History of Ebola Virus Disease
Emergence of Ebola in Humans
Ebola virus disease (EVD), one of the deadliest viral diseases, was discovered in 1976 when two consecutive outbreaks of fatal hemorrhagic fever occurred in different parts of Central Africa. The first outbreak occurred in the Democratic Republic of Congo (formerly Zaire) in a village near the Ebola River, which gave the virus its name. The second outbreak occurred in what is now South Sudan, approximately 500 miles (850 km) away.
Initially, public health officials assumed these outbreaks were a single event associated with an infected person who traveled between the two locations. However, scientists later discovered that the two outbreaks were caused by two genetically distinct viruses: Zaire ebolavirus and Sudan ebolavirus. After this discovery, scientists concluded that the virus came from two different sources and spread independently to people in each of the affected areas.
Viral and epidemiologic data suggest that Ebola virus existed long before these recorded outbreaks occurred. Factors like population growth, encroachment into forested areas, and direct interaction with wildlife (such as bushmeat consumption) may have contributed to the spread of the Ebola virus.
Since its discovery in 1976, the majority of cases and outbreaks of Ebola Virus Disease have occurred in Africa. The 2014-2016 Ebola outbreak in West Africa began in a rural setting of southeastern Guinea, spread to urban areas and across borders within weeks, and became a global epidemic within months.
Identifying a Host
Following the discovery of the virus, scientists studied thousands of animals, insects, and plants in search of its source (called reservoir among virologists, people who study viruses). Gorillas, chimpanzees, and other mammals may be implicated when the first cases of an EVD outbreak in people occur. However, they – like people – are “dead-end” hosts, meaning the organism dies following the infection and does not survive and spread the virus to other animals. Like other viruses of its kind, it is possible that the reservoir host animal of Ebola virus does not experience acute illness despite the virus being present in its organs, tissues, and blood. Thus, the virus is likely maintained in the environment by spreading from host to host or through intermediate hosts or vectors.
African fruit bats are likely involved in the spread of Ebola virus and may even be the source animal (reservoir host). Scientists continue to search for conclusive evidence of the bat’s role in transmission of Ebola. 1 The most recent Ebola virus to be detected, Bombali virus, was identified in samples from bats collected in Sierra Leone.2
Understanding Pathways of Transmission
The use of contaminated needles and syringes during the earliest outbreaks enabled transmission and amplification of Ebola virus. During the first outbreak in Zaire (now Democratic Republic of Congo – DRC), nurses in the Yambuku mission hospital reportedly used five syringes for 300 to 600 patients a day. Close contact with infected blood, reuse of contaminated needles, and improper nursing techniques were the source for much of the human-to-human transmission during early Ebola outbreaks.3
In 1989, Reston ebolavirus was discovered in research monkeys imported from the Philippines into the U.S. Later, scientists confirmed that the virus spread throughout the monkey population through droplets in the air (aerosolized transmission) in the facility. However, such airborne transmission is not proven to be a significant factor in human outbreaks of Ebola.4 The discovery of the Reston virus in these monkeys from the Philippines revealed that Ebola was no longer confined to African settings, but was present in Asia as well.
By the 1994 Cote d’Ivoire outbreak, scientists and public health officials had a better understanding of how Ebola virus spreads and progress was made to reduce transmission through the use of face masks, gloves and gowns for healthcare personnel. In addition, the use of disposable equipment, such as needles, was introduced.
During the 1995 Kikwit, Zaire (now DRC) outbreak, the international public health community played a strong role, as it was now widely agreed that containment and control of Ebola virus were paramount in ending outbreaks. The local community was educated on how the disease spreads; the hospital was properly staffed and stocked with necessary equipment; and healthcare personnel was trained on disease reporting, patient case identification, and methods for reducing transmission in the healthcare setting.5
In the 2014-2015 Ebola outbreak in West Africa, healthcare workers represented only 3.9% of all confirmed and probable cases of EVD in Sierra Leone, Liberia, and Guinea combined.6 In comparison, healthcare workers accounted for 25% of all infections during the 1995 outbreak in Kikwit.7 During the 2014-2015 West Africa outbreak, the majority of transmission events were between family members (74%). Direct contact with the bodies of those who died from EVD proved to be one of the most dangerous – and effective – methods of transmission. Changes in behaviors related to mourning and burial, along with the adoption of safe burial practices, were critical in controlling that epidemic.8
1 Baseler L., Chertow D, et. Al. The Pathogenesis of Ebola Virus Disease. Annu. Rev. Pathol. Mech. Dis. 2017. 12:387–418.
2 Goldstein T. et al. The discovery of Bombali virus adds further support for bats as hosts of ebolavirusesexternal icon. Nature Microbiology. 2018 Aug 27. [Epub ahead of print]
3 Amundsen, S. Historical Analysis of the Ebola Virus: Prospective Implications for Primary Care Nursing Today. Clinical Excellence for Nurse Practitioners. Vol 2. No 6. 1998. 343-351.
4 Baseler L., Chertow D, et. Al. The Pathogenesis of Ebola Virus Disease. Annu. Rev. Pathol. Mech. Dis. 2017. 12:387–418.
5 Amundsen, S. Historical Analysis of the Ebola Virus: Prospective Implications for Primary Care Nursing Today. Clinical Excellence for Nurse Practitioners. Vol 2. No 6. 1998. 343-351.
6 WHO. Health worker Ebola infections in Guinea, Liberia and Sierra Leone: A Preliminary Report 21 May 2015. Accessed June 20, 2017. http://www.who.int/hrh/documents/21may2015_web_final.pdf pdf icon[917 – KB]external icon
7 Khan A. et al. The Reemergence of Ebola Hemorrhagic Fever, Democratic Republic of the Congo, 1995. J Infect Dis (1999) 179 (Suppl 1): S76-86.
8 Baseler L., Chertow D, et. Al. The Pathogenesis of Ebola Virus Disease. Annu. Rev. Pathol. Mech. Dis. 2017. 12:387–418.