EBOLA in Australia? Unlikely, you might think. After all, Australia is an
island continent, well isolated from the rest of the world’s dangerous diseases.
Rabies, known in the Middle East since 2300 BC and present on every other
inhabited continent, has never made the leap to Australia. And as far as anyone
knows, the nearest Ebola has got is the Philippines, about 3000 kilometres to
the north. But to virologist John Mackenzie, from the University of Queensland
in Brisbane, the prospect is not nearly as far-fetched as it sounds. He believes
the disease, which in its worst form can cause uncontrollable haemorrhaging and
an agonising death, could turn up at any time, courtesy of a roving fruit
bat.
Mackenzie is one of a small band of scientists who are trying to work out why
Australia is suffering from a rash of “zoonoses”, diseases that jump from
animals to humans, and why, in almost every case, a bat features somewhere in
the story. In the past four years, Australia has seen three new diseases make
the leap from bat to human, two of them with fatal consequences. Virologists
suspect that at least one of these diseases came from outside Australia,
crossing the narrow divide from Papua New Guinea aboard a bat. The others could
be recent imports or may have lurked unnoticed in Australia’s bats, jumping
species only when people and their domestic animals began to encroach further
into bat habitat.
So far, no one has isolated Ebola virus from any wild bat—either in the
Philippines or in Africa, where the worst outbreaks have been. The virus’s
natural host remains a mystery. And yet, when Bob Swanepoel and his colleagues
from South Africa’s National Institute of Virology in Sandringham inoculated 19
animal species with Ebola, the virus replicated and circulated only in the bats.
And if wild bats carry Ebola then it can certainly travel. Bats fly up to 200
kilometres in a single journey, and often share roosts with other species of
bat, encouraging the virus to spread into new bat hosts and new regions. If bats
in the Philippines are harbouring Ebola, then it is perfectly plausible for the
disease to make its way via Papua New Guinea and other islands to northern
Australia.
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“The thought of Ebola reaching Australia is pretty scary,” says Mackenzie.
“It is unlikely, but it is a scenario we can’t ignore. Some unusual and so far
unexplained things have been happening.” According to one expert, Ebola reston,
the strain from the Philippines, may already have arrived. “We just don’t know
it,” says Clarence Peters, head of the Special Pathogens Branch at the Centers
for Disease Control in Atlanta and a leading figure in two bestsellers about the
threat of newly emerging diseases.
The two bestsellers, The Coming Plague by Laurie Garrett and The
Hot Zone by Richard Preston, have alerted the public to a disquieting fact
that has troubled scientists for some time. Despite all the advances in the
control and eradication of disease, new diseases, or old ones in new guises,
have been turning up with worrying regularity. In Australia, new diseases seem
to be emerging from one particular source: bats. “We have three new viruses,
each one capable of infecting humans and each one associated with bats,” says
Mackenzie. “And we have ever increasing contact between bats and humans.”
Fruit bats, or flying foxes, are the biggest worry. Human diseases have been
associated with the four most common fruit bats in Australia. Between them,
these bats cover two-thirds of Australia
(see Maps). These large animals,
which have a wingspan of up to 1.6 metres, congregate in trees, sometimes in
huge colonies, or “camps” of up to 100 000 animals. Colonies have recently
become established in the popular botanic gardens close to the heart of
Melbourne and Sydney. The animals even roost in suburban back gardens, forced to
take refuge there as their natural habitat disappears. Women concerned about the
plight of orphaned bats have been known to express their breast milk to feed the
helpless animals, says Chris Tidemann, an ecologist at the Australian National
University in Canberra.
“By and large, people don’t understand zoonosis,” says Tidemann, who has been
monitoring bat populations for more than 20 years. “They don’t understand how
wild animals can spread diseases to humans.” People who care for sick and
injured bats may well have a baby in one hand and a bat in the other, he
says.
The first sign of trouble came on 22 September, 1994, when government
veterinary officials in Queensland and the Australian Animal Health Laboratory
(AAHL) near Melbourne were told about an outbreak of severe respiratory disease
in thoroughbred horses from stables in the Brisbane suburb of Hendra. Within
four days, 13 horses were dead; 11 from the stables, one from a neighbouring
property and one which had been moved from Hendra to a stable 50 kilometres
away. The horses’ trainer, Vic Rail, was on a respirator in a Brisbane hospital
fighting for his life. He died on 27 September. A stablehand was more fortunate:
he was seriously ill for six weeks but recovered.
