An experimental Ebola serum yet untested on humans seemed to show promise in healing   American health workers who had contracted the deadly virus while in Africa, according to medical practitioners who administered the drugs.

Three vials of the experimental drug ZMapp were flown from the U.S. to Liberia last week to try to save Dr. Kent Brantly, Nancy Writebol and another American missionary who all tested positive for Ebola, and whose health conditions were said to have rapidly deteriorated last week.

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The drug, developed by San Diego-based biotechnology firm Mapp, had only been tested on monkeys.

The Ebola-infected Americans had to sign a waiver to agree to take the treatment even if it has not been approved for safe administration on humans.

Now sources say Dr. Kent Brantly's condition seemed to have improved after taking the experimental serum prior to being flown back to the U.S. where he is now confined in isolation quarters at the Emory Hospital in Atlanta.

He had manifested Ebola symptoms on July 22, with the symptoms worsening last week.

Brantly had volunteered to first let Writebol take the experimental serum that was ready for use last Friday, but when he suddenly developed breathing difficulties and felt close to dying, he was given the drug that first thawed from its sub-zero frozen state.

Doctors said Brantly's condition "miraculously improved" within a day of taking the drug.

Writebol also took two doses of the serum and is set to come home to the U.S. anytime soon. Doctors said her condition did not show any significant improvement after taking the first dose of ZMapp, but showed promising results after the second dose.

ZMapp was developed by harvesting the antibodies of experimental mice that were infected with Ebola. Antibodies are produced by living organisma to combat disease-causing foreign bodies that enter their system such as viruses and bacteria.

ZMapp was tested on six monkeys that were all infected with Ebola.  Four of them were given the drug within a day from infection, while two were treated two days after they manifested Ebola symptoms.  

All six monkeys survived.  A "control" monkey that was not treated with the drug died.

While the experimental serum proved successful on monkeys, its danger, according to researchers, lies in the fact that there is yet no known dosage of the drug that is safe for human use.

In the last four decades since Ebola was discovered, there has been no approved or licensed drugs to combat it.

Scientists point to two reasons that are largely responsible for the lack of advancement in developing anti-Ebola drugs.

For one, they say the virus is difficult to work with as it does not grow well in a petri dish-cultured environment, and that very few facilities are securely equipped to handle the deadly virus.

For another, Ebola has been largely feared as a potential bio-terror weapon, so only governments that are concerned about terrorism have been willing to give funding to Ebola research.

"It's not economically viable for any company to do this kind of research because they have stockholders to think about," said Ben Neuman, a virologist at the University of Reading in Britain.

TagsEbola drug, ZMapp