Ebola Virus Disease (EVD) is a severe, frequently fatal illness that affects humans and other primates. The disease is caused by one of several species of the Ebolavirus genus and has been historically confined to outbreaks in Central and West Africa. Its appearance in human populations is characterized by a high case fatality rate. Understanding the origins and transmission pathways of this virus is paramount to controlling outbreaks and clarifying its initial jump from wildlife into the human population.
Defining Zoonotic Disease and Ebola’s Classification
A zoonotic disease is an infectious disease that can be transmitted from animals to humans. This transmission from a non-human vertebrate host forms the basis of the disease’s emergence in people. Ebola is explicitly classified as a zoonotic pathogen, meaning its outbreaks are rooted in an animal-to-human transfer.
The virus responsible for EVD belongs to the family Filoviridae, a group of viruses named for their characteristic long, filamentous shape. Within this family is the genus Ebolavirus, which includes multiple species like the highly virulent Zaire ebolavirus, responsible for the largest and deadliest outbreaks. A disease is considered zoonotic even if it can spread extensively between humans afterward, as long as its initial source is non-human.
Animal Reservoirs and Initial Spillover Events
The natural reservoir for the Ebola virus is believed to be certain species of fruit bats, particularly those belonging to the Pteropodidae family. These bats are thought to carry the virus without developing symptoms, allowing them to harbor the pathogen. While the virus has not been definitively isolated from a bat in the wild, strong evidence, including the detection of viral RNA and antibodies, points to them as the primary host.
The virus jumping from its animal host to the first human is known as a spillover event. Spillover often occurs through human interaction with other infected mammals, which serve as intermediate hosts. Non-human primates, forest antelopes like duikers, and gorillas are known to become infected and may transmit the virus to people.
The initial transmission to humans is frequently linked to the handling or consumption of infected wildlife, often referred to as bushmeat. People can become infected by slaughtering or preparing sick or dead animals found in the forest. Exposure to infected animal fluids, such as feces or blood, provides a pathway for the virus to cross into the human population, initiating an outbreak.
How Ebola Spreads Among Humans
After a spillover event infects the first person, the disease is maintained and spread through person-to-person transmission. The virus is transmitted through direct physical contact with the bodily fluids of someone who is sick or has died from EVD. A person is only considered contagious once they begin showing symptoms of the disease.
The most infectious body fluids include:
- Blood
- Vomit
- Feces
- Urine
- Breast milk
- Saliva
- Semen
Direct contact means the infected fluids must enter the body through broken skin or mucous membranes in the eyes, nose, or mouth. Indirect transmission is also possible through contact with surfaces or objects, such as contaminated bedding or medical equipment.
Specific cultural practices, such as traditional funeral rites that involve close contact with the deceased, have been major factors in accelerating human-to-human spread. Healthcare settings that lack proper infection control measures can become centers of transmission when staff are exposed to the fluids of symptomatic patients. Public health responses focus on isolating sick individuals, conducting rigorous contact tracing, and ensuring safe burial practices to contain the outbreak.