A strange virus
Virologists from the AAHL and from the Queensland Department of Health in
Brisbane isolated a strange virus from four of the dead horses and from one of
Rail’s kidneys. Under the electron microscope, it was identified as a member of
the paramyxovirus family. Virologists thought it was a morbillivirus, the group
of viruses that includes measles, canine distemper and rinderpest, and they
called it equine morbillivirus (EMV). They have since changed their
minds—although the name has stuck. Based on the large size of the virus
and the organisation of its genome, the virologists say it is different enough
to warrant a genus of its own, which they want to call megamyxovirus.
Although no one knew it when Rail and his horses died, the disease had struck
before, more than 800 kilometres further north. A month earlier, in the coastal
town of Mackay, a sugar-cane farmer called Mark Preston had helped his wife, a
vet, treat two sick horses. The horses died shortly after. A year later, in
September 1995, Preston became dangerously ill with severe muscular seizures. He
developed meningoencephalitis, an inflammation of the membranes around the
brain, and within a month he was dead. EMV was detected in his cerebrospinal
fluid and in preserved tissue from the two horses.
What was going on? Scientists had to unravel what these cases had in common,
where the virus had come from and how the three men became infected. The team
tested more than 2000 horses and dozens of domestic and farm animals, but found
no trace of the virus. Somewhere out there, there had to be a “reservoir host”,
an animal that didn’t necessarily fall sick itself but was a permanent carrier
of the disease. It had to be an animal that could spread the disease between
Brisbane and Mackay. Peter Young from the Animal Research Institute of the
Department of Primary Industry in Brisbane reasoned that the most likely
candidates were wading birds and fruit bats. The chances of a virus crossing
from a bird to a mammal seemed low, so he focused on bats.
The evidence came from bat carers. Two of three orphaned bats they sent to
the institute had antibodies to EMV. Then Kim Halpin, a PhD student working with
Young and Mackenzie, isolated the virus from the uterine fluid of a grey-headed
flying fox that had become tangled in a barbed wire fence in Brisbane. The
presence of the virus in the uterine fluid may explain how the disease crossed
from bats to horses and then to humans.
The first sick horses at both Hendra and Mackay fell ill in the middle of the
fruit bat breeding season. Young and Halpin suggest that when the bats gave
birth, infected fluids and membranes dropped onto the horses below. The horses
might even have licked infected fetuses or eaten placenta on the ground. When
Rail force-fed his sick mare, his arms were repeatedly scratched and the cuts
exposed to the animal’s saliva. So Rail may have contracted the virus this way.
No one knows how the disease was spread among the horses, but one theory is that
Rail used the same syringe to inject his animals with vitamins.
Recent experiments at the AAHL have shown that the virus targets the cells
lining the blood and lymph vessels, so it can spread rapidly through the body of
susceptible animals. But the AAHL has also shown that although the virus is
extremely virulent once it strikes, it is not highly infectious. In laboratory
tests, EMV did not spread readily from horse to horse, supporting the theory
that Rail had helped to spread the virus between his horses at Hendra.
There is now no doubt that fruit bats are an important reservoir for the
virus. In a study of hundreds of fruit bats of three different species from
Melbourne to Cape York, and across to Darwin, Hume Field, a researcher in
Young’s lab, found that around half the animals had antibodies to EMV. Such a
high figure suggests that the virus has been in the bat population for a long
time, possibly decades. Field and others have also found antibodies to EMV in
six species of fruit bat from Papua New Guinea and the island of New Britain,
suggesting that it could have migrated to Australia from the north, then spread
widely among the local bat population.
Secretive sheathtails
The smaller and more secretive insectivorous bats do not seem to be part of
the EMV story. Not one of the hundreds of animals examined had antibodies to the
virus. But the emergence of another new and deadly virus—Australian bat
lyssavirus (ABL)—is linked to both fruit bats and insectivorous bats.
ABL was first isolated from a black flying fox in New South Wales in May
1996. Six months later, the virus claimed its first human victim, a woman from
Rockhampton in Queensland, who had tended sick and orphaned bats. Because the
woman regularly handled flying foxes, they were blamed for her death. But the
AAHL has recently discovered that the woman almost certainly contracted the
disease from the bite of yellow-bellied sheathtail, a small insectivorous
species. Virus isolated from four sheathtails is identical to that found in the
woman’s spinal fluid and brain, says Allan Gould from the AAHL. ABL has also
been isolated from 42 flying foxes, including one from the botanic garden in
Melbourne.
