Ebola virus disease (EVD), formerly known as Ebola haemorrhagic fever, is a severe and often fatal infection caused by viruses in the genus Ebolavirus.
The largest Ebola outbreak to date took place between 2014 and 2016 and primarily affected countries in West Africa, including Guinea, Liberia and Sierra Leone, according to the Centers for Disease Control and Prevention (CDC). Limited cases also occured in Italy, Mali, Nigeria, Senegal, Spain, the U.K. and the U.S. In total, 28,652 suspected, probable and confirmed Ebola cases were reported during the outbreak, 11,325 of which were fatal.
Ebola virus belongs to the same family of viruses as the Marburg virus, which causes similarly severe disease to its cousin, according to the World Health Organization (WHO). Marburg virus disease was first discovered in 1967 during two major outbreaks in Germany and Serbia, which were linked to monkeys imported from Africa. Since then, sporadic cases have been reported, mostly in African countries; in July 2022, the WHO reported two fatal cases in Ghana, where the disease hadn't been seen before.
Related: The deadliest viruses in history
When was Ebola discovered?
Ebola was first discovered in 1976 when outbreaks erupted in the areas that are now South Sudan and the Democratic Republic of the Congo (DRC), according to the CDC. The outbreak in DRC — then the Republic of Zaïre — took place in a village near the Ebola River, after which the virus was named.
Although initially thought to be part of the same large outbreak, the two 1976 outbreaks were later found to be caused by genetically distinct viruses that likely originated from different sources and independently spread through each locale. These species of Ebola virus were named Zaire ebolavirus and Sudan ebolavirus after the areas where they were identified.
What are Ebola viruses?
EVD is caused by viruses in the genus Ebolavirus within the family Filoviridae; these viruses most commonly cause disease in humans and nonhuman primates, including monkeys, gorillas and chimpanzees, according to the CDC.
To date, six species of Ebolavirus have been identified, four of which cause severe disease in humans. These include:
- Ebola virus (species Zaire ebolavirus)
- Sudan virus (species Sudan ebolavirus)
- Taï Forest virus (species Taï Forest ebolavirus, formerly Côte d’Ivoire ebolavirus)
- Bundibugyo virus (species Bundibugyo ebolavirus)
The remaining two Ebola virus species include the Reston virus (species Reston ebolavirus) and Bombali virus (species Bombali ebolavirus). The first has been reported to cause illness in nonhuman primates and pigs, but it's not yet known whether the latter causes disease in either people or animals, other than bats.
Scientists discovered the Bombali virus fairly recently, in 2018, while they were analyzing oral and rectal swab samples taken from free-tailed bats (Chaerephon pumilus and Mops condylurus) in Sierra Leone, according to a 2018 report in the journal Nature Microbiology.
Where did Ebola viruses come from?
Ebola viruses are zoonotic, meaning they spread to humans from infected animals during what's called a "spillover event." Animals that can carry Ebola virus include chimpanzees, gorillas, monkeys, forest antelope, porcupines and bats, according to the WHO.
Current evidence suggests that African fruit bats likely serve as the natural "reservoir" for Ebola viruses in the wild, according to the CDC. That means the animals likely carry the viruses without falling ill and instead act as the primary vessels that the viruses replicate in. In theory, the viruses can then spread from bats to other animal species and to humans.
While fruit bats are suspected to be the virus' reservoir host, however, scientists have not pinpointed a specific species that serves as the main host. In addition, Ebola viruses have also been detected in insect-eating bat species, such as the free-tailed bats of Sierra Leone.
How does Ebola spread?
Ebola viruses jump from animals to humans when people come into close contact with the blood, secretions, organs or other bodily fluids of infected animals, according to the WHO.
Similarly, Ebola spreads from person to person via direct contact; that means that the blood, secretions, organs or other body fluids containing the virus must come into contact with broken skin or the mucous membranes of a healthy individual in order for the virus to be transmitted. The fluids may come directly from the infected patient or from surfaces touched by the sick individual, such as bedding or clothing.
A WHO Ebola Situation Assessment for Oct. 6, 2014 states that the virus is most easily transmitted through blood, feces and vomit. The virus can also be transmitted through urine, saliva, sweat, breast milk, amniotic fluid and semen, according to the CDC.
