As West Africa battles a deadly Ebola outbreak, the World Health Organisation (WHO) has approved the use of untested drug ZMapp for people infected by the virus.

ZMapp is an experimental treatment developed by US company Mapp Biopharmaceutical based off years of US and Canadian research.

It is not a vaccine against the Ebola virus but rather can be used to treat individuals already infected with the virus.

ZMapp had never been used to treat humans prior to being used on two US aid workers who fell sick with Ebola working in Liberia.

The two Americans - Nancy Writebol, 59, and Dr Kent Brantly, 33 - saw their conditions improve by varying degrees after they received the experimental drug.

The lead investigator who worked to develop ZMapp, Dr Gene Olinger, said in 2012 the antibodies in the treatments were the safest for animals and most successful in protecting against Ebola.

"It is rare that an antiviral compound prevents Ebola virus infection with limited to no morbidity in treated animals at any point of treatment following infection by this lethal virus," he said.

The ZMapp Ebola treatment has been tested on Rhesus monkeys with a high success rate. ( Flickr: Ellery Chen )

"Until recently, attempts to utilize antibodies to provide protection against Ebola virus have been met with failure.

"The level of protection against disease that we saw with MB-003 [an early version of ZMapp] was impressive."

In early trials using Rhesus monkeys all animals survived being infected with the Ebola virus when the treatment was administered one hour after infection.

Two-thirds of the animals were protected even when the treatment was administered 48 hours after infection.

The drug antibodies were initially developed using a mouse model, then it was successfully humanised and produced in the tobacco plant-based production system. Current doses of ZMapp were produced using tobacco plants.

Developer's supply of ZMapp exhausted

On Tuesday Mapp Biopharmaceutical announced their supply of the experimental drug had run out.

"The available supply of ZMapp has been exhausted. We have complied with every request for ZMapp that had the necessary legal/regulatory authorization," Mapp Biopharmaceutical said in a statement.

"It is the requestors' decision whether they wish to make public their request, acquisition, or use of the experimental drug."

WHO said only about 10 to 12 doses of the drug had been made.

Despite WHO ruling people infected in the West African Ebola outbreak can be offered the untested drug it still has not been approved by any government as safe for humans.

"Any decision to use ZMapp must be made by the patients' medical team," Mapp Biopharmaceutical said.

The company says the drug has been provided at no cost in all cases where it has been used.

Australian expert says there may be no way to prove ZMapp is effective

Infectious disease expert Associate Professor Sanjaya Senanayake from Australian National University says it is difficult to test Ebola treatments for human use.

"Even if the drug is tested in healthy volunteers to show that it’s safe the next step to show that it works is difficult because we can't predict when Ebola outbreaks are going to be," he said.

Associate Professor Senanayake says drugs to treat infectious diseases are hard to test due to the ethical issues of giving placebos to patients suffering from a disease with a high mortality rate.

"Even though this hasn't been tested in the usual way, given that this is such a lethal infection and in animal models it seems to be effective, shouldn't we give it to humans?" he said.

"You've got those ethical questions to balance."

He said aside from the difficulties of testing on humans there are a number of reasons why there may be so few doses available of the potentially life saving Zmapp drug are available to infected patients.

"Perhaps the process to produce the drug is very extensive and expensive, so costs may have been prohibitive," he said.

Associate Professor Senanayake said while the two US aid workers who have received the medication are showing signs of improvement it could be impossible to prove the link between the treatment and recovery.

"The question is: is it because of the drug, because of ZMapp or in spite of ZMapp?" he said.

"After all, despite its high fatality rate, around 40 per cent of people survive Ebola infection by themselves.

"It's hard to know but I suspect the drugs could be ready fairly shortly, perhaps within the next couple of years, but in terms of how we test it if it’s effective in humans will always remain a difficult issue."

Canada donates its own Ebola treatment to WHO

Canada's health minister Rona Ambrose said the country was donating a small quantity of another experimental Ebola treatment to the WHO for use in Africa.

The Canadian government will donate between 800 to 1,000 doses of the drug, with the final number given dependent on how much Canada holds back for research and clinical trials.

The government will also keep a small supply in case it is needed domestically.

Deputy chief public health officer of the Public Health Agency of Canada Dr Greg Taylor says the country has about 1,500 animal doses of the treatment.

It would take between four and six months to create a large quantity of the treatment.

"We see this as a global resource, something we need to put on the global table to say 'how can we make best use of this asset'," he said.

"We're looking to do that as fast as we can."



Dr Taylor said deciding whether to use an experimental drug on humans was very difficult.

"You really don't know how safe it is, you don’t know what the side effects are going to be," he said.

"But in this extraordinary circumstance in Africa right now, we're trying to do everything we can to assist."

ABC/Reuters