Image caption Ebola is one of the most feared viruses known to science

Scientists researching the lethal ebola virus have told the BBC that a commercial vaccine to prevent the onset of infection may never be developed.

Two companies with leading vaccine candidates have had their funding from the Pentagon suspended in recent weeks.

An expert said it was now "unlikely" a prophylactic vaccine would ever be used to prevent outbreaks of the disease.

Ebola is often described as the most frightening disease on Earth.

The virus causes a severe haemorrhagic fever, where victims bleed both internally and externally.

The virus attacks white blood cells and blood vessels, causing a rash, red eyes, severe abdominal pain and vomiting.

Overall we've made great progress but unfortunately we are still far away from commercial use. It is hard to say when a vaccine might be available, if ever Dr Heinz Feldmann, Rocky Mountain Laboratories

In recent weeks, an outbreak in western Uganda claimed the lives of at least sixteen people. There is no specific treatment and the virus can kill up to 90% of those who become infected.

Efforts to develop a vaccine have been funded in the main by the US Department of Defense and the National Institutes of Health. They have poured millions of dollars into scientific research because of concerns that the virus could be turned into a biological weapon.

As a result of this funding, several vaccine candidates have been developed and have shown themselves effective in animal trials.

Two companies, Sarepta and Tekmira have begun human safety trials of their vaccines.

But in recent days, both companies have been told by the Defense Department to temporarily stop work on their vaccines due to funding constraints. It is expected that a decision to either resume testing or completely terminate the contracts will be made by early September.

Scientists say their understanding of the nature of the virus has markedly improved over the past decade. But the chances of turning that knowledge into a vaccine are very dependent on money.

All about the money

Gene Olinger, a virologist at the US Army Research Institute of Infectious Disease at Fort Detrick in Maryland, told BBC News: "With the current funding, if it doesn't change, I would say there should be a vaccine in five to seven years. It could double or triple it if the funding goes away."

There is also a big concern over the lack of a large pharmaceutical company which might be willing to develop and market a vaccine for ebola. Since the disease was first discovered in 1976, slightly more than 2,200 people have been infected. And outbreaks have been almost impossible to predict.

Where does Ebola come from? Scientists at the World Health Organisation believe that fruit bats from the Pteropodidae family are the natural hosts of the ebola virus. Although non human primates are often the source of the infection they are not thought to be a reservoir but rather an accidental host. The virus gets into humans through the consumption of infected animals or through contact with their blood, secretions and organs. It can pass from human to human via bodily secretions and fluids. Burial ceremonies in some parts of Africa where mourners have direct contact with the body of the deceased have played an important role in spreading the disease. There are five different strains of ebola, three of which have caused outbreaks in Africa. The Reston species discovered in monkeys imported from the Philippines can infect humans but doesn't appear to have caused illness or death to date. Due to its high mortality rate, ebola is classified as a category A bio terrorism agent by the US Centers for Disease Control.

Larry Zeitlin is the president of Mapp Biopharmacueticals, a small company involved in the development of a therapy for ebola. He says that since the disease is sporadic and extremely deadly, it limits the numbers of people who are affected. But it also limits the commercial potential of any therapy.

"I think it's unlikely that a large pharmaceutical company would get involved," he explained.

"There isn't a huge customer base and big pharma is obviously interested in big profits. So these niche products which are important for biodefence are really driven by small companies."

This uncertainty over funding and the absence of interest from large pharmaceutical firms is leading some scientists to argue that a preventative vaccine might never see the light of day.

Heinz Feldmann is one of the world's leading experts on ebola. He oversees haemorrhagic fever research at Rocky Mountain Laboratories in the United States. He strikes a realistic note about the prospects for a vaccine.

"Overall we've made great progress but unfortunately we are still far away from commercial use. It is hard to say when a vaccine might be available, if ever," he said.

Practical steps

Others are even more sceptical on the basis of the science. Larry Zeitlin says that the challenges in developing a working therapy and worries over a mass innoculation programme will severely limit any prospective ebola vaccine.

Image caption Ebola virus needs careful handling in highly secure labs

"I think it's unlikely you'll see a vaccine that's used in millions and millions of people to prevent the disease, that might only come about if the nature of the disease changed, if it became something spread through airborne contact more like the flu," he said.

While progress has been slow in developing new therapies or vaccines, experts say that big steps have been taken in learning how to manage outbreaks and trace the contacts of those who come down with the disease.

Gene Olinger believes the recent outbreak in Uganda shows that developing new drugs is not the only approach that can be successful against ebola.

"I think they have the infrastructure now to do their own diagnostics, they can then call in help when they need it," he explained.

"The rapid response we saw in the Uganda outbreak indicates they have a greater control over their ability to respond. That makes a big difference. They've done a really good job."