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Ebola virus a threat to great ape populations

The Ebola virus, identified for the first time in 1976 in the Democratic Republic of Congo (ex-Zaire), has unleashed several lethal epidemics in Central Africa. For several years, many outbreaks have been occurring simultaneously in the Republic of Congo and Gabon, making the control of Ebola virus infection a major public health priority for these countries. In humans infection triggers haemorrhagic fever. In 80% of cases it leads to death in a few days. High mortality generated by this particularly virulent virus, transmitted by direct contact, engenders serious social and economic consequences for the countries affected. There is no therapy or vaccine available and preventive measures and rapid abatement of epidemics by isolating patients are the only weapons that can hinder spread of the disease.

The IRD researchers and their co-workers from other organizations (1), who have been studying the virus since 2001 in West central Africa, have been investigating the virus transmission mechanisms from the still-unidentified animal reservoir to humans. Human epidemics appear to result from two successive waves of contamination between species. The first propagates from the reservoir to certain susceptible species, such as gorillas, chimpanzees and duikers (Bovidae). A second stems from humans infected through contact with infected carcasses of animal victims of the virus (2). Epidemiological data obtained during human epidemics that occurred between 1976 and 2001 shows that each of these developed from a single-animal source, then spread from person to person. However, in the study undertaken between 2001 and 2003, epidemiological findings suggest on the contrary that several different and concomitant epidemic chains exist, each stemming from a different single-animal source. Gene sequencing analyses of the virus performed on patients' blood samples confirmed these observations, and showed in addition that these chains do not result from a common transmitted viral strain but from several strains.

A further discovery came from an organized count of carcasses found in the forest coupled with calculations of rates of presence of these animals (from occurrence of dung, tracks, nests, and so on). There have been large increases in mortality in some animal species before and during the human epidemics. The populations of gorillas and duikers plummeted 50% between 2002 and 2003 in the Lossi animal sanctuary (320 km2) in the Republic of Congo and Chimpanzee numbers fell by 88%. Hundreds or even thousands of animals may well have died during the latest epidemics that hit the region. Laboratory analyses of samples taken from animal carcasses during human epidemics confirmed that these carcasses were Ebola-virus infected. The rapid decline in animal populations in this part of Africa could therefore be due to Ebola epidemics. Gene analyses conducted on the samples have also shown the involvement of several infecting viral strains, as has been found in humans.

Ebola virus epidemics in the large apes hence appear not to stem from propagation of a single epidemic from one individual to another, but rather from massive simultaneous infections of these primates from the original carrier animal facilitated by a particular set of environmental conditions. Infection of humans occurs in a second phase, usually by contact with animal carcasses. The discovery of infected carcasses can therefore be taken as a warning sign of a human epidemic. Detection and diagnosis of Ebola infection on these carcasses could help define Ebola human transmission prevention and control programmes before any human epidemics arise, and thus increase the possibilities for containing or avoiding them.

Outbreaks among the great apes seem to hit mainly at times of seasonal changes. However, the exact environmental conditions that foster their emergence are not known. Neither is the host, the natural reservoir of the virus, which contaminates the animals. Research is under way to identify the factors involved.

Institut de Recherche pour le Développement, Paris (IRD). January 2004.

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