Ebola: overview, history, origins and transmission
Updated 18 March 2021
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1. History of the disease
Ebola virus disease (EVD) is a severe disease caused by Ebola virus, a member of the filovirus family, which occurs in humans and other primates. The disease emerged in 1976 in almost simultaneous outbreaks in the Democratic Republic of the Congo (DRC) and Sudan (now South Sudan).
Between 1979 and 1994 no cases or outbreaks were detected, however since that time outbreaks have been recognised with increasing frequency (see table below).
The largest outbreak to date took place in West Africa between March 2014 and June 2016, affecting primarily Guinea, Liberia and Sierra Leone. Over 28,000 cases were recorded.
There are 6 species of Ebola virus, 4 of which have caused disease in humans:
- Zaïre ebolavirus (EBOV)
- Sudan ebolavirus (SUDV)
- Tai Forest (TAFV) (formerly known as Ebola Ivory Coast)
- Bundibugyo ebolavirus (BDBV)
Reston ebolavirus (RESTV), has caused severe illness in non-human primates but not in humans. RESTV was first detected in October 1989 in Reston, Virginia (USA) in a colony of monkeys imported from the Philippines, and has subsequently caused outbreaks in non-human primates in Pennsylvania (Philadelphia), Texas (Alice) and Italy (Sienna). Several research workers became infected with the virus during these outbreaks, but did not become ill.
Investigations traced the source of all outbreaks caused by RESTV to one export facility in the Philippines, but how the facility was contaminated was not determined. In December 2008, RESTV was found in sick pigs in the Philippines. A number of workers developed antibodies but none had had any symptoms.
A 6th species of ebolavirus was discovered in bats in Sierra Leone in 2018, and named Bombali ebolavirus. It is not yet known if this species is pathogenic for humans.
In Africa between 1976 and 2014, outbreaks of EVD primarily occurred in remote villages close to tropical rainforests in Central and West Africa. Most confirmed cases were reported from the Democratic Republic of the Congo (DRC, formerly Zaire), Sudan, Gabon, Uganda, and the Republic of Congo.
In 2014, Ebola outbreaks occurred for the first time in West Africa (Guinea, Liberia and Sierra Leone), and in these countries there was intense transmission in urban areas. Associated with this extensive outbreak, Ebola cases were imported into Italy, Nigeria, Mali, Senegal, Spain, the UK and the USA.
See information on current Ebola outbreaks.
1.1 Map showing the location of all known cases and outbreaks in Africa to December 2016
1.2 Confirmed cases and outbreaks of Ebola virus disease to 2020 (excluding the West Africa outbreak)
| Year | Country | Ebola virus species | Cases | Deaths | Case fatality rate |
|---|---|---|---|---|---|
| 1976 | Zaire (Democratic Republic of Congo) | Zaire | 318 | 280 | 88% |
| 1976 | Sudan (a) | Sudan | 284 | 151 | 53% |
| 1976 | England | Sudan | 1 | 0 | 0% |
| 1977 | Zaire (DRC) | Zaire | 1 | 1 | 100% |
| 1979 | Sudan (a) | Sudan | 34 | 22 | 65% |
| 1989 | USA | Reston | 0 (b) | 0 | 0% |
| 1990 | USA | Reston | 0 (b) | 0 | 0% |
| 1992 | Italy | Reston | 0 (b) | 0 | 0% |
| 1994 | Gabon | Zaire | 52 | 31 | 60% |
| 1994 | Côte d’Ivoire | Tai Forest | 1 | 0 | 0% |
| 1995 | Democratic Republic of Congo | Zaire | 315 | 254 | 79% |
| 1996 | Gabon | Zaire | 31 | 21 | 57% |
| 1996 | Gabon | Zaire | 60 | 45 | 75% |
| 1996 | South Africa | Zaire | 1 | 1 | 100% |
| 2000-1 | Uganda | Sudan | 425 | 224 | 53% |
| 2001-2 | Gabon and Republic of Congo | Zaire | 124 | 97 | 78% |
| 2002-3 | Republic of Congo | Zaire | 143 | 128 | 89% |
| 2003 | Republic of Congo | Zaire | 35 | 29 | 83% |
| 2004 | Sudan (a) | Sudan | 17 | 7 | 41% |
| 2005 | Republic of Congo | Zaire | 12 | 10 | 75% |
| 2007 | Democratic Republic of Congo | Zaire | 264 | 187 | 71% |
| 2007 | Uganda | Bundibugyo | 149 | 37 | 25% |
| 2008 | Philippines | Reston | 0 (c) | 0 | 0% |
| 2008-9 | Democratic Republic of Congo | Zaire | 32 | 14 | 47% |
| 2011 | Uganda | Sudan | 1 | 1 | 100% |
| 2012i | Uganda | Sudan | 24 | 17 | 70% |
| 2012ii | Uganda | Sudan | 7 | 4 | 40% |
| 2012 | Democratic Republic of Congo | Bundibugyo | 57 | 29 | 55% |
| 2014 (d) | Democratic Republic of Congo | Zaire | 66 | 49 | 74% |
| 2017 | Democratic Republic of Congo | Zaire | 8 | 4 | 50% |
| 2018 | Democratic Republic of Congo | Zaire | 54 | 33 | 61% |
| 2018-20 | Democratic Republic of Congo | Zaire | 3470 | 2287 | 66% |
| Total (e) | 5986 | 3963 | 66% |
(a) now South Sudan
(b) Ebola-Reston was introduced into quarantine facilities by monkeys imported from the Philippines. A total of 8 people developed antibodies but did not become ill
(c) Ebola-Reston was detected in pigs on 3 farms. Antibodies were detected in 6 workers on the farms, but none had clinical illness
(d) not linked to the 2014 to 2016 West Africa outbreak
(e) all data from WHO
1.3 Cases that occurred during the West Africa outbreak 2013 to 2016
| Country | Ebola virus species | Cases | Deaths | Case fatality rate |
|---|---|---|---|---|
| Guinea, Liberia and Sierra Leone | Zaire | 28,616 | 11,310* | 40% |
| Mali | Zaire | 8 | 6 | 75% |
| Nigeria | Zaire | 20 | 8 | 40% |
| Senegal | Zaire | 1 | 0 | |
| Italy | Zaire | 1 | 0 | |
| Spain | Zaire | 1 | 0 | |
| UK | Zaire | 1 | 0 | |
| USA | Zaire | 4 | 1 | 25% |
| Total | 28,652 | 11,325* | 39.50% |
*Note that the number of deaths reported during this outbreak were likely to have been underestimated.
