Science

What is the Ebola virus, and can it be stopped?

The Ebola virus causes a disease that kills up to 90 percent of those who contract it, but a promising vaccine could provide protection. Wednesday, 10 April 2019

By Michael Greshko

Ebola virus, formally called Zaire ebolavirus, is a rare virus that infects humans and nonhuman animals such as pigs and other primates. It is one among several viruses within the genus Ebolavirus, only four of which are known to infect humans: Ebola, Sudan, Taï Forest, and Bundibugyo.

Some viruses within the genus Ebolavirus do not cause symptoms in humans, such as Reston ebolavirus, the variant at the center of the book and TV series The Hot Zone(though it does cause disease in pigs and non-human primates). A variant called Bombali virus was recently found in bats, but at present it's unclear whether it infects other animals. Zooming out, Ebolavirus sits within a group called the filoviruses, which includes similar pathogens such as Marburgvirus and Cuevavirus.

Ebola is a zoonosis, or a disease that can “spill over” into humans from nonhuman animals in the wild that carry the sickness. Researchers don't know for sure which animals are Ebola carriers, but there's evidence that fruit bats may play a role in spreading the virus to other animals, such as chimpanzees, gorillas, and small antelopes called duikers. Humans, in turn, can come into contact with the virus by interacting with infected animals, such as by hunting or preparing bushmeat.

The virus causes Ebola virus disease (EVD), a severe and sometimes fatal illness that can cause fever, weakness, diarrhea, fatigue, vomiting, stomach pain, and unexplained bleeding and bruising. On average, symptoms appear between eight and 10 days after exposure.

Ebola spreads through contact with bodily fluids—such as blood, urine, feces, vomit, breast milk, and saliva—from people who have fallen ill or died from EVD. The virus gets into the body through breaks in the skin or through mucous membranes, such as those in the eyes, nose, or mouth. Contaminated needles or syringes also can transmit the virus, and there's a strong chance that it can also spread via sexual contact. The virus can persist in semen, even after a man has recovered from EVD.

How dangerous is Ebola?

Depending on the patient's immune response and access to medical care, Ebola can be fatal 35 to 90 percent of the time, which is why local and global health officials work so hard to contain outbreaks.

The 2014-2016 outbreak in Guinea, Liberia, and Sierra Leone—the worst on record—infected 28,600 people and killed 11,325 directly. But as one 2016 study notes, the affected regions’ fragile health-care systems and existing poverty meant that the epidemic had devastating ripple effects. In 2014, an estimated five million children from the ages of three to 17 were out of school because of the Ebola epidemic. Childhood vaccination programs also temporarily halted, leaving hundreds of thousands of children exposed to other fatal diseases such as measles. One 2015 estimate held that the epidemic led to as many as 120,000 maternal deaths, partly because of the health-care system's collapse.

As serious as Ebola and similar viruses are, though, it's dangerous to misrepresent or exaggerate their threat. For one, fears of Ebola “going airborne,” as some commentators declared during the 2014-2016 outbreak, are unfounded: There is no evidence that Ebola is evolving to more effectively spread through the air.

There is also no strong evidence that strict travel bans, such as those suggested during the 2014 outbreak, are effective at slowing down the spread of viruses such as Ebola. Travel bans could actually make some outbreaks worse by isolating and stigmatizing the very communities that need the most help. During the 2014-2016 outbreak, the U.S. Centers for Disease Control and Prevention (CDC) ran information campaigns and other efforts to limit risks of spreading Ebola while minimizing the disruption to international trade and movement.

Ebola is also far from the only lethal virus that humans face. From October 2018 to March 2019 alone, between 28,000 and 46,800 people in the United States died from influenza, according to CDC estimates. In 2017, measles killed 110,000 people worldwide; before the measles vaccine, some 2.6 million people died each year from that virus.

How does Ebola virus work?

The exterior of the Ebola virus is a snakelike filament less than a millionth of a meter long. The filament houses the virus's RNA, a string of genetic material about 19,000 base pairs long that encodes seven proteins. The virus's outer membrane is studded with specific complexes of proteins and carbohydrates, called glycoproteins, that act like skeleton keys for the various “locks” on the outsides of our bodies' cells.

Ebola uses glycoproteins as a devious disguise: The virus's exterior lets it chemically imitate the debris that results from apoptosis, the orderly, programmed death of a cell. Normally, nearby cells can detect the remains of their dead neighbors and absorb the wreckage for disposal—which means that when they detect Ebola, they inadvertently welcome it into the cell's interior. At first, Ebola is stuck within a kind of cellular tour bus, a membraned bubble called a vesicle. But proteins on one end of the virus let it eject its RNA out of the vesicle and into the cell's innards.

Once the Ebola RNA is loose inside the cell, it hijacks the cell's machinery to make many copies of its building blocks, which then assemble. By recruiting the cell's very own membrane machinery, they bud off to form new viruses. Eventually, being a viral factory becomes too much for the cell to bear, and it dies.

Ebola causes the symptoms it does because of the types of cells it targets, which include a group of immune cells called dendritic cells that act a bit like the body's security cameras. In their absence, other classes of immune cells are left flying blind—so the virus can replicate rapidly. In addition, Ebola can disrupt cells' ability to make interferon—an anti-viral signaling molecule—and can even cause some immune cells to self-destruct.

