Correction: An earlier version of this article incorrectly characterized Professor Thomas Geisbert as saying that rhesus macaques infected with Ebola by researchers had had the drug injected into their bodies. It should have described him as saying that the virus was injected into their bodies. The article has been updated.

As the Ebola outbreak spread to a fifth African country Friday, researchers announced that the experimental drug pressed into emergency use in recent weeks had cured a group of 18 monkeys of the deadly disease, including some that didn’t receive the treatment for five days after they were injected with the virus.

The research raised new hope for the eventual use of the cocktail of monoclonal antibodies against Ebola, which has no cure or vaccine. It has been fatal to more than half of the 3,069 people who have acquired the virus in Liberia, Guinea, Sierra Leone, Nigeria, and now Senegal, which confirmed its first case Friday.

The research unveiled Friday, in the online version of the journal Nature, is “a huge step forward,” said Thomas Geisbert, a professor at the University of Texas Medical Branch at Galveston, who has conducted similar research on the lethal Marburg virus and reviewed the Ebola study for Nature. “It’s very, very hard for me to believe that this would not have substantial utility in treating human disease.”

The drug, ZMapp, “was able to reverse severe [Ebola] disease . . . leading to full recovery of all treated” nonhuman primates within 28 days of their being infected, the researchers wrote.

The treatment has not been tested on humans, although it was given on an emergency basis to seven people who fell ill in recent weeks, including two American missionaries who recovered. Of the seven who received the drug, two have died — an elderly Spanish priest and a Liberian doctor. Two other doctors in Liberia and a British nurse are still being treated. But it is impossible to gauge the drug’s impact based on a small sample of people who became infected at different times and received care under differing circumstances.

During the deadliest Ebola outbreak ever in western Africa, two American missionaries received an experimental drug called ZMapp. An Ebola expert explains how ZMapp is derived and how it fights the deadly virus. (Gillian Brockell and Pamela Kirkland/The Washington Post)

The small supply of the drug, perhaps 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 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 number that may be needed.

Kobinger said he thinks additional doses should be available for humans by summer 2015.

But David Howard, a spokesman for Reynolds American Services, which owns Kentucky BioProcessing, said in an e-mail 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.” Geisbert, the professor, noted that the rhesus macaques studied were given proportionately much larger doses of Ebola than most infected humans receive, and the virus was injected directly into their bodies, not absorbed through breaks in the skin.

As a result, he said, healing all 18, especially after some had become severely ill with the hemorrhagic disease, means the drug shows promise.

The researchers did not test ZMapp on the highly lethal Zaire strain of the Ebola virus that is spreading across West Africa, but they said the version of the virus they used is similar and the treatment should work on it. Kobinger said the monkeys tested appear to be completely healthy, with no side effects that other drugs had caused. Three infected monkeys that were not treated died within eight days.

The study marked the first time that test subjects were effectively treated five days after acquiring the infection. Geisbert and another group of researchers reported last week that they had used a different drug to successfully treat monkeys three days after the animals were infected with Marburg.

On Thursday, U.S. government researchers, in collaboration with British drugmaker GlaxoSmithKline, announced that they will begin human trials of an experimental Ebola vaccine next week at the National Institutes of Health in Bethesda, Md. Officials said they want to rush the drug to health workers and others at risk in West Africa.

More than 1,500 people have died in the worst Ebola outbreak in history, according to the World Health Organization, which predicts that the current epidemic could ultimately total 20,000 ­cases. (A handful of cases, which officials have said is a separate outbreak, also have emerged in Congo.)

Senegal’s health minister, Awa Marie Coll-Seck, said the Ebola patient discovered there — a Guinean national who traveled to Senegal — was put in isolation.

As the outbreak worsened in West Africa, Senegal had taken aggressive measures to prevent the virus from spreading to the country. Last week, Senegal closed its borders with neighboring Guinea, preventing flights headed to Guinea, Sierra Leone or Liberia from touching down in Senegal, a major travel hub for the region.