2. Natural reservoir
Ebola is believed to be zoonotic, however, the natural reservoir is unknown, despite extensive investigations. Non-human primates have been a source of human infection, however, they are not thought to be the reservoir as they develop severe, fatal illness when infected.
High numbers of animal carcasses were noted in surrounding areas prior to outbreaks in Gabon and DRC, and recovered carcasses were infected with a variety of strains of Ebola virus suggesting they were not the reservoir but had been infected by more than one source.
Harvesting of migrating fruit bats was thought to be the source of a large outbreak in the DRC in 2007.
3. Transmission
The first human case in an Ebola outbreak is acquired through contact with blood, secretions organs or other bodily fluids of an infected animal. EVD has been documented in people who handled infected chimpanzees, gorillas, and forest antelopes, both dead and alive, in Cote d’Ivoire, the Republic of Congo and Gabon. The first case in the West Africa outbreak was likely acquired via exposure to bats.
The virus is then transmitted from person to person through direct contact with the blood, secretions, organs or other bodily fluids of infected persons. People can also become infected through contact with objects, such as needles or soiled clothing, that have been contaminated with infected secretions.
Outbreaks have been fuelled by traditional burial practices, in which mourners have direct contact with the bodies of the deceased. Acquisition via sexual contact with a convalescent case or survivor has been documented - the virus can be present in semen for many months after recovery.
Hospital workers have frequently been infected during Ebola outbreaks through close contact with infected patients, and insufficient use of correct infection control precautions and barrier nursing procedures. Needlestick exposures in research laboratory workers who subsequently became infected were documented in England (in 1976), and Russia (in 2004).
Ebola is not spread through routine, social contact (such as shaking hands or sitting next to someone) with asymptomatic individuals. There is no evidence of transmission of Ebola virus through intact skin or through small droplet spread, such as coughing or sneezing.
4. Symptoms
The incubation period of Ebola virus disease ranges from 2 to 21 days.
The onset of illness is sudden, with fever, headache, joint and muscle pain, sore throat and intense weakness. Diarrhoea and vomiting may be significant.
Some patients may develop a rash, red eyes, hiccups, impaired kidney and liver function, and internal and external bleeding.
Ebola virus disease is fatal in between 40 to 90% of all clinically ill cases, depending on the virus species, patients’ age and many other factors.
5. Diagnosis
Clinical diagnosis of EVD in the early stages of infection is difficult, as early symptoms are non-specific and similar to those of other infections. However, once later symptoms develop, a clinical diagnosis of Ebola may be made.
Laboratory diagnosis must be carried out under maximum biological containment conditions. Tests are available to detect viral RNA or antibodies to the virus, and it can also be isolated in tissue culture.
In the UK, laboratory diagnosis is performed by the Rare and Imported Pathogens Laboratory.
6. Treatment
There are clinical trials in progress for experimental treatments and vaccines, but none are currently licensed for use.
Patients require intensive supportive therapy including intravenous fluids or oral rehydration with solutions including electrolytes, maintaining their oxygen status and blood pressure.
7. Prevention
To avoid person to person transmission of Ebola virus, great care needs to be taken when nursing patients, to avoid contact with infected bodily fluids.
Patients should be isolated, and strict barrier nursing techniques should be used, including wearing masks, gloves and gowns. Invasive procedures such as the placing of intravenous lines, handling of blood, secretions, catheters and suction devices are a particular risk and strict infection control is essential.
See the detailed guidance for the management of Ebola patients, including infection prevention and control.
Non-disposable protective equipment must be properly disinfected before re-use. Other infection control measures include proper use, disinfection, and disposal of instruments and equipment used in caring for patients.
The bodies of those that have died of Ebola virus infection remain highly infectious and should be promptly and safely buried or cremated.
8. UK guidelines
The UK has specialist guidance on the management (including infection control) of patients with Ebola and other viral haemorrhagic fevers.
This guidance provides advice on how to comprehensively assess, rapidly diagnose and safely manage patients suspected of being infected, within the NHS, to ensure the protection of public health.