Ebola 101: Complete Guide to Biology, Transmission & Strains

What is Ebola?

Ebola virus disease (EVD) is a rare and deadly infectious disease caused by the Ebola virus, a member of the family Filoviridae. According to the CDC, Ebola virus was first discovered in 1976 near the Ebola River in what is now the Democratic Republic of the Congo.

Since its discovery, multiple outbreaks have occurred across sub-Saharan Africa. Five species of Ebola virus have been identified, with Zaire ebolavirus being the most lethal and the cause of the majority of outbreaks.

Biology and Virology

Viral Structure

Ebola viruses are enveloped, single-stranded, negative-sense RNA viruses with a characteristic filamentous shape:

• Nucleoprotein (NP): Encapsidates the viral RNA genome
• Viral proteins (VP35, VP40): VP35 is the polymerase cofactor; VP40 drives viral budding
• Glycoprotein (GP): Mediates entry into host cells via macropinocytosis
• RNA-dependent RNA polymerase (L): Replicates and transcribes the viral genome

Replication Cycle

Ebola virus enters host cells primarily through macropinocytosis and receptor-mediated endocytosis. The glycoprotein (GP) binds to host cell surface receptors including NPC1 (Niemann-Pick C1) within endosomes. Viral replication occurs in the cytoplasm, and new virions bud from the cell surface.

Pathophysiology

Ebola virus infection causes a massive immune dysregulation, including a "cytokine storm" that damages the endothelial lining of blood vessels. This leads to increased vascular permeability, internal and external bleeding, and multi-organ failure. The virus also infects and impairs dendritic cells and macrophages, crippling the adaptive immune response.

How Ebola Spreads

Primary Transmission Route: Human Contact

Ebola virus is transmitted through direct contact with the blood, secretions, organs, or other bodily fluids of infected individuals. Key transmission routes include:

• Direct contact with blood or bodily fluids of symptomatic patients (most common)
• Contact with contaminated surfaces, bedding, or medical equipment (fomites)
• Sexual transmission from recovering patients (virus can persist in semen for months)
• Traditional burial practices involving direct contact with the deceased

Animal Reservoir: Fruit bats (Pteropodidae family) are considered the natural reservoir of Ebola virus. Humans can become infected through contact with infected bats or other wildlife (primates, duikers). Once a human is infected, secondary transmission can occur through human-to-human contact.

High-Risk Activities

• Caring for Ebola patients without proper personal protective equipment (PPE)
• Participating in traditional burial practices involving the body of an Ebola victim
• Contact with fruit bats or non-human primates in endemic areas
• Healthcare work in outbreak settings without adequate infection control

Major Ebola Virus Species

1. Zaire ebolavirus (EBOV)

Geographic Distribution: Democratic Republic of the Congo, Republic of the Congo, Gabon, and other Central/West African countries

Reservoir: Fruit bats (Pteropodidae family)

Clinical Disease: Ebola Hemorrhagic Fever (EHF)

Mortality Rate: Up to 90% in outbreaks without supportive care; ~50% with treatment

Significance: Responsible for the majority of outbreaks, including the 2014-2016 West Africa epidemic (28,000+ cases) and the largest outbreak to date.

2. Sudan ebolavirus (SUDV)

Geographic Distribution: Uganda, Sudan, South Sudan, Democratic Republic of the Congo

Reservoir: Fruit bats (suspected)

Clinical Disease: Ebola Hemorrhagic Fever (EHF)

Mortality Rate: 40-60% in confirmed cases

Historical Note: First identified in southern Sudan in 1976. No approved vaccine specifically for SUDV, though cross-reactive vaccines are being evaluated.

3. Bundibugyo ebolavirus (BDBV)

Geographic Distribution: Uganda (Bundibugyo District)

Reservoir: Fruit bats (suspected)

Clinical Disease: Ebola Hemorrhagic Fever (EHF)

Mortality Rate: 25-40% in confirmed cases

Clinical Features: Identified in 2007-2008 outbreak in Uganda; generally lower mortality than Zaire species.

