What makes Ebola one of the world's deadliest viruses?

The Ebola outbreak in the Democratic Republic of Congo and Uganda prompted the World Health Organization (WHO) to declare an international public health emergency in May.

The rare Bundibugyo variant behind the outbreak is not quite as deadly as the more common type of Ebola. Even so, data from previous outbreaks show that as many as one in three infected people die.

So what makes Ebola so dangerous?

Transformed into virus factories

An Ebola outbreak begins when the virus is transmitted from animals to humans, says Ørjan Olsvik. He is a retired professor at UiT The Arctic University of Norway.

To truly understand how Ebola wreaks havoc in the body, we need to zoom in.

Under the microscope, the Ebola virus looks like long threads.

“Viruses are not alive,” says Olsvik, who has personally come face to face with the disease during previous outbreaks in African countries.

However, it can latch onto your cells and inject its genetic material into them. That transforms your cells into tiny virus factories.

“You produce your own death,” says the retired professor.

But even a common cold virus hijacks our cells to copy itself. So why does it go so terribly wrong when the Ebola virus does it?

The virus sets off a kind of vicious circle, says consultant Joakim Øverbø. He researches viruses at the Norwegian Institute of Public Health (NIPH).

Attacks the immune system’s conductor

“The first cells that the virus attacks after entering through a mucous membrane or a break in the skin are often the immune cells,”  says Øverbø.

And not just any immune cells. The victims belong to an ancient branch of our immune system: the innate immune system.

“These initial cells are the conductors that determine the direction of the subsequent immune response,” he says.

Like the dendritic cells.

Their job is to present pieces of viruses and other enemies to the specialised part of the immune system.

But after being hijacked by Ebola, they are unable to sound the alarm.

Like Trojan horses

The virus not only prevents the innate immune system from doing its job. The hijacked cells create chaos.

“These immune cells circulate throughout the body,” says Øverbø.

Like tiny Trojan horses, they enter the lymphatic system, spleen, liver, and kidneys.

Once inside these organs, an army of new Ebola viruses springs from the occupied immune cells. The virus then takes over new types of cells, which are also turned into virus factories.

Like this monkey kidney cell.

The red threads are Ebola viruses emerging from the infected cell.

Releases decoy spikes

But now something even more serious is about to happen.

“What really makes it dangerous over time is that fluid begins leaking from the blood vessels into the body,” says Øverbø.

This leakage from the blood vessels is caused by several factors. The body realises it is under attack by a virus far too late.

“The body overreacts, and you get a very strong inflammatory response,” he says.

The immune cells release a cytokine storm. This is an army of small signalling molecules that trigger an inflammatory response that gets out of control.

The inflammation contributes to making the blood vessels leaky. But the Ebola virus also has another trick, Øverbø explains.

The outside of the virus is covered with small spikes called glycoproteins.

The virus uses them to attach to its target. But these spikes can also be released on their own.

The free-floating spikes act as a decoy that misleads the immune system. And when they reach blood vessels, they also help to weaken them.

“Fluid from the bloodstream begins to leak into parts of the body where it doesn't belong. This can lead to low blood pressure, circulatory failure, and shock,” says Øverbø.

Some Ebola patients also bleed from the nose, ears, and eyes.

Forms tiny blood clots

Your organs don’t just lose fluid, nutrients, and oxygen from the blood. They are also damaged by the virus and attacked by your own immune system.

And while the blood vessels leak, the hijacked immune cells try to make the blood clot. It only works partially.

“This can put the body in a dangerous imbalance where tiny blood clots form at the same time as bleeding increases,” says Øverbø.

All of this combined creates a vicious circle with more and more virus and increasing destruction inside the body.

Eventually, the disease either takes your life – or you survive it.

Not airborne

Even though Ebola is an extremely dangerous virus, it is unlikely to spread to the rest of the world.

WHO is adamant that the risk remains very high locally and regionally, but low internationally. That’s what WHO Director-General Tedros Adhanom Ghebreyesus said at a press conference on June 3.

According to Ørjan Olsvik, there are two reasons why the disease does not spread easily.

“It’s not airborne. So you don't infect other people very easily,” he says.

So you must come into direct contact with bodily fluids of a sick person to become infected. Transmission through respiratory droplets from coughing or sneezing is also considered unlikely, according to Norway's public health information service (link in Norwegian).

Healthcare workers are especially vulnerable

The other reason the disease does not spread easily is that it progresses so rapidly.

“The time from infection to illness is among the shortest. And you become so incredibly sick that you die quickly. That's terrible for the person affected, but it actually helps limit the spread of the disease,” says Olsvik.

Still, some groups are at greater risk of infection. This is especially true for healthcare workers and relatives of infected patients.

“We have a problem with funeral rituals in some communities where people touch and kiss the deceased,” says Olsvik.

Healthcare workers may also not have enough protective equipment.

Three vaccine candidates under development

So what can be done to help those at risk?

There is an approved vaccine against the most common form of Ebola, called the Zaire strain. Vaccinating people who have come in contact with the infected person has proven effective against this variant.

One of the researchers behind the 2017 study demonstrating this was Gunnstein Norheim, a senior adviser at the Norwegian Institute of Public Health.

But the Bundibugyo virus that's currently raging is so different from the Zaire virus that the existing vaccine is likely to have little effect, Norheim points out.

That's why healthcare authorities, researchers, and pharmaceutical companies from many countries are working to develop a new vaccine.

“There’s now tremendous activity and competition to come up with the best possible solution,” says Norheim.

Could take many months

But how long will it take before a new vaccine is ready?

On June 1, the global vaccine collaboration CEPI, which Norway helped start, announced that they will support three vaccine candidates.

Norheim believes it could take six to nine months before a vaccine is ready for clinical trials. The first step will be determining whether it is safe and produces a strong immune response.

“And then it depends on whether it actually protects against the disease. To find that out, field trials in Africa will have to be carried out afterwards,” says Norheim.

Less deadly in the West

However, treatment involves more than just vaccines and drugs. Even without Ebola-specific medicines, infected people have a much better chance of survival in a well-equipped hospital, Norheim explains.

That’s one reason why the disease is less deadly in Western countries than in many parts of  Africa, where patients may be treated in temporary tent clinics.

“There are many other factors that determine the fatality rate, such as access to diagnostics and treatment to replace lost fluids,” he says.

Treatment with intravenous fluids can save many lives. This can compensate for all the fluid that leaks out of the blood vessels.

But providing this kind of treatment in rural clinics in the Democratic Republic of Congo is far from straightforward, Norheim points out.

References:

Baseler et al. The Pathogenesis of Ebola Virus Disease, Annual Review Pathology: Mechanisms Disease, 2017. DOI: 10.1146/annurev-pathol-052016-100506

Henao-Restrepo et al. Efficacy and effectiveness of an rVSV-vectored vaccine in preventing Ebola virus disease: final results from the Guinea ring vaccination, open-label, cluster-randomised trial (Ebola Ça Suffit!), The Lancet, 2017.

———

Translated by Nancy Bazilchuk

Read the Norwegian version of this article on forskning.no

Subscribe to our newsletter

The latest news from Science Norway, sent twice a week and completely free.

Sign up