- Ebola is one of the deadliest viruses known to man. But
scientists have, so far, been unable to discover one vital fact about the
disease: which species is spreading it?
-
- 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 published
in the current issue of the journal Science describes the extraordinary
power of the Ebola virus to jump from this unknown species and decimate
wild populations of chimpanzees and gorillas and in the process threaten
local human populations with a frightening illness.
-
- The Ebola virus causes haemorrahagic 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 that medical staff caring for patients have to practise strict "barrier
nursing" techniques wearing 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,
CÙte d'Ivoire, the Republic of Congo and Uganda. Overall there have
been about 1,500 cases of Ebola and more than 1,000 deaths since the virus
was first identified.
-
- The mystery of where the Ebola 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 nor vaccines.
If they could discover its natural hiding place between epidemics they
would stand a better chance of predicting - and perhaps preventing - any
future outbreaks.
-
- Tests on wild animals soon revealed that non-human primates
were also susceptible to the Ebola virus. Eric Leroy of L'Institut de Recherche
pour le DÈveloppement 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 lethality of the Ebola virus for great apes can be
judged by how many carcasses have been found during some outbreaks. It
takes a month for the carcass of a full-grown gorilla to rot away to nothing.
"Considering that we have observed that a gorilla carcass decomposes
within one month in the tropical forest and that the carcasses were found
in the vicinity of villages, hundreds or even thousands of animals may
have died in the thinly populated 3,000 square km of this forest region,"
the researchers say.
-
- 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.
-
- But 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. "The slow
reproductive cycle of the great apes, together with hunting and poaching,
may lead to their extinction in western central Africa," say scientists.
-
- Dr 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 Nineties 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 subsequent transmission occurred by direct person-to-person
contact," says Dr Leroy. Scientists identified at least 10 separate
chains of transmission, each originating from one index case that occurred
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 by
separate introductions of several strains of Ebola. They found that 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 flu, 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. "Thus, 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," Dr 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.
-
- © 2003 Independent Digital (UK) Ltd
-
- http://news.independent.co.uk/world/science_medical/story.jsp?story=483126
|
