A trial of an experimental drug to treat Ebola has raised hopes of breakthrough in the fight against disease after all animals in the study recovered from the disease, even if they were treated five days after being infected.

As the outbreak continues to spread in west Africa, researchers said the experimental drug ZMapp cured the animals even those displaying severe symptoms.

The treated monkeys were exposed to a lethal level of Ebola before receiving three doses of ZMapp starting three, four and five days after infection.

The treatment reversed their symptoms including excessive bleeding, rashes, and liver damage.

Three weeks after they were infected, no trace of the virus could be detected in the infected animals' blood.

The Belgian scientist who discovered Ebola in 1976 said it was vital to start clinical trials of ZMapp. Prof Peter Piot, now director of the London School of Hygiene & Tropical Medicine, said: "This well designed trial in non-human primates provides the most convincing evidence to date that ZMapp may be an effective treatment of Ebola infection in humans. It is now critical that human trials start as soon as possible."

"I never thought that 40 years after I encountered the first Ebola outbreak, this disease would still be taking lives on such a devastating scale."

Two US doctors who made a full recovery after contracting the disease in Liberia were given the drug, but doctors at the American hospital where they were received care have said they did not know whether the drug helped or hindered their recovery.

ZMapp is also being used as part of the treatment of the British nurse Will Pooley who is being treated in London after being evacuated from Sierra Leone.

With no cure for the virus that is ravaging west Africa on an unprecedented scale, there is a renewed impetus in the US and the UK to find a vaccine. On Thursday, GlaxoSmithKline said it was bringing forward clinical trials on humans of a promising potential vaccine.

The development of ZMapp and its success in treating advanced stages of Ebola infection was described as a "monumental achievement" by Prof Thomas Geisbert, from the University of Texas, writing in Nature. He added: "The next crucial step will be to formally assess its safety and effectiveness. Testing the latter is clearly difficult, because intentional infection of human subjects in clinical trials is not possible."

WHO's latest figures put the death toll at 1,552, almost as many as the combined death toll of the 26 outbreaks since Piot's discovery 48 years ago and has warned that the current spread could affect 20,000 before it is contained.

A team of scientists led by Dr Gary Kobinger, from the public health agency of Canada, which is developing the drug with Californian company LeafBio wrote in Nature: "We hope that initial safety testing in humans will be undertaken soon, preferably within the next few months, to enable the compassionate use of ZMapp as soon as possible."

ZMapp is a blend of three laboratory-made antibodies designed to neutralise the virus and was first identified as a drug in January 2014. It is manufactured using the low nicotine tobacco plant, Nicotiana benthamiana, which is one of the most widely used hosts in plant virology. The plants are grown for several weeks before being treated with the antibody. They are then grown for a further week and the antibody proteins generated harvested.

Prof David Evans, of the virology department at the University of Warwick described the trials as "extremely encouraging". But he added that the results did not prove that the health care workers who have already received ZMapp and recovered did so due to the therapy. Others who also received ZMapp succumbed to the virus, including a 75-year old Spanish priest who was evacuated from Liberia.

Prof Martin Hibberd, from the London School of Hygiene & Tropical Medicine, said he hoped the researchers behind ZMapp could "receive sufficient funding to undertake these clinical trials straight away as this is by far the most advanced potential treatment option available to my knowledge".