The experimental drug pressed into emergency use in the West African Ebola epidemic cured a group of 18 monkeys of the deadly disease, including some who didn't receive the treatment until five days after they were injected with the virus, researchers reported Friday.

The finding, published online in the journal Nature, raises new hope for use of the cocktail of monoclonal antibodies, called ZMapp, against Ebola, which has no known cure or vaccine. It has been fatal to more than half the people who have contracted the virus in Liberia, Guinea, Sierra Leone and Nigeria.

During the current outbreak, more than 1,500 people have died and 3,069 people have become infected in five countries, the latest of them Senegal, according to the World Health Organization. The current epidemic is worse than all previous Ebola outbreaks combined. A small number of cases, believed to be a separate outbreak, have surfaced in the Democratic Republic of Congo.

"The highlight of these experimental results is undoubtedly ZMapp, which was able to reverse severe [Ebola] disease...leading to full recovery of all treated [non-human primates]" within 28 days after they were infected, the researchers wrote. They added that their work marked the first time that test subjects, in this case rhesus macaques, were effectively treated five days after acquiring the infection. Another group of researchers reported last week that they had used a different drug to successfully treat monkeys for the deadly and similar Marburg virus three days after the animals were infected.

Though it had never been tested on humans, the drug was given to seven people who fell ill with Ebola in Africa in recent weeks, but it is impossible to determine its impact on a small sample of people who became infected at different times and received their care under differing circumstances. Two American missionaries, who were rushed home from Liberia and placed in isolation, recovered. An elderly Spanish priest and a Liberian doctor have died. Two more Liberian doctors and a British nurse are still being treated.

The small supply of the drug, perhaps about 20 doses, is exhausted, according to Mapp Biopharmaceutical, the tiny San Diego company that developed it.

In a telephone news conference, Gary Kobinger of Canada's Public Health Agency, one of the authors of the newly-released paper, said human subjects should receive three doses of the drug to maximize its effectiveness. He estimated that Kentucky Bioprocessing, the company that actually manufactures the drug in tobacco plants, could produce 20 to 40 doses a month, once it is working at full speed -- a small fraction of the amount that may be needed in an outbreak that the WHO predicts could leave 20,000 people infected.

Kobinger said he believed additional doses should be available for humans by the summer of 2015.

In an email, David Howard, a spokesman for Reynolds American Services, which owns Kentucky Bioprocessing, said that "any estimate of output or capacity regarding production of ZMapp would be pure speculation at this time, since what constitutes a human dose hasn’t been determined yet. The important issue right now is to focus on the need to proceed through clinical and production process development steps. Until these steps are completed, we can’t know things like dose size or dose frequency."

Asked whether his research shows that ZMapp will work on people, Kobinger said: "I think it strongly supports that concept, but it's not proven." In addition to further study, he said, researchers should learn valuable information from tests on the people who have been treated with ZMapp.

The researchers did not test the highly lethal Zaire strain of the Ebola virus which is currently spreading across West Africa, but said the version they used is similar and that ZMapp holds promise to work on the Zaire version. Kobinger said that the monkeys tested appear to be completely healthy, with no side-effects from the drugs like the ones that appeared when previous treatments were tested. Three infected macaques that were not treated died within eight days.

"With the new cocktail, with all the animals, we haven't seen any side effects," Kobinger said.

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