravel the virus's biological cycle, in other words the whole range of ways in which the virus circulates in its natural environment, from its natural host (or reservoir) right up to humans. They showed that strong epidemics of Ebola have decimated populations of large primates over the past several years in the border regions between Gabon and the Republic of Congo. Human infection appears to occur only in a secondary way, through contact with carcasses of dead animals (2). However, the virus's natural cycle is not restricted just to transmission from the reservoir to the non-human primate and then to humans. It is quite possible that several reservoir species co-existent and that many other animal species can become infected, thus contributing to propagation of the virus in nature.
A serological investigation conducted from 1980 to 2000 on 790 nonhuman primates from Cameroon, Gabon and the Republic of Congo, belonging to 20 different species, hence revealed that 12.9 % of wild chimpanzees carry Ebola virus antibodies, several of the positive samples dating from before the first epidemics in these countries. These results therefore indicate that chimpanzees are regularly in contact with the animal virus reservoir and that some of them develop non-fatal infections. The presence of specific antibodies in the animals taken before the epidemics means that the Ebola virus has probably been circulating for a long time in Central African forests. The detection of such antibodies in other primate species (including 5 drills, 1 baboon and 1 mandrill) suggests that circulation of the virus involved many contamination events between distinct animal species. Thus, the multiplicity of infected species, their different susceptibilities to the virus and the great differences in their ways of life, are indicators of the complexity of Ebola virus's circulation in its natural environment. These observations also show that an epidemic or sporadic cases can appear at any moment in the sub-region of Central Africa as a whole.