ABL belongs to the same group of viruses as rabies, one of the most feared
diseases. Rabies kills between 40 000 and 100 000 people each year, mostly in
developing countries. The other five lyssaviruses have been associated with bats
in Europe, America and Africa and all have caused rabies-like illnesses. Genetic
studies show that ABL is the most closely related to the rabies virus, differing
by just 8 per cent. “The virus is a hair different, that’s all,” says Fred
Murphy, a world expert on viral diseases based at the University of California,
Davis. As far as he is concerned, the woman died from rabies. Officially,
Australia’s rabies-free status remains, but that may change. “Sooner or later a
dog or cat will be scratched by a bat with ABL,” says Young. “It could cause a
disease that to all and intents and purposes is rabies.”
In October, the AAHL began tests to find out if cats and dogs can contract
the disease. “I suspect they will be susceptible,” says Ken McColl, who is
conducting the experiments. The crucial experiment will be to see if they
excrete the virus, which would mean they can infect other animals—or their
owners. The good news, if there is any, is that because the two viruses are so
similar, rabies vaccine also protects against ABL. Those who handle bats,
including carers, vets and researchers, have been advised to have a rabies
vaccine.
Old or new?
So far, there have been few clues to the origin of ABL. It has now been
isolated from bats all the way from Melbourne to Darwin, but shows such limited
genetic variation over this huge range that some researchers believe it is a
recent arrival that has not had time to mutate. Others point to a slight
difference between the virus found in fruit bats and sheathtails, which they
say, suggests it has been around for several hundred years at least, spilling
over to humans because of their increasing contact with bats. Virologists are
hunting for it among the bats of Papua New Guinea and the Philippines.
The third and most recent disease emerged in June last year, when sows at a
piggery near Sydney gave birth to deformed or mummified piglets. A new virus,
again a paramyxovirus, was isolated from the dead piglets. Two workers at the
piggery fell ill with flu-like symptoms, but recovered. Both had antibodies to
the virus in their blood, indicating that they have been exposed to the virus.
Antibodies—but no virus—have also been found in little red and
grey-headed flying foxes, which roosted close to the piggery.
And as well as incubating new diseases, bats may be a key factor in the
spread of known diseases too. A study in northern Queensland suggests that bats
are an important reservoir for the virus that causes Ross River fever, a
debilitating disease transmitted by mosquitoes. David Harley, a student in
tropical health at the University of Queensland in Brisbane, has found that
mosquitoes captured in Cairns near a colony of spectacled flying foxes are far
more likely to carry the virus than mosquitoes trapped elsewhere in the area,
suggesting that they acquire it from bats. “Something is going on,” he says. “It
requires further investigation.”
Mackenzie is also investigating the possibility that fruit bats from Papua
New Guinea could introduce another potentially fatal disease, Japanese
encephalitis, into Australia. In the meantime, Halpin has recently found three
more paramyxoviruses in fruit bats. “We have no idea what they are or where they
fit in,” says Mackenzie.
The human cases to date could have been freak accidents, a result of an
unusual combination of circumstances. Or they might be a warning that worse is
yet to come. “All bats and flying foxes should be considered infective,” says
Linda Selvey, manager of the Communicable Diseases Unit with Queensland Health,
the state health department. Data collected by her department indicate that many
people who are at risk are not being vaccinated against ABL, the one disease for
which there is a vaccine. Between July 1996 and April this year, 110 people in
Queensland needed treatment after being bitten or scratched by bats.
Despite the alarming nature of these viruses, Mackenzie cautions that bats
should not become public enemy number one. Fruit bats play a vital ecological
role by spreading pollen and seeds. After the first fatal case of ABL hit the
news, there was a spate of indiscriminate shootings. Yet, as Mackenzie points
out, in the past four years only three people have died from bat-borne diseases,
far fewer than the annual toll from snake or spider bite. Of the hundreds of
people who may been in contact with bats, only a tiny number have fallen ill, so
these diseases are not highly infectious to humans—at least not yet.
Australia cannot afford to be complacent, however, says Murphy, nor can the
rest of the world. “We’re all in this together,” says Murphy. “There will always
be emerging diseases and viruses. We may conquer one but another will pop up,
normally more complicated and difficult to deal with than the one we have
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