Death on the wing

BATS are to blame for the latest case of a lethal virus jumping from animals to humans.

DURING the Middle Ages, flea-infested rats spread bubonic plague across Europe. Seven centuries later, it's fruit bats that are being blamed for a 21st-century scourge.

Queensland opposition health spokesman Mark McArdle reckons they are "flying petrie dishes" and wants them moved from residential areas, a policy rejected by the state government, which has banned bat culling.

"I'm not questioning for one second the importance of bats in the environment," McArdle says. "But there are risks with having bats living so close to humans. We can't stop development, and if a decision has to be made between people and bats, then the bats have to be moved."

Today in Rockhampton, a funeral will be held for vet Alister Rodgers, who died last week of the lethal bat-borne Hendra virus. He caught the rare illness while treating a dying horse which had been infected - researchers presume - by a disease-bearing bat.

Rodgers was the fourth person in the world to have died from the animal-borne illness, known to have infected only seven people (all of them Queenslanders) since its discovery in the Brisbane racetrack suburb of Hendra 15 years ago. Hendra virus is just one of more than 860 known diseases that originated in animals before jumping species to infect human hosts. Some 75 per cent of new diseases are zoonotic, that is, of animal origin.

AIDS came out of Africa in the 1980s, but it was only three years ago that scientists pinned its origins on wild chimpanzees in a corner of Cameroon. The first known human case was of a Congolese man whose blood was stored as part of a 1959 medical study.

The deadly SARS (severe acute respiratory syndrome) - the first severe and rapidly-transmitted disease of the new century - is believed to have originated from civet cats in the Chinese markets where wild animals are stacked in cages and slaughtered on the spot.

By feeding cattle the rendered remains of other animals, farmers created the health scare of the 1990s: mad cow disease. Humans can catch the incurable and brain-wasting disorder by eating contaminated beef, although Australian livestock remains free of the disease.

The H1N1 swine flu sweeping the planet is a genetic mix of human, bird and pig flu viruses, interchangeable between the species. Flu-ridden farm workers have twice infected herds of pigs with the flu in Australia this year.

Hume Field is Australia's internationally renowned bat expert. A specialist on emerging infectious diseases associated with wildlife, he works as the principal veterinary epidemiologist with Queensland's Department of Primary Industries and has been researching Hendra virus since its discovery.

"Why is it we are seeing more diseases emerging from wildlife?" he asks.

"Fundamentally it's the opportunity for contact. There are less and less wilderness areas in the true sense - whether it be through clearing for development or building roads for logging and farming - so it's harder and harder for little ecological niches to exist in a wildlife population."

Ankle-deep in mangrove mud, Field has seen first-hand the bat's role as a bearer of disease. A raucous colony of fruit bats - commonly called flying foxes - has taken roost in a mangrove swamp in the coastal town of Yeppoon, in central Queensland.

The colony is a half-hour drive from the horse stud where Rodgers treated a Hendra-infected horse, which he had mistakenly, but understandably, diagnosed as snakebite. The J4S stud is shaded by towering gum trees in full flower, where flying foxes flock at dusk to feast on the blossoms.

Field's research team has discovered that urine collected from the Yeppoon colony has been contaminated with the Hendra virus. He believes infected bats spread the virus to horses when their urine, droppings or placental matter falls on to railings, into water or feed bins.

The broad symptoms of Hendra virus in horses - ranging from flu-like to neurological, and easily mistaken for other illnesses - make it feasible that infected horses have been dropping dead in paddocks for decades before the disease was recognised.

Field has detected antibodies to Hendra virus in bat blood stored at the Queensland Museum from the 1970s. Yet the disease was not discovered until Brisbane trainer Vic Rail and 14 horses died in 1994.

"Hendra-like viruses have evolved in fruit bats over tens of thousands of years but we've only been aware of them for 15 years," Hume says. "There's some trigger for the emergence. My thinking is it's an ecological impact; that there's been increasing ecological pressure on bats in Australia as a result of clearing for farms, logging for timber, drought and climate change.

"Their food resources in the natural environment have become very patchy, so bats are moving with increasing numbers into what they perceive as urban oases in rural and coastal towns.

"Over the last 20 or 30 years it seems flying foxes have become urban animals."

Although bats have never been known to directly infect a human with Hendra virus, horses seem particularly susceptible and replicate the virus to a concentration that can infect people exposed to blood or mucus.