"The virus can remain in certain body fluids (including semen) of a patient who has recovered from EVD, even if they no longer have symptoms of severe illness," the CDC notes. For this reason, the WHO recommends that EVD survivors with penises practice abstain from sex or practice correct and consistent condom use until their semen has twice tested negative for Ebola or until 12 month have passed since their initial symptoms.
"There is no evidence that Ebola can be spread through sex or other contact with vaginal fluids from a woman who has had Ebola," the CDC states. That said, the virus can persist in the placenta, amniotic fluid and fetus of people who became infected while pregnant, even after they've recovered, and the virus may also hang around in the breast milk of people who become infected while breastfeeding, according to the WHO.
Ebola does not spread through the air, meaning a person cannot contract the virus from breathing the same air as an infected individual. However, if an infected individual directly sneezes on a person and the mucus from that sneeze comes into contact with an open cut or the eyes, nose or mouth of someone else, there is a chance of infection. WHO has not documented cases of this actually happening, though.
What are the symptoms of Ebola?
Symptoms of EVD may appear two to 21 days after exposure to the Ebola virus, and on average, symptoms tend to appear within eight to 10 days, the CDC says.
"The symptoms are extremely nonspecific in the beginning — Ebola looks like almost anything," Dr. Bruce Hirsch, an infectious-disease specialist at North Shore University Hospital in Manhasset, New York, told Live Science. For example, the early stages of an Ebola infection may resemble influenza (flu), malaria or typhoid fever, the CDC states.
According to the CDC, the patient will usually experience the following symptoms:
- Aches and pains, such as severe headache and muscle and joint pain
- Weakness and fatigue
- Sore throat
- Loss of appetite
- Gastrointestinal symptoms including abdominal pain, diarrhea, and vomiting
- Unexplained hemorrhaging, bleeding or bruising; bleeding can be both internal and external
Patients often progress from the "dry" symptoms (fever, aches and pains, fatigue) to the "wet" symptoms (diarrhea, vomiting) over the course of their illness, the CDC says. Hemorrhagic syndrome — where patients bleed from the nose and mouth — usually occurs only in the late stages of the disease. Typically, the Ebola virus causes hemorrhagic syndrome in 30% to 50% of patients, said Angela Rasmussen, a research assistant professor of microbiology at the University of Washington.
Other symptoms that occur in advanced stages of EVD include rash and symptoms of impaired kidney and liver function, according to WHO. People may also develop red eyes and hiccups in late stages of the disease, the CDC says.
Because the symptoms of Ebola resemble those of other diseases, medical professionals use a series of tests to diagnose EVD. According to WHO, common tests include:
- Virus isolation by cell culture
- Antibody-capture enzyme-linked immunosorbent assay (ELISA)
- Serum neutralization test
- Antigen-capture detection tests
- Electron microscopy
- Reverse transcriptase polymerase chain reaction (RT-PCR) assay
How deadly is Ebola?
EVD is often fatal. The average case fatality rate of the disease is estimated at 50%, and in past outbreaks, the case fatality rates have ranged from 25% to 90%, according to the WHO.
During the West Africa Ebola epidemic, the largest Ebola outbreak in history, 11,308 deaths were reported out of 28,610 total cases, meaning about 39% of cases were fatal, according to the CDC.
In response to a virus, the human body produces antibodies. "Antibodies are the protein 'workhorses' of the immune system; in less severe infections (like the flu), antibodies stop the virus from spreading and help clear it from the body," said Jonathan Lai, associate professor of biochemistry at Albert Einstein College of Medicine in New York. The problem with Ebola is that the virus spreads so rapidly that it can easily overcome the body's immune response," he told Live Science.
How to prevent Ebola transmission
The CDC offers the following tips to avoid EVD when living in or traveling to a region where Ebola virus is potentially present:
- Avoid contact with blood and body fluids (such as urine, feces, saliva, sweat, vomit, breast milk, amniotic fluid, semen, and vaginal fluids) of people who are sick with EVD.
- Avoid contact with semen from a man who has recovered from EVD, until testing shows that the virus is gone from his semen.
- Avoid contact with items that may have come in contact with an infected person’s blood or body fluids (such as clothes, bedding, needles, and medical equipment).
- Avoid funeral or burial practices that involve touching the body of someone who died from EVD or suspect EVD.
- Avoid contact with bats, forest antelopes, and nonhuman primates, such as monkeys and chimpanzees, blood, fluids, or raw meat prepared from these or unknown animals.
