Hunting the home of the Ebola virus

By STEVE CONNOR

Somewhere in the tropical heat of an African rainforest there lives an animal that harbours one of the most deadly viruses on Earth.

Scientists have yet to discover what kind of animal it is but every now and again the virus emerges from its natural "reservoir" to infect other species - including humans - with lethal effect.

A study in the latest Science describes the extraordinary power of the Ebola virus to jump from this unknown species and decimate chimpanzees and gorillas and, in the process, threaten humans with a frightening illness.

The Ebola virus causes haemorrhagic fever and kills up to 90 per cent of the people it infects. They die in excruciating agony culminating in heavy internal and external bleeding. The virus is transmitted by direct contact with body fluids such as blood, and its deadly infectiousness means medical staff caring for patients have to wear full gowns, gloves and masks.

Ebola first emerged in 1976 when 284 people became infected in Sudan and 318 fell ill in Zaire (now the Democratic Republic of Congo). By the time the outbreaks had subsided, nearly 400 people had died of the illness.

A further outbreak in Sudan in 1979 was followed by a period where the disease seemed to have disappeared, only for it to re-emerge in 1995 when 355 people became infected and 244 died in an epidemic at Kikwit in Zaire.

Since then a number of smaller outbreaks have occurred in Gabon, Cote d'Ivoire, the Republic of Congo and Uganda. Overall, there have been about 1500 cases of Ebola and more than 1000 deaths since the virus was first identified.

The mystery of where the virus resides when it is not causing a human epidemic has intrigued scientists desperate to find ways of combating a disease for which there are no cures or vaccines. If they could discover its natural hiding place between epidemics they would stand a better chance of predicting - and perhaps preventing - future outbreaks.

Tests on wild animals soon revealed that non-human primates were also susceptible to the Ebola virus. Dr Eric Leroy, a researcher in Franceville, Gabon, and the lead author of the Science study, says that up to 100 per cent of chimpanzees and gorillas infected with the virus probably die from it.

The effect on local apes could be catastrophic.

Sightings of gorillas and chimpanzees have declined dramatically in recent years - despite a survey of mountain gorillas showing a 17 per cent increase in numbers - and conservationists fear that man's closest living relatives are at serious risk of extinction within a few decades.

The causes are primarily loss of forest habitats and poaching for bush meat. However, Ebola puts extra pressure on apes and can abruptly wipe out local populations. One group of 143 gorillas, for instance, which had been closely monitored by zoologists for 10 years, disappeared between October 2002 and January 2003 following an Ebola outbreak.

Leroy and his colleagues have scrutinised the Ebola outbreaks in both humans and animals that have occurred over the past four years in Gabon and the Republic of Congo and have collected samples of virus from carcasses of gorilla, chimpanzee and duiker - a small antelope common in this part of Africa.

Investigations into the outbreaks of Ebola in humans during the late 1990s pointed to a link with great apes. The origins of many of these epidemics could be traced to direct human contact with dead chimpanzees or gorillas, either through hunting bush meat or from handling carcasses found in the forest.

"The index [first] cases were mainly hunters and transmission occurred by direct person-to-person contact," says Leroy. Scientists identified at least 10 separate chains of transmission, each originating from one index case occurring between October 2001 and May 2003.

They went on to analyse the genetic material of the virus that they had isolated from each outbreak to see whether these outbreaks had resulted from multiple introductions of a single viral strain or separate introductions of several strains of Ebola. They found there were at least eight different strains of Ebola involved, showing that for this relatively short period under study the mode of transmission of the disease was more complex than previously imagined. Because Ebola is a genetically stable virus - unlike say influenza, which mutates rapidly - the fact that many strains are involved suggested that there have been multiple independent introductions of the virus from the reservoir species into apes and humans.

"Different strains of Ebola virus may be widespread throughout the forests of central Africa, with simultaneous infection of great apes occurring from unknown natural hosts under particular but unknown environmental conditions," the scientists say. "Ebola outbreaks probably do not occur as a single outbreak spreading throughout the Congo basin as others have proposed but are due to multiple episodic infection of great apes."

The great unknown, of course, is the name of this reservoir species. Dr Leroy says there are few real clues. "We are working on that. We aren't near to identifying the animal but we have some ideas, in particular fruit bats. We don't have much evidence at all, just observations and ideas," he says. Both apes and fruit bats eat the same kind of food so it is not unreasonable to assume that they may come into close contact with one another at certain times of the year.

Ebola outbreaks in wild animals seem to occur at the beginning of the dry season. But no one has yet shown that it is possible to find Ebola virus in wild bats. "In South Africa a scientist succeeded in infecting fruit bats experimentally and he observed rapid development of the virus," Leroy says.

So although it is technically possible to infect fruits bats with Ebola, there is still no evidence that this is the mystery reservoir species. Until this animal is found, the sole measure that scientists can take in predicting and preventing an Ebola outbreak in humans is to watch what is happening to gorillas and chimpanzees in the wild.

Ebola: speed and stealth

* Ebola causes rapid death through massive blood loss in up to 90 per cent of those infected.

* It spreads through body fluids, generally kills rapidly and has so far afflicted remote regions of Africa.

* The World Health Organisation says more than 1000 people have died of Ebola since the virus was first identified in 1976.

- INDEPENDENT