As the West African epidemic of Ebola virus grows, so does misinformation about the virus, particularly how it is transmitted from person to person. Ebola virus is transmitted from human to human by close contact with infected patients and virus-containing body fluids. It does not spread among humans by respiratory aerosols, the route of transmission of many other human viruses such as influenza virus, measles virus, or rhinovirus. Furthermore, the mode of human to human transmission of Ebola virus is not likely to change.
What is aerosol transmission? Here is a definition from Medscape:
Aerosol transmission has been defined as person-to-person transmission of pathogens through the air by means of inhalation of infectious particles. Particles up to 100 Î¼m in size are considered inhalable (inspirable). These aerosolized particles are small enough to be inhaled into the oronasopharynx, with the smaller, respirable size ranges (eg, < 10 Î¼m) penetrating deeper into the trachea and lung.
All of us emit aerosols when we speak, breathe, sneeze, or cough. If we are infected with a respiratory virus such as influenza virus, the aerosols contain virus particles. Depending on their size, aerosols may travel long distances, and when inhaled they lodge on mucosal surfaces of the respiratory tract, initiating an infection.
Viral transmission can also occur when virus-containing respiratory droplets travel from the respiratory tract of an infected person to mucosal surfaces of another person. Because these droplets are larger, they cannot travel long distances as do aerosols, and are considered a form of contact transmission. Ebola virus can certainly be transmitted from person to person by droplets.
Medical procedures, like intubation, can also generate aerosols. It is possible that a health care worker could be infected by performing these procedures on a patient with Ebola virus disease. But the health care worker will not transmit the virus by aerosol to another person. In other words, there is no chain of respiratory aerosol transmission among infected people, as there is with influenza virus.
In the laboratory, machines called nebulizers (which are used to administer medications to humans by inhalation) can be used to produce virus-containing aerosols for studies in animals. A human would likely be infected with an Ebola virus-containing aerosol generated by a nebulizer (theoretically; such an experiment would be unethical).
A variety of laboratory animals have been infected with Ebola virus (Zaire ebolavirus) using aerosols. In one study rhesus macaques were infected with aerosolized Ebola virus using a chamber placed over the animals’ heads. This procedure resulted in replication of the virus in the respiratory tract followed by death. Virus particles were detected in the respiratory tract, but no attempts were made to transmit infection from one animal to another by aerosol. In another study, cynomolgous macaques, rhesus macaques, and African Green monkeys could be infected with Ebola virus aerosols using a head-only chamber. Virus replicated in the respiratory tract, and moved from regional lymph nodes to the blood and then to other organs. Virus titers in the respiratory tract appeared to be lower than in the previous study. No animal to animal transmission experiments were done.
When rhesus macaques were inoculated intramuscularly with Ebola virus, virus could be detected in oral and nasal swabs; however infection was not transmitted to animals housed in separate cages. The authors conclude that ‘Airborne transmission of EBOV between non-human primates does not occur readily’.
Pigs can also be infected with Ebola virus. In one study, after dripping virus into the nose, eyes, and mouth, replication to high titers was detected in the respiratory tract, accompanied by severe lung pathology. The infected pigs can transmit infection to uninfected pigs in the same cage, but this experimental setup does not allow distinguishing between aerosol, droplet, or contact spread.
In another porcine transmission experiment, animals were infected oronasally as above, and placed in a room with cynomolgous macaques. The pigs were allowed to roam the floor, while the macaques were housed in cages. All of the macaques became infected, but their lungs had minimal damage. However it is not known how the virus was transmitted from pigs to macaques. The authors write: ‘The design and size of the animal cubicle did not allow to distinguish whether the transmission was by aerosol, small or large droplets in the air, or droplets created during floor cleaning which landed inside the NHP cages’. The authors also indicate that transmission between macaques in similar housing conditions was never observed.
While these experimental findings show that animals can be infected with Ebola virus by aerosol, they do not provide definitive evidence for animal to animal transmission via this route. It is clear is that the virus does not transmit via respiratory aerosols among nonhuman primates.
We do not know why, in humans or non-human primates, Ebola virus does not transmit by respiratory aerosols. The virus might not reach sufficiently high titers in the respiratory tract, or be stable in respiratory secretions, to be efficiently transmitted by this route. There are many other possibilities. A careful study of Ebola virus titers in the human respiratory tract, and in respiratory secretions, would be valuable. However during Ebola virus outbreaks the main concern is to save people, not conduct experiments.
These experiments reveal the large gaps in our understanding about virus transmission in general, and specifically why Ebola virus is not transmitted among primates by respiratory aerosols.
24 thoughts on “Transmission of Ebola virus”
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I think it is important the findings that ebov can replicate in some airway epithelium cells in non-human primates disease models. That the replication may be conductive of a respiratory transmission is another question in need of clarification. For example, what is the viral subunit responsible for these cells attachment? Is the gene coding this structure changing during current outbreaks?
I find this is about as useful as anything else coming via traditional media (hint, its funny).
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Of course…. what good is a bioweapon you can’t control?
