By Julie Steenhuysen

CHICAGO (Reuters) - A targeted antibody treatment was effective at keeping monkeys alive even three days after being exposed to the deadly Hendra virus, a promising sign that the treatment may also work in people, U.S. researchers said on Wednesday.

Although rare, the Hendra virus kills 60 percent of those it infects and has caused sporadic outbreaks in Australia since it was first identified in 1994.

The experimental treatment -- an engineered version of a human immune system protein -- has already been used in an emergency situation when a 12-year-old Australian girl and her mother were exposed after the girl's horse died last year from a Hendra infection.

But the drug, known as a monoclonal antibody, still needs to go through clinical trials before it can be licensed.

In the latest study, the team infected 14 African green monkeys -- chosen because they respond to Hendra in much the same way humans do.

Twelve of the monkeys were given doses of the human antibody, known as m102.4, starting at different intervals after exposure to the virus.

"We were able to delay treatment for three days and still completely protect the animals," said Thomas Geisbert of the University of Texas, who worked on the study published in Science Translational Medicine.

"If you translate that to human outbreaks, certainly you have a bigger window than really we thought you would," he said in a telephone interview.

All of the animals in the study who got the treatment survived, but the virus killed the untreated control animals.

"Taking this drug into a human setting really would have a chance to do a lot of good," Geisbert said.

The treatment, which works by sticking to the virus and keeping it from entering cells, also appears to be effective on a related virus called Nipah, which kills up to 75 percent of the people it infects.

Nipah virus emerged in 1998 in Malaysia, and also has been found in Bangladesh and India. Nipah appears to infect humans more easily than Hendra and can be transmitted from person to person.

The viruses can cause brain swelling and acute respiratory illness. Both viruses are carried by a type of fruit bat commonly called flying foxes.

Geisbert said more testing needs to be done but the findings are "a significant stepping stone" toward the beginning of human clinical trials.

(Editing by Eric Walsh)