Are there vaccines for Ebola?
Recently, two vaccines were developed for the prevention of EVD, according to the WHO.
The first vaccine, called Ervebo or rVSV-ZEBOV, was approved in November 2019 by the European Medicines Agency and in December 2019 by the U.S. Food and Drug Administration (FDA). Since then, it's been cleared for use in Burundi, Central African Republic, the Democratic Republic of the Congo, Ghana, Guinea, Rwanda, Uganda and Zambia.
The single-dose vaccine is approved for use in individuals 18 years of age and older, excluding pregnant and breastfeeding women, and has been found to be highly safe and against Zaire ebolavirus, the Ebola virus species responsible for the largest and most deadly Ebola outbreaks to date.
The second vaccine is administered in two doses approximately eight weeks apart; the first dose is a formulation called Zabdeno (Ad26.ZEBOV) and the second is called Mvabea (MVA-BN-Filo). Multiple clinical studies have demonstrated that the two-dose vaccine is safe and induces an immune response against Zaire ebolavirus; however, the exact level of protection the vaccine provides is unknown, according to the WHO.
In 2020, the European Medicines Agency recommended granting "marketing authorization" to Zabdeno-and-Mvabea for use in individuals as young as 1 year old. This vaccine has not been approved for routine use by the FDA but it has been used under a research protocol, according to the CDC.
How is Ebola treated?
In 2020, the FDA approved two treatments for EVD caused by Zaire ebolavirus, according to the CDC. One, called Inmazeb, contains three monoclonal antibodies — lab-made proteins that latch onto the virus and prevent it from infecting cells. The second treatment, Ebanga, contains a single monoclonal antibody that works the same way.
Both treatments were tested during a clinical trial held during the 2018-2020 Ebola outbreak in the Democratic Republic of the Congo and were shown to increase patients' chance of survival. (Neither treatment has been evaluated for efficacy against virus species other than Zaire ebolavirus.)
In the event that neither Inmazeb nor Ebanga is available, supportive care measures can improve patients' chances of survival. Such care includes:
- Providing fluids and electrolytes (body salts) orally or through infusion into the vein (intravenously).
- Using medication to support blood pressure, reduce vomiting and diarrhea, and to manage fever and pain.
- Treating other infections, if they occur.
What recovery from Ebola looks like
EVD survivors are often faced with long-lasting medical problems and other changes in their bodies.
Ebola virtual particles can persist in certain areas of the body, including the testicles, the inside of the eye and the central nervous system, according to the WHO. "Whether the virus is present in these body parts and for how long varies by survivor," the CDC states.
And as previously stated, the viruses can also linger in semen; in the placenta, amniotic fluid and fetus of people who have been infected while pregnant; and in the breast milk of people who have been infected while breastfeeding. (The WHO provides specific guidance for managing pregnancy and breastfeeding in the context of EVD.)
EVD survivors often report tiredness; headaches; muscle and joint pain; stomach pain or loss of appetite; and eye and vision problems, including blurry vision, pain, redness, and light sensitivity, the CDC says. Other long-term complications of the infection include:
- Memory loss
- Neck swelling
- Dry mouth
- Tightness of the chest
- Hair loss
- Hearing problems (ringing in the ears and hearing loss)
- Pain or tingling in the hands and feet
- Inflammation of the pericardium (tissue around the heart)
- Inflammation of one or both testicles
- Decreased or lost interest in sex
- Changes in menstruation
- Difficulty falling or remaining asleep
- Depression, anxiety, and post-traumatic stress disorder (PTSD)
One survivor experienced a change in eye color due to inflammation and very high blood pressure in his left eye, according to a 2015 report in The New England Journal of Medicine. When fluid in the eye was tested, it was found to contain Ebola virus, but was believed to not be contagious.
EVD survivors can develop antibodies that will likely protect them from the virus for at least 10 years, or possibly even longer, the CDC states. However, it is not known if people who recover are immune for life or if they can later become infected with a different species of Ebola virus, the agency notes.
Learn about the FDA-approved treatments for Ebola with SciShow. Learn about why bats carry so many dangerous viruses in Science Magazine. Learn about the strategies uses to prevent and control Ebola outbreaks from Doctors Without Borders.
This article is for informational purposes only, and is not meant to offer medical advice. This article was originally published on Live Science in 2016 and was partially rewritten July, 28 2022.
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