I am finding that we, as Humans, know absolutely nothing as to how Ebola is transmitted. Last week [the week of 10-5 through 10-11-14] there where 3 states reporting the virus. This week [the very next] there is now detection of the virus in 47 states. It is now here in the US and will become an uncontrollable epidemic within weeks. I am sick of our so called health care professionals guessing or out and out lying to us about this insidious disease. I believe Humanity is now doomed.
The potential for transmission via inhalation of aerosols, therefore, cannot be ruled out by the observed risk factors or our knowledge of the infection process. Many body fluids, such as vomit, diarrhea, blood, and saliva, are capable of creating inhalable aerosol particles in the immediate vicinity of an infected person. Cough was identified among some cases in a 1995 outbreak in Kikwit, Democratic Republic of the Congo,11 and coughs are known to emit viruses in respirable particles.17 The act of vomiting produces an aerosol and has been implicated in airborne transmission of gastrointestinal viruses.18,19 Regarding diarrhea, even when contained by toilets, toilet flushing emits a pathogen-laden aerosol that disperses in the air.20-22
I’ve seen those studies too. Furthermore epidemiological reports warned mode of transmission is increased by sores or ingestion of aerosol particles plus other studies showed the strength of Virus primates were infected by were low in studies showing no transferrence bh aerosol. The study stated transfer aerosol “could not be ruled out.”
Even CDC admits in one report ” … the reported Ebola virus aerosol transmission among nonhuman primates had been demonstrated in laboratory experiments. …” in reference to a research on possible transmission modes of a particular family.
Furthermore CDC stated “By contrast simple physical contact with a sick person appears to be neither necessary or sufficient for contracting EHF.”
SPUTUM is a body fluid. It just needs a mode of entrance. The Union discussing readiness conditions of Dallas hospital stated they used basic Standard Contact Precautions which obviously is not appropriate. Epidemiology experts are recommending respirators, not masks.
There are loads of .edu research out there to refer to rather than media which are better and have more authority on the subject. The media is too political and worries about Wallstreet. You’ll just get bias.
That’s the same information I found on the CDC site and it made me mad as hell when, even today, the feds are still saying this can’t be spread through the air. I certainly realize the issue of not wanting to panic the people, but when you blatantly lie to us, that’s when the panic starts. And, you’re right this is about money. Thank you for such a thoughtful reply.
I noticed that the NIH was quick to condemn the Republicans for reducing funding, but they don’t have one ebola project listed on their site…lol
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Huh? What states are those? Credible citations for your incredible claims, please…
I read recently that patients in Spain and Dallas has higher viral titers compared to other outbreaks. Do you feel there is reason for concern. Can this increased viremia favor conditions where the viral titer in respiratory secretions would allow for increased probability of aerosolized droplets. I also read a report that said one of the Spanish victims experienced rather violent coughing, which has not been a common symptom in previous outbreaks in Africa…or during the initial months of this outbreak.
This is a very scary epidemic that nobody seems to be able to stop.
At the moment we don’t exactly know it transmits. I was ready on the web and the pictures are really scary. http://whatisebolavirusinfection.com/ebola-symptoms-pictures.htm . What are we going to do if it becomes out of control in the USA?
No question Ebola is a terrible virus and disease. However, it does not appear to be easily transmissable. Evidence of this is the disease transmission in Dallas where scores of people were exposed to the active virus and two have been infected. Granted two is too many but it does illustrate the point about transmission.
Here are some points to consider:
1. Viruses are “stupid.” They attempt to overwhelm the human immune system with great numbers. But inidividually they are ineffective.
2. The Ebola virus is actually quite large. The average size is about 0.9 microns. This is about 10 to 50 times the average size of other viruses such as the common flu.
3. In order to exist viruses must have a “host.” They generally exist in an “unattached” state for just a few seconds at best.
4. So when you add the need for a host with the large size, the infectious particles will be 5 microns or larger. This means they will settle out of the air relatively quickly – even if they become airborne. In addition, these particles will have very few numbers to do their infectious damage.
5. On the other hand, one drop of liquid contains about 65 billion microns! In other words, one drop of sweat or other bodily fluid could contain about the same number of infectious particles as a 50,000 square foot building of airborne particles.
So, is it possible on the basis of the math and physics involved to contract Ebola virus infection through airborne means? Yes, but one has about the same odds as winning the lottery – about 250,000,000 to one and that would be for someone within a few feet of the infected individual.
I just looked up the Rhys monkey aerosol study. The monkey became infected.
The Ebola reston strain was airborne in non-human primates in the Reston, VA, outbreak. Arch Virol, 1996. Jahrling et al.
Replying on your number 3, I have heard and read in multiple sources that the Ebola virus can be infectious outside the body for weeks under the “right” circumstances (4 degrees Celcius, smooth surface). So I have to disagree with you on this one.
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To say that the Ebola virus can live “for weeks” under the right circumstances is like saying humans can live underwater for longer than 10 minutes without oxygen. Both true but not relevant. Remember, viruses need high numbers to be infectious. Viruses are not self sustaining organisms. They need a “host” to reproduce and, ultimately, survive.
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