Ebola (Human)

Primary reference(s)

WHO, 2020. Ebola virus disease. World Health Organization (WHO). Accessed 19 November 2019.

Additional scientific description

The Ebola virus causes an acute, serious illness which is often fatal if untreated. Ebola virus disease (EVD) first appeared in 1976 in two simultaneous outbreaks, one in what is now Nzara, South Sudan, and the other in Yambuku, Democratic Republic of the Congo. The latter occurred in a village near the Ebola River, from which the disease takes its name (WHO, 2020a).

The 2014–2016 outbreak in West Africa was the largest Ebola outbreak since the virus was first discovered in 1976. The outbreak started in Guinea and then moved across land borders to Sierra Leone and Liberia. The current 2018–2019 outbreak in eastern Democratic Republic of the Congo is highly complex, with insecurity adversely affecting public health response activities (WHO, 2020a).

The virus family Filoviridae includes three genera: Cuevavirus, Marburgvirus, and Ebolavirus. Within the genus Ebolavirus, six species have been identified: Zaire, Bundibugyo, Sudan, Taï Forest, Reston and Bombali. The virus causing the current outbreak in Democratic Republic of the Congo and the 2014–2016 West African outbreak belongs to the Zaire ebolavirus species (WHO, 2020a).

Ebola is introduced into the human population through close contact with the blood, secretions, organs or other bodily fluids of infected animals such as fruit bats, chimpanzees, gorillas, monkeys, forest antelope or porcupines found ill or dead or in the rainforest (WHO, 2020a).

Ebola then spreads through human-to-human transmission via direct contact (through broken skin or mucous membranes) with blood or body fluids of a person who is sick with or has died from Ebola, or objects that have been contaminated with body fluids (such as blood, faeces, vomit) from a person sick with Ebola or the body of a person who died from Ebola (WHO, 2020a).

Health-care workers have frequently been infected while treating patients with suspected or confirmed EVD. This occurs through close contact with patients when infection control precautions are not strictly practiced. Burial ceremonies that involve direct contact with the body of the deceased can also contribute in the transmission of Ebola. People remain infectious as long as their blood contains the virus. Pregnant women who get acute Ebola and recover from the disease may still carry the virus in breastmilk, or in pregnancy related fluids and tissues. This poses a risk of transmission to the baby they carry, and to others. Women who become pregnant after surviving Ebola disease are not at risk of carrying the virus (WHO, 2020a). 

The incubation period is from 2 to 21 days. A person infected with Ebola virus cannot spread the disease until they develop symptoms. The symptoms of EVD can be sudden and include flu-like symptoms followed by diarrhoea, vomiting, rash, internal and external bleeding (such as oozing from the gums or blood in stools), and symptoms of impaired kidney and liver function. The average EVD case fatality rate is around 50%. Case fatality rates have varied from 25% to 90% in past outbreaks (WHO, 2020a).

It can be difficult to clinically distinguish EVD from other infectious diseases such as malaria, typhoid fever and meningitis. Laboratory confirmation that symptoms are caused by Ebola virus infection are made using diagnostic serological and virological tests. Careful consideration should be given to the selection of diagnostic tests, which take into account technical specifications, disease incidence and prevalence, and social and medical implications of test results. It is strongly recommended that diagnostic tests that have undergone an independent and international evaluation be considered for use (WHO, 2020a).

The World Health Organization (WHO) has published case definitions for EVD (WHO, 2014).

Metrics and numeric limits

The 2014–2016 epidemic in West Africa was the largest Ebola outbreak since the virus was first discovered in 1976. A total of 28,616 Ebola cases were reported in Guinea, Liberia and Sierra Leone, with 11,310 deaths (WHO, 2016a).

Examples of drivers, outcomes and risk management

There is no proven treatment for EVD but simple interventions early on can significantly improve chances of survival. This includes rehydration with fluids and body salts (given orally or intravenously), and treatment of specific symptoms such as low blood pressure, vomiting, diarrhoea and infections. A range of potential treatments including blood products, immune therapies and drug therapies are currently being evaluated. Health-care workers have frequently been infected while treating patients with suspected or confirmed EVD. This occurs through close contact with patients when infection control precautions are not strictly practiced (WHO, 2020a). 

Community engagement is key to successfully controlling outbreaks. Working with communities to reduce risk factors for Ebola transmission is critical to controlling outbreaks. Burial ceremonies that involve direct contact with the body of the deceased can contribute in the transmission of Ebola (WHO, 2020a).

Good outbreak control relies on applying a package of interventions, including case management, surveillance and contact tracing, a good laboratory service, safe burials and social mobilisation. Community engagement is key to successfully controlling outbreaks. Raising awareness of risk factors for Ebola infection and protective measures (including vaccination) that individuals can take is an effective way to reduce human transmission. Risk reduction messaging should focus on several factors: reducing the risk of wildlife-to-human transmission; reducing the risk of human-to-human transmission; outbreak containment measures; reducing the risk of possible sexual transmission; reducing the risk of transmission from pregnancy related fluids and tissue; and controlling infection in health-care settings (WHO, 2020a).

In May 2019, the WHO published a roadmap aiming to coordinate partners’ actions and contributions to the licensing and roll-out of Merck’s Ebola vaccine (VSV-ZEBOV) in African countries. The vaccine was developed during the West Africa Ebola epidemic of 2014–2016, during which more than 11,000 people lost their lives to the disease. The vaccine was tested in European and African countries at the time and was used under an ‘expanded access’ protocol in the Democratic Republic of Congo (WHO, 2020b).

Based on regulatory evaluation by the European Medicines Agency and the United States Food and Drug Administration (FAO), the WHO expedited prequalification and coordinated work with countries at risk of Ebola outbreaks to streamline regulatory licensing of the vaccine for use in those countries (WHO, 2020b).

On 12 November 2019, the WHO prequalified an Ebola vaccine for the first time, a critical step that will help speed up its licensing, access and roll-out in countries most at risk of Ebola outbreaks. This is the fastest vaccine prequalification process ever conducted by the WHO. The injectable Ebola vaccine, Ervebo, is manufactured by Merck (known as MSD outside the USA and Canada). The vaccine has been shown to be effective in protecting people from the Ebola Zaire virus and is recommended by the WHO Strategic Advisory Group of Experts (SAGE) for vaccines as part of a broader set of Ebola response tools. The decision is a step towards greater availability of the vaccine in the future, although licensed doses will only be available mid-2020 (WHO, 2020b).

In February 2020, the WHO published a roadmap aiming to coordinate partners’ actions and contributions to the licensing and roll-out of Janssen’s Ebola vaccine (Ad26.ZEBOV, MVA-BN®-Filo) in African countries. This second vaccine was developed during the West Africa Ebola epidemic of 2014–2016, during which more than 11,000 people lost their lives to the disease. The vaccine was tested in European and African countries at the time. The WHO will expedite prequalification and licensing of the vaccine for use in countries at risk of Ebola outbreaks and will coordinate work between those countries’ regulatory authorities and the European Medicines Agency and FAO (WHO, 2020b).