The symptoms, although slow to show, are severe and life-threatening. Rodgers and Brisbane vet Ben Cunneen - who died just over a year ago - suffered blinding headaches, before slipping into a coma with untreatable brain inflammation.

A veterinary nurse who worked alongside Cunneen on a sick horse at a Brisbane vet clinic last year survived after weeks in intensive care, but still suffers from pounding headaches and severe lethargy.

The bats, however, seem immune to the effects of the killer virus.

"Our immune systems are totally at a loss with how to deal with the bat viruses; they have such a dramatic effect," Field says.

"There have been, over time, quite a list of viruses that have been detected in bats.

"Typically they've been novel diseases that haven't been known to cause infections in people. If you're a mammalian virus it makes a lot of sense in an evolutionary context to have a mammalian reservoir that's mobile, that can fly."

Bats also carry lyssavirus, a rabies-like disease that killed two bat carers in Australia a decade ago. The disease is transmitted through untreated bites or scratches from infected bats. The public has been warned against handling sick bats, and vets and carers need to be immunised before handling them.

More worrying, though, is the untreatable Nipah virus, closely related to Hendra virus but seemingly more contagious.

When first detected in India and Bangladesh it could only infect humans after incubating in pigs. But now it has mutated to jump directly from bats to humans, and from person to person. Although not yet detected in Australia, experts fear it is a matter of time.

Australian virologist John MacKenzie, who has advised the World Health Organisation about ways to deal with swine flu, sees bats as a natural spreader of new diseases. Bats cross from northern Australia to colonies in Papua New Guinea and East Timor, which in turn mingle with bats from Malaysia.

"You can go almost around the world and find overlapping populations," he says. "It's an open conduit for these viruses to move around." Even Hendra virus, he warns, might adapt to start infecting humans directly.

"The risk as it stands is negligible but things can change. Nipah has gone from bat to human, and now human to human."

Martyn Jeggo, director of the CSIRO's Australian Animal Health Laboratory, says the frequency - and not simply the diagnosis - of animal-borne diseases is growing.

He singles out China as a giant "cooking pot" for exotic new viruses to jump species, with the world's biggest concentration of humans mingling more with exotic wildlife and intensive livestock.

"It's all about the opportunity for viruses to jump or switch hosts," he says.

"We are seeing an increased risk from zoonotic disease and if we are going to have any chance of managing the risks we have to understand the viruses and continue to invest in the research."

Funding is fast drying up, however, with the looming closure of the Biosecurity Co-operative Research Centre, which will close next June after the federal government knocked back its application for $40 million in funding over the next six years.

Its executive director, Stephen Prowse, warns against complacency despite Australia's isolation and strict quarantine.

"We need to have a better understanding of these diseases," he says. "We need more information to be able to make an assessment of the risks, because without research we make intuitive decisions."

With zoonotic diseases on the march, veterinarians remain at the front line of diagnosing and contracting them.

Ausvet director Nigel Perkins, who is advising the Queensland government on its response to the latest Hendra outbreak, is calling on vets to routinely use protective gear, in the same way dentists and doctors began safeguarding against AIDS in the 1980s.

"Vets need to be taking precautions for every sick horse, not just with those they think might have Hendra," he says.

"Vets could potentially get infected while doing something to a horse that's apparently healthy, and veterinary guidelines should reflect that." Rodgers was not wearing a mask or gloves when he examined the sick horse that infected him. Nor were two other Rockhampton vets who are being tested for Hendra after treating other horses on the J4S property.

"Hendra is so rare, it means a lot of individuals will think 'it can't happen to me'," Perkins says. "But we're asking them to change their behaviour."

A vaccine to protect humans against the Hendra virus is a good decade away, although one for horses may be available within four years. Meantime, horse owners are being advised to keep horses stabled at night, away from flowering or fruiting trees, and to protect their food and water from bat droppings.

Despite their encroachment into suburbia, flying fox numbers have been declining steeply and they remain a protected species vital to spreading the seeds and pollen of native flora. Although NSW still permits farmers a limited cull, it looks set to stop culls pending the results of a review announced this month.

Field believes moving the winged mammals would be counterproductive; other bats, possibly bearing new diseases, would simply fly in to take their place.

"By disrupting the system and culling animals you're only increasing the turnover and by doing that you run the risk of just bringing in new groups," he says.

"It's better to know your enemy, have a high level of awareness across industry and have people realise they need to use protective equipment if they suspect a horse has Hendra virus.

"We can't just sit around and wait for a vaccine or effective treatment."