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The deadly Hendra virus was first detected in Australia nearly two decades ago but it still baffles scientists.

So far, they know that fruit bats carry the virus which can infect horses, dogs and humans.

But they do not know exactly what causes it, how it spreads or why it is so deadly.

There is also no known treatment.

Hendra virus first rang alarm bells in Australia with the sudden deaths in Brisbane of 14 horses, prominent race horse trainer Vic Rail and one of his stablehands in September 1994.

Initially called equine morbillivirus, scientists later renamed it Hendra virus (HeV) - after the Brisbane suburb where the first outbreak was detected - when they discovered it was a completely new genus of the paramyxoviridae family, which includes measles, mumps and canine distemper.

But unlike most paramyxo viruses, Hendra can infect more than one species.

It is carried by fruit bats, which excrete the virus in their urine, faeces and saliva.

Scientists are not sure exactly how horses become infected.

One theory is that they contract the virus after coming into contact with a bat's urine, faeces, blood or other body fluids, while grazing under fruit trees the bats love to feed on.

Infected horses can display various symptoms including high temperatures, depression, nasal discharge, problems breathing and problems with their co-ordination.

Humans have been only been infected after being in close contact with horses with the virus.

However Hendra is not considered to be as highly infectious as equine influenza and has only been spread between horses which have been in close contact with each other.

While potentially deadly to horses and humans, it is not yet known whether bats can die from Hendra.

Scientists believe bats act as hosts for the virus, so they can carry the infection, but seem to recover if they fall ill.

Not all infected horses die from Hendra but they are generally euthanased because scientists are not exactly sure of how they become infected or pass it on.

The University of Melbourne's associate professor of veterinary microbiology James Gilkerson said Hendra has been tricky to diagnose in horses since the first outbreak.

In 1994 the infected horses which died did so quickly from what appeared to be an acute respiratory disease.

However since then, many infected horses have appeared to have neurological problems.

"That's partly why we are seeing more cases in recent years because with a lot of the sudden deaths we would have said before we weren't sure why they died, so now we are testing them and finding Hendra virus," he said.

"So we are being more alert to the fact that horses may have Hendra and because we are testing more cases we are seeing more cases."

Scientists at the CSIRO are working on a vaccine to protect horses against the virus.

It is expected it could be available as early as 2012.