Ebola virus disease (EVD), commonly known as Ebola, is a severe, often fatal illness in humans caused by the Ebola virus. The disease, characterized by sudden onset of fever, intense weakness, muscle pain, headache, and sore throat, is followed by vomiting, diarrhea, rash, and, in many cases, internal and external bleeding. The mortality rate for Ebola can be exceedingly high, with some outbreaks reporting case fatality rates of up to 90%.
The Ebola virus is a member of the Filoviridae family, which also includes the Marburg virus. Ebola viruses are filamentous virions that appear as long, thread-like structures. They are zoonotic pathogens, meaning they are transmitted to humans from animals, but they can also spread between humans through direct contact with infected bodily fluids.
History and Discovery of Ebola
The Ebola virus was first identified in 1976 during two simultaneous outbreaks in Nzara, Sudan, and Yambuku, Democratic Republic of the Congo (then Zaire). The latter occurred near the Ebola River, from which the disease takes its name. These initial outbreaks had high mortality rates and caused significant fear and disruption.
Since its discovery, the Ebola virus has been the cause of numerous outbreaks, primarily in Central and West Africa. The most significant outbreak occurred between 2014 and 2016 in West Africa, affecting Guinea, Liberia, and Sierra Leone, and resulting in more than 28,000 cases and 11,000 deaths.
The Ebola Virus and Its Strains
Ebola virus is classified into five distinct species, four of which are known to cause disease in humans. Each species is associated with different levels of virulence and epidemiological characteristics:
- Zaire ebolavirus (EBOV)
- Sudan ebolavirus (SUDV)
- Taï Forest ebolavirus (TAFV)
- Bundibugyo ebolavirus (BDBV)
- Reston ebolavirus (RESTV)
1. Zaire ebolavirus (EBOV)
Zaire ebolavirus (EBOV) is the most virulent and deadly of the Ebola virus species, with case fatality rates ranging from 60% to 90% in past outbreaks. It was the species responsible for the first recognized Ebola outbreak in 1976 in the Democratic Republic of the Congo (DRC).
- Geographical Distribution: EBOV outbreaks have primarily occurred in the DRC, Republic of Congo, and Gabon.
- Epidemiological Features: EBOV is highly transmissible and causes severe disease in humans. It has been responsible for some of the largest and deadliest Ebola outbreaks, including the West African outbreak of 2014-2016, which was unprecedented in its scale and impact.
- Pathogenicity: EBOV targets various cell types, including endothelial cells, hepatocytes, and immune cells. It causes a severe inflammatory response, leading to widespread tissue damage, vascular leakage, and multi-organ failure.
2. Sudan ebolavirus (SUDV)
Sudan ebolavirus (SUDV) was also discovered in 1976 during an outbreak in Sudan. SUDV has caused several outbreaks with varying case fatality rates, typically between 40% and 60%.
- Geographical Distribution: SUDV outbreaks have occurred in Sudan and Uganda.
- Epidemiological Features: SUDV is less virulent than EBOV but still causes significant morbidity and mortality. It has been responsible for multiple outbreaks, including notable ones in Gulu, Uganda, in 2000 and 2012.
- Pathogenicity: Similar to EBOV, SUDV infects a variety of cell types and causes a severe inflammatory response. The pathogenesis involves extensive tissue damage and immune system dysregulation.
3. Taï Forest ebolavirus (TAFV)
Taï Forest ebolavirus (TAFV), previously known as Côte d’Ivoire ebolavirus, was first identified in 1994 when a researcher became infected while performing an autopsy on a chimpanzee in the Taï Forest, Côte d’Ivoire.
- Geographical Distribution: TAFV has only been identified in Côte d’Ivoire.
- Epidemiological Features: TAFV is the least studied of the Ebola virus species, with only one confirmed human case to date. The infected researcher survived, suggesting a potentially lower virulence compared to other species.
- Pathogenicity: Due to the limited number of cases, less is known about TAFV’s pathogenicity. However, it is presumed to have similar mechanisms of infection and disease progression as other Ebola virus species.
4. Bundibugyo ebolavirus (BDBV)
Bundibugyo ebolavirus (BDBV) was discovered in 2007 during an outbreak in the Bundibugyo District of Uganda. BDBV has caused outbreaks with case fatality rates around 25% to 40%.
- Geographical Distribution: BDBV outbreaks have occurred in Uganda and the Democratic Republic of the Congo.
- Epidemiological Features: BDBV is less virulent than EBOV and SUDV but still causes significant disease. The discovery of BDBV highlighted the genetic diversity of Ebola viruses and the potential for new species to emerge.
- Pathogenicity: BDBV causes similar symptoms and disease progression as other Ebola viruses. The pathogenesis involves immune system evasion and triggering of an inflammatory response.