When Ebola enters other immune cells, the infection causes blood vessels to weaken and triggers the formation of many tiny blood clots, leading to the hemorrhages and bleeding seen in some, but not all, Ebola cases. Cells in the liver, adrenal gland, and gastrointestinal tract suffer immensely, throwing an infected person's body into chaos.

What are the treatments?

Health officials emphasize the importance of preventing contact with Ebola in the first place through hand-washing and avoiding contact with people or animals infected with the virus. Once a person is infected, treatments center on taking care of the symptoms. Oxygen and intravenous fluids help, as do medications that address diarrhea and blood pressure loss.

Researchers have made rapid progress in developing an experimental Ebola vaccine, called rVSV-ZEBOV. In 2015, international researchers tested the vaccine among 11,841 people living in Guinea's Basse-Guinée region, and it proved 100-percent effective. The CDC says that the vaccine should become officially licensed by U.S. officials sometime in 2019.

In addition, experimental antiviral drugs that can stop Ebola replication in its tracks are under development.

A brief history of Ebola outbreaks

The Ebola virus was first formally identified in the fall of 1976, after an outbreak that struck near Yambuku, a village near the Ebola river in the northern Democratic Republic of the Congo (then Zaire). From September 1 to October 24 of that year, 318 villagers succumbed to the fever; about eight out of nine died. Meanwhile, 284 people in Sudan—including 37 percent of a cotton factory's cloth room workers—fell ill to a similar virus, 151 of whom died within weeks of being infected. Between 1977 and 1988, health officials tracked a total of 35 cases in Sudan and the DRC, 23 of which ended in fatalities.

In 1989, people in the Philippines and the U.S. faced the scenario that later inspired The Hot Zone. On October 2, 1989, a hundred monkeys were shipped from the Philippines to New York City and were driven to Hazelton Research Products, a company in Reston, Virginia, that kept and sold animals for lab testing. By November 12, 14 monkeys either died or were euthanized after showing signs of hemorrhagic fever.

Concern mounted once tests revealed that the monkeys had some kind of Ebola virus. Four workers at the facility also tested positive for Ebola, one of whom seems to have contracted the virus by accidentally cutting himself with a scalpel while studying a monkey that died of infection. Alarmed, the facility brought in U.S. Army scientists to stabilize the situation. Without causing panic or a threat to public health, scientists had to quietly euthanize the remaining monkeys and sterilize the facility.

Fortunately, the people exposed to the virus didn't show symptoms. And back in the Philippines, studies of animal handlers showed that while some people had antibodies to the virus, they didn't show symptoms. In 1992, when the same strain of Ebola reared its head at a primate facility in Italy, the humans there were also healthy. This species of Ebola—now called Reston ebolavirus—does not cause any symptoms in humans.

Several years later, in 1995, health officials had to confront the largest Ebola outbreak to date: 315 cases in and around Kikwit, a town in the DRC. Initially, the virus spread among families and hospitals; a quarter of all cases were among health-care workers. But as medical workers donned protective gear such as face masks and gowns, cases among medical staff thankfully plummeted. In all, 250 people died, 79 percent of those infected.

For the next 20 years, isolated cases and outbreaks would flare up from time to time, from a 425-case outbreak in Uganda in 2000 to the 2005 death of a Russian lab technician exposed to the virus. The largest outbreak to date began in late 2013 and rose to official status in March 2014, after health officials identified 49 cases and 29 deaths in the West African nation of Guinea.

By July 2014, Ebola had spread to Conakry, Guinea's capital, as well as Monrovia and Freetown, the capitals of nearby Liberia and Sierra Leone. By June 2016, when the outbreak officially drew to a close, seven other countries—Italy, Mali, Nigeria, Senegal, Spain, the United Kingdom, and the United States—also had reported several cases of EVD, some from infections sustained among medical workers.

The 2014 outbreak marked the first time that Ebola had infiltrated highly populated urban areas, making it especially serious and difficult to bring under control. Nearly three-fourths of cases were spread among family members, showing that coming in contact with the bodies of people who had died of Ebola was one of the most effective ways of spreading the virus.

As a result, the World Health Organization and other health bodies strongly urge that only professionals wearing protective gear should bury people who have died of Ebola—and that burials should be done as soon as possible. But acting on that advice with dignity is challenging, as it disrupts one of our most profound, intimate experiences: laying our departed loved ones to rest.

Now, local and global health officials are facing one of the worst-ever Ebola outbreaks in parts of the Democratic Republic of the Congo. As of April 4, 2019, a total of 1,100 people—a quarter under 18 years old—have had confirmed or probable cases of EVD, and 690 have died. Some 81 cases—and 27 fatalities—are among health-care workers trying to fight the epidemic.

Containing the outbreak has proved challenging, in no small part because of community distrust. Reports from on-the-ground media and the World Health Organization say that communities have felt alienated by the heavy-handed approach used by aid groups and the local government. Officials says they are actively working to rebuild trust and stem the tide of infections.

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