Scientists are still trying to unlock the mysteries of the Hendra virus that has killed nine horses and required the testing of dozens of people in Queensland and New South Wales in the past three weeks.

In rare cases, the virus can cross to humans, at times lethally. Seventeen people have been infected since the virus first appeared in the Brisbane suburb of Hendra in 1994, and four of them have died.

It is deadly to horses, with a reported mortality rate of 70 per cent.

Researchers now know it is linked to another virus native to Malaysia, and are studying bat ecology and immune systems to try to unravel the mechanisms that allow similar viruses to spread to humans and other animal species.

When the virus first struck, panic spread. In September 1994 prominent Queensland trainer Vic Rail, a stablehand and most of Rail's horses fell ill at his property at Hendra, a suburb in Brisbane's northeast close to the Doomben and Eagle Farm racecourses.

Rail and 14 horses died. Later outbreaks were to claim the lives of a Mackay farmer and two vets, the most recent two years ago.

Researchers at the CSIRO's high-security animal health laboratory at Geelong, south of Melbourne, isolated and identified the cause as a previously unknown virus within the Paramyxoviridae family.

This family includes the viruses that cause mumps and measles in humans, and Newcastle disease, Rinderpest and canine distemper in animals.

Researchers also found that Hendra was related to - but distinct from - Nipah virus, named after Sungai Nipah, a Malaysian village hit by outbreaks in 1998 and 1999.

Although related, the two viruses infect different species and use different methods of transmission.

Hendra is unique. Unlike other Paramyxoviridae viruses, it can infect more than one other animal species, using fruit bats as carriers.

The virus does not affect the bats.

Scientists still do not know exactly how the virus moves from bats to horses and humans, although CSIRO research has shown viruses can be excreted in the urine of horses, cats and guinea pigs. Researchers believe horses might be infected by bat urine, aborted bat fetuses or reproductive fluids.

Animal health experts involved in the latest outbreaks have warned owners to keep horses away from trees where flying foxes could be roosting or feeding, and yesterday officials isolated parts of Sydney's Royal Randwick Racecourse to protect against the virus.

Trainers were instructed to keep all feed and water under cover, and areas of grass that horses could reach were roped off.

But scientific detective work has been paying off.

Researchers first identified the part of the cell membrane the Hendra and Nipah virus used to infect cells, speeding up studies in other areas that include the hope of a human vaccine.

In May, CSIRO produced a prototype vaccine for horses that successfully prevented Hendra infection, potentially enabling a break in the cycle of the virus' transmission from flying foxes to horses and on to humans.

A joint Queensland-NSW taskforce is now looking at ways to accelerate the distribution of the vaccine.

Breaking the path from flying foxes is crucial.

Hendra can spread only from flying foxes to horses, and from horses to humans.

CSIRO says there is no evidence of human-to-human, human-to-horse or bat-to-human spread of Hendra virus, allowing animal health officials to isolate affected properties without banning or restricting other horse movements.

"These incidents are not contagious incidents," Queensland chief veterinary officer Rick Symons told ABC radio.

"They're virtually at random, relating to the interaction of flying foxes and horses. It's not a disease that's going from property to property."

Officials are also rejecting calls to cull fruit bats, which are widely spread throughout Australia.

"Culling or shifting flying foxes does not work," Symons said.

Meanwhile, a third NSW horse - a companion to one put down at Wollongbar two weeks ago - has died from Hendra, joining others killed as far north as Cairns.

Tracking a killer
* Prominent Queensland horse trainer Vic Rail and 14 horses die from an unknown disease in Hendra, Brisbane, in September 1994.

* The same year CSIRO and Queensland researchers discovered a new virus now named after the suburb.

* The carriers are identified as fruit bats.

* Although not fully understood, Hendra virus is not highly contagious but can be lethal if transmitted to horses and humans.

* CSIRO has developed a prototype vaccine to protect horses.

* A joint Queensland-NSW taskforce is working to contain the latest outbreaks that have so far killed nine horses.