5. Reston ebolavirus (RESTV)
Reston ebolavirus (RESTV) is unique among Ebola viruses as it does not cause disease in humans. RESTV was first identified in 1989 during an outbreak in Reston, Virginia, USA, among monkeys imported from the Philippines.
- Geographical Distribution: RESTV has been identified in the Philippines, China, and the United States.
- Epidemiological Features: RESTV primarily infects non-human primates and pigs. While it can infect humans, it does not cause symptomatic disease. The discovery of RESTV raised concerns about the potential for zoonotic transmission.
- Pathogenicity: In non-human primates and pigs, RESTV causes severe disease. The lack of pathogenicity in humans is not fully understood, but it may be related to differences in host immune responses or virus-host interactions.
Transmission of Ebola
Ebola is transmitted to humans through close contact with the blood, secretions, organs, or other bodily fluids of infected animals, including fruit bats, chimpanzees, gorillas, monkeys, forest antelope, and porcupines found ill or dead or in the rainforest. Once introduced into the human population, the virus spreads through human-to-human transmission via direct contact (through broken skin or mucous membranes) with the blood, secretions, organs, or other bodily fluids of infected people, and with surfaces and materials (e.g., bedding, clothing) contaminated with these fluids.
Symptoms and Progression of Ebola
The symptoms of Ebola begin with a sudden onset of fever, fatigue, muscle pain, headache, and sore throat. This is followed by vomiting, diarrhea, rash, symptoms of impaired kidney and liver function, and in some cases, both internal and external bleeding (e.g., oozing from the gums, blood in the stools). Laboratory findings include low white blood cell and platelet counts and elevated liver enzymes.
The incubation period, or the time interval from infection with the virus to onset of symptoms, is 2 to 21 days. People remain infectious as long as their blood contains the virus.
Diagnosis of Ebola
Diagnosing Ebola can be challenging, especially in the early stages, as its symptoms are similar to those of other diseases, such as malaria, typhoid fever, and meningitis. Laboratory tests used to diagnose Ebola include:
- Antigen-capture enzyme-linked immunosorbent assay (ELISA) testing.
- IgM ELISA.
- Polymerase chain reaction (PCR).
- Virus isolation.
Later in disease or after recovery, tests can be conducted to detect antibodies to Ebola, including IgM and IgG.
Treatment of Ebola
Supportive care – rehydration with oral or intravenous fluids – and treatment of specific symptoms improve survival. There is no proven treatment for Ebola, but a range of potential treatments, including blood products, immune therapies, and drug therapies, are currently being evaluated. Recently, the U.S. Food and Drug Administration (FDA) approved two monoclonal antibody treatments, Inmazeb (atoltivimab, maftivimab, and odesivimab-ebgn) and Ebanga (ansuvimab-zykl), for the treatment of Zaire ebolavirus infection.
Prevention of Ebola
Good outbreak control relies on applying a package of interventions, namely case management, surveillance and contact tracing, a good laboratory service, safe burials, and social mobilization. Community engagement is key to successfully controlling outbreaks. Raising awareness of risk factors for Ebola infection and protective measures 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: Animals should be handled with gloves and other appropriate protective clothing. Animal products (blood and meat) should be thoroughly cooked before consumption.
- Reducing the risk of human-to-human transmission: This involves wearing gloves and appropriate personal protective equipment when taking care of ill patients at home. Regular handwashing is required after visiting patients in the hospital, as well as after taking care of patients at home.
- Outbreak containment measures: These include prompt and safe burial of the dead, identifying people who may have been in contact with someone infected with Ebola, monitoring the health of contacts for 21 days, separating the healthy from the sick to prevent further spread, and maintaining good hygiene and a clean environment.
Vaccination for Ebola
The rVSV-ZEBOV (Ervebo) vaccine has been shown to be effective in protecting people from the Zaire ebolavirus. It was used in the 2018-2020 Ebola outbreak in the Democratic Republic of the Congo and has been approved by the FDA and the European Medicines Agency (EMA).
Conclusion
Ebola virus disease is a severe and often fatal illness caused by the Ebola virus, a member of the Filoviridae family. The virus is classified into five distinct species: Zaire ebolavirus, Sudan ebolavirus, Taï Forest ebolavirus, Bundibugyo ebolavirus, and Reston ebolavirus. Each species varies in its geographical distribution, virulence, and impact on human health. Ebola is transmitted through direct contact with the bodily fluids of infected individuals or animals, and it causes a range of symptoms that can lead to severe complications and death.
Diagnosis of Ebola involves laboratory tests such as ELISA and PCR, while treatment primarily consists of supportive care and experimental therapies. Preventive measures, including public health interventions, vaccination, and community engagement, are critical for controlling outbreaks and reducing the spread of the virus. The lessons learned from past outbreaks have informed current strategies for managing Ebola, highlighting the importance of early detection, effective treatment, and robust public health measures. By continuing to advance research and improve preparedness, we can enhance our ability to respond to Ebola and protect global health.