Giving the ZMapp cocktail to rhesus macaque monkeys up to five days after infection with the Ebola virus completely protected the animals — a finding that a Canadian-led research team says supports compassionate use of the treatment during the current outbreak in West Africa.

ZMapp, developed with involvement of the Public Health Agency of Canada and U.S. researchers, is a cocktail of three monoclonal antibodies that is designed to bind to the protein of the Ebola virus, neutralizing the virus so it can’t do any further damage.

In Friday’s online issue of the journal Nature, researchers led by Gary Kobinger of the National Microbiology Laboratory in Winnipeg showed that giving ZMapp resulted in the survival of 18 rhesus macaques infected with the Ebola virus.

"The level of improvement was at least beyond my own expectation," Kobinger told reporters Friday from Montreal.

The researchers haven’t observed any long-term side-effects in the animals that received ZMapp.

"ZMapp exceeds the efficacy of any other therapeutics described so far, and results warrant further development of this cocktail for clinical use," the study’s authors concluded.

Kobinger cautioned that the animals received a very high dose of the Ebola virus that could resemble an accidental laboratory exposure, but differs from natural exposures in humans.

A Phase 1 safety study is scheduled to begin in healthy humans in early 2015. Mapp BioPharmaceuticals, which has licensed the drug, is conducting the next stages of research needed to seek regulatory approval for ZMapp.

If safety data from a Phase 1 trial in humans in the U.S. supports the compassionate use of ZMapp, Kobinger expects it could be used under Health Canada’s special access program possibly by spring. But scaling up production in tobacco plants to stockpile thousands of doses is another matter.

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Gary Kobinger of the National Microbiology Laboratory in Winnipeg said the level of improvement in the infected monkeys treated with ZMapp exceeded his expectations. The drug cocktail may prove important in treat Ebola in West Africa. (CBC)

In the study, the treatment reversed severe Ebola virus disease symptoms such as excessive bleeding, rashes and elevated liver enzymes, even when ZMapp was given as late as five days after exposure to the virus.

In contrast, the three rhesus macaques that did not receive ZMapp all succumbed to the infection with a central African strain of Ebola virus by Day 8. Kobinger said the laboratory strain is comparable to one causing an outbreak now in West Africa.

There is no licensed vaccine or treatment against the Ebola virus.

Moving the most promising interventions forward is an urgent matter, said Thomas Geisbert, a professor of microbiology and immunology at the University of Texas Medical Branch in Galveston.

Geisbert is working on another potential treatment for the Ebola virus, as well its cousin, Marburg virus, and wrote a journal commentary that accompanies the study.

"I think it's a monumental achievement in the field," Geisbert said of the ZMapp findings in monkeys. "The fact that they were able to treat these monkeys in a time in the disease course when you can actually detect virus in the blood of the animals and when the animals are quite ill and then be able to use the ZMapp and still be able to protect the animals against a lethal Ebola virus infection is a very stunning and certainly a promising finding."

So far, seven people infected with the Ebola virus have received ZMapp. Two have died, a Liberian doctor and a Spanish priest, but it’s not clear how many doses they received and if it was early enough in the course of the illness. At least two doses are needed in non-human primates, Kobinger said.

Doctors have stressed it's unclear whether ZMapp made a difference for human patients. So far, about 45 per cent of people infected in the West African outbreak have recovered without treatment, Geisbert said.

If clinical trials are successful, the manufacture of ZMapp could require investment to make monoclonal antibodies at an industrial scale, assuming there is funding.

"Unfortunately, it's not like Hollywood where they make a drug or a vaccine in 24 hours and save the world. You know, it does take some time," Geisbert said.

The study was funded by the Defence Threat Reduction Agency, the U.S. National Institutes of Health and the Public Health Agency of Canada (PHAC), and a Canadian Safety and Security Program grant.

Lab worker safety

Meanwhile, PHAC says the Canadian scientists returning home from an Ebola virus lab in Sierra Leone over concerns for their health and safety aren't showing any signs of illness, and will come back to Canada on a private charter plane.

Ebola drug ZMapp tobacco plants

Indicator proteins glow under ultraviolet light on the leaves of a plant related to tobacco that will be used to grow antibodies for ZMapp, an experimental treatment for the Ebola virus. (Sean Gallup/Getty)

The agency said the three workers are in good health and are returning as a precaution.

"The risk that any of them were infected is very low," the agency said in an email Thursday night. "As an added precaution, the employees will be returning on a private charter plane to Canada and will be monitored closely on their journey home and after they return."

Once they arrive, a quarantine officer and border officials will board the plane to assess their health.

The agency announced late Tuesday that it was pulling the team from a World Health Organization outpost at Kailahun, in eastern Sierra Leone. The agency said three people at their hotel complex tested positive for the Ebola virus.

Once the workers have been cleared for entry, they will travel to private residences where they will stay in voluntary isolation for the rest of the 21-day incubation period, avoiding their families and the community.

Meinie Nicolai, who co-ordinates the Ebola outbreak response for Doctors Without Borders in Brussels, doesn't blame the team for leaving.

"If there's stress in a team, you'd better take them out because they can make mistakes and mistakes can be fatal," Nicolai said.

The agency has said it's committed to helping in the response to the outbreak and is preparing to send another team to Sierra Leone once appropriate steps have been taken to ensure a safe living environment.

With files from CBC's Amina Zafar and Karen Pauls