How is the Ebola virus transmitted?
It is agreed that the most likely carrier source of Ebola virus (also referred to as a natural reservoir host) is that of infected bats who can transmit the disease to both human populations and those of non-human primates. The mechanism of transmission is believed to be through direct contact with infected bodily tissues (including organs), substances (like faeces or vomit) and fluids (like that of blood, saliva, mucous, urine, semen, sweat, tears and even breast milk) or mucous membranes of an infected animal.
Direct contact merely means that people or animals (such as monkey) have inadvertently touched or come into contact with infected substances which then enter their body – often through the mucous membranes of the eyes, nose, mouth or via broken skin (a wound, cut or abrasion).
Transmission exposure may be characterised as:
- Primary: This usually refers to a known presence of the Ebola virus in a confirmed endemic / outbreak area. A person may be classified in the primary category if they frequently travel to or reside in an endemic / outbreak area, like the DRC, Gabon, Sudan or Côte d’Ivoire (Ivory Coast), or have a history of exposure in tropical African forests.
- Secondary: This typically refers to identified transmission modes involving human-to-human (most often healthcare personnel, family or caregivers falling ill after being exposed to an infected individual while alive or deceased) or primate-to-human contact (usually animal care workers exposed to infected primates). Individuals who prepare bushmeat (sourced from wild animals) for human consumption and fall ill may also be classed as secondary transmission sufferers.
Transmission modes of the Ebola virus include:
1. Animal / wildlife-to-human transmission:
- Direct contact between humans and animals (dead or alive), which can include bats, primates, porcupines and even forest antelope, results in cross-species transmission. This is also known as a spill-over event. Contact is made through the handling of an infected animal which may involve preparations for cooking and consuming it.
2. Human-to-human transmission:
- Direct contact between humans can easily begin spreading the Ebola virus within community populations. An infected person can transmit the infectious disease via direct contact with bodily secretions and fluids, mucous membranes (like the eyes, nose and mouth) or even broken skin. Caregivers and medical personnel in close contact with infected individuals (without adequate barrier protection) are at high risk of infection themselves.
- Transmission is also possible through contact that is made between objects and surfaces and the bodily fluids and secretions of an infected person. Surfaces and objects, like clothing or bed linens and even countertops and doorknobs can become contaminated by an infected person, and subsequent contact can result in disease transmission. The Ebola virus is capable of surviving on surfaces (outside of the body) for several hours at a time. Contact with contaminated surfaces and objects within this timeframe can still transmit the virus, causing infection. Non-disposable needles and syringes, and other medical equipment have also been known to contribute to disease transmission if not properly sterilised before re-use or if accidental contact occurs.
- Transmission is possible during the handling of the body of a person who has died from an Ebola infection. The viral level remains high in a person’s bloodstream and various bodily fluids even after death, and any contact that is made with these can still transmit the disease. Burial ceremonies in many West African regions involve direct contact with the body of a loved one, placing mourners at a greater risk of infection.
Recovery from and the transmission risk of Ebola virus disease (EVD)
Precautions are advised even after recovery from the Ebola virus. This is because it is not yet well understood just how much of the virus can remain in certain areas of the body that are not easily reached by the immune system. It may thus take some time for the virus to be completely cleared from the body. How long this takes is still questionable and part of ongoing research.
A person may show signs of great improvement within a few weeks of falling ill but may still carry some of the virus in portions of their body, such as in the spinal column fluid, testes (in the semen) or even the eyes (in the ocular fluid). The virus may also linger in breast milk for a period of time, making it possible to perhaps pass it on to a nursing baby (although this has not been conclusively proven, and breastfeeding is not entirely discouraged among nursing mothers in the months following recovery from Ebola).
Viral levels may remain high in semen, making sexual transmission (via oral, vaginal or anal sexual activity) possible, for several months after recovering from an infection. It is believed that the amount of virus present in men’s semen may decrease over time (eventually becoming non-existent), but quantities appear to vary from one man to another. Precise levels and time periods of risk are yet to be conclusively determined through study. It has also not yet been determined if a recovering female can transmit the disease to a healthy sexual partner through vaginal fluids.
A persistent viral presence in the bodily fluids is still an area of research (3) and researchers have yet to determine just how long it takes the body to be completely clear of an infection with the Ebola virus, and whether it is still transmissible during the recovery stages.
Following previous outbreaks, it has been noted that survivors of the Ebola virus have tested positive for antibodies in their system once they have recovered. These antibodies may be detectable for several years, somewhat reducing the chances of contracting EVD again. These individual are, however, not entirely immune to reinfection and may still be able to fall ill via exposure to any one of the other Ebola species.
As the scale of lives lost during Ebola outbreaks is reduced with improved response procedures, researchers and medical professionals are able to gain a better understanding of the longer-term effects and make more accurate assessments through ongoing studies. The more lives that can be spared during an outbreak, the more medical researchers are able to learn about the disease and its effects.
The Ebola virus is not believed to be transmissible through the following mechanisms:
- Elements, such as air or water (i.e. air or water droplet inhalation or the consumption of contaminated). Since the virus is not thought to be an airborne contagion, the Ebola virus may not be contracted from the coughing or sneezing activity of an infected individual. Direct contact with respiratory droplets or secretions are however, not considered to be entirely non-transmissible mechanisms and thus precautions (known as standard contact and droplet precautions) are always taken when handling ill individuals suspected as having been infected.
- Food consumption (the handling of an infected animal may transmit Ebola virus via exposure to its bodily fluids, but illness as a direct result of consumption only is not entirely proven)
- Insects such as mosquitoes
- Casual contact with an Ebola survivor (only direct contact with bodily fluids increases transmission risk during or shortly after a recovery period)
- Blood and blood products used in blood transfusions are not regarded as a transmission risk factor. To date, no such incidence of transfusion-transmitted Ebola has occurred. Health departments around the world have strict monitoring systems and viral clearance steps in order to maintain blood use safety.
3. U.S. National Library of Medicine - National Institutes of Health. September 2017. Implementation of a study to examine the persistence of Ebola virus in the body fluids of Ebola virus disease survivors in Sierra Leone: Methodology and lessons learned: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5593174/ [Accessed 23.05.2018]