4. Taï Forest ebolavirus (TAFV)

Geographic Distribution:Côte d'Ivoire

Reservoir: Fruit bats (suspected)

Clinical Disease: Limited human cases (one known)

Mortality Rate: Low (single human case recovered)

Clinical Features: First identified in 1994 after a dead chimpanzee was found in Taï Forest; only one known human case.

5. Reston ebolavirus (RESTV)

Geographic Distribution: Philippines, United States (imported)

Reservoir: Fruit bats; also causes outbreaks in domestic pigs

Clinical Disease: Non-pathogenic in humans

Mortality Rate: 0% in humans (fatal in non-human primates)

Clinical Features: Discovered in laboratory monkeys imported from the Philippines to Reston, Virginia in 1989. Causes no illness in humans but is lethal to primates.

Comparison Table

Clinical Manifestations

Ebola Hemorrhagic Fever (EHF)

Incubation Period: 2-21 days (typically 8-10 days)

Early Phase (Days 1-3)

• Sudden onset of fever, fatigue, and malaise
• Muscle pain (myalgia) and headache
• Sore throat and abdominal pain

Progressive Phase (Days 4-7)

• Persistent high fever
• Vomiting and diarrhea (often severe)
• Rash (maculopapular)
• Impaired kidney and liver function

Severe/Hemorrhagic Phase (Days 7-14+)

• Internal and external bleeding (gums, nose, bloody stools)
• Multi-organ failure
• Shock and death (if untreated)
• Recovery: Survivors develop antibodies 2-4 weeks after symptom onset

Mortality: 25-90% depending on strain, access to supportive care, and quality of care (average ~50%)

Long-Term Sequelae in Survivors

Incubation Period: Recovery can take weeks to months

Post-Ebola Syndrome (PES)

1. Joint pain: Chronic arthralgia affecting multiple joints
2. Ocular problems: Uveitis, vision changes, potentially permanent eye damage
3. Neurological issues: Headaches, fatigue, memory problems
4. Persistent viral shedding: Virus can persist in semen (up to 12 months), breast milk, and other fluids
5. Pregnancy complications: High risk of fetal loss and maternal death

Long-term outcome: Many survivors experience debilitating sequelae that significantly impact quality of life for months to years after recovery.

Prevention & Safety Strategies

Individual Protection Measures

• Avoid Contact with Infected Individuals: Do not touch patients or their bodily fluids
• Personal Protective Equipment: Wear full PPE (gowns, gloves, face shields, masks) when caring for Ebola patients
• Proper Disinfection: Use approved disinfectants; Ebola virus can be killed by bleach, UV light, and heat (60°C for 60 minutes)
• Safe Burial Practices: Avoid traditional burial practices that involve contact with the deceased
• Avoid Bushmeat: Do not handle or consume non-human primates or fruit bats in endemic areas

Public Health Measures

• Isolation of suspected and confirmed Ebola patients in dedicated treatment centers
• Contact tracing and monitoring of all close contacts for 21 days
• Vaccination of contacts and contacts-of-contacts (ring vaccination strategy)
• Safe and dignified burial practices
• Community engagement and risk communication

Vaccine & Treatment Status

Vaccine: rVSV-ZEBOV (Ervebo) — an FDA-approved live-attenuated vaccine for Zaire ebolavirus. Administered as a single dose for pre-exposure prophylaxis in outbreak settings. Also approved: Ad26.ZEBOV/MVA-BN-Filo (Zabdeno/Mvabea) two-dose regimen.

Treatment: Inmazeb (atoltivimab/maftivimab/odesivimab) — the first FDA-approved treatment for Zaire ebolavirus infection (2020). Additionally, Ebanga (ansuvimab) — a monoclonal antibody approved for treatment. Supportive care (IV fluids, electrolyte management) remains critical for survival.