- Scientists have found that the Ebola epidemics that periodically hit Congo Brazzaville and Gabon are usually following Ebola outbreaks among great apes populations. While the disease is threatening these populations, the knowledge could facilitate early warning procedures, thus avoiding outbreaks among humans.
The Ebola virus causes epidemics that wreak havoc in some tropical forest areas in Central Africa. Its control is therefore a major public health priority. The virus, transmitted to humans mainly by carcasses of gorillas, chimpanzees and duikers (Bovidae), provokes haemorrhagic fever which is usually fatal.
Researchers from the Paris-based Institut De Recherche Pour Le Développement (IRD) and co-workers from partner organizations in Gabon and other countries have just shown that these animals can also be victims of Ebola epidemics, according to an IRD release, diffused through 'Science Daily'.
- Such outbreaks appear to result from multiple simultaneous transmission events starting from an as-yet unidentified animal reservoir, triggering a drastic decline in great ape populations which has been observed in these areas of Africa, IRD scientists found. These findings, published in the journal 'Science', "indicate clearly that the animal Ebola epidemics often precede the human ones."
According to IRD, "the occurrence of infected animal carcasses found in an area would therefore be an indicator of a forthcoming outbreak about to hit nearby villages. Such finds could therefore aid in attempts at more effective prevention of Ebola virus transmission to humans."
The Ebola virus, identified for the first time in 1976 in Congo Kinshasa (DRC), has unleashed several lethal epidemics in Central Africa. For several years, many outbreaks have been occurring simultaneously in Congo Brazzaville and Gabon, making the control of Ebola virus infection a major public health priority for the two countries. IRD partner CIRMF in Libreville (Gabon) has become a major centre of Ebola research.
In humans, infection triggers haemorrhagic fever. In 80 percent 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. Mortality is highest for the Zaire strain of the virus, which is common in Gabon and the Congos.
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. Several research institutes and pharmaceutical companies however report to be in the process of developing an Ebola vaccine - one type is currently being tested on humans.
The IRD researchers and their co-workers in Gabon and other countries - who have been studying the virus since 2001 in western 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, IRD reports. "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."
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 organised 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, IRD says. "The populations of gorillas and duikers plummeted 50 percent between 2002 and 2003" in the Lossi animal sanctuary (320 km2) in Congo Brazzaville and "chimpanzee numbers fell by 88 percent." Therefore, 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 had confirmed that these carcasses were indeed Ebola-virus infected. "The rapid decline in animal populations in this part of Africa could therefore be due to Ebola epidemics," IRD concludes. Gene analyses conducted on the samples had 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, the study found. "Infection of humans occurs in a second phase, usually by contact with animal carcasses."
The discovery of infected carcasses could therefore be taken as a warning sign of a human epidemic, the French and Gabonese scientists hold. "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."
Many questions however remain unanswered. For example, outbreaks among the great apes seem to hit mainly at times of seasonal changes but the exact environmental conditions that foster their emergence are not known. Neither is the host - the natural reservoir - of the virus, which contaminates the great apes. Research is under way to identify the factors involved, IRD reports.
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