Ebolavirus will not become a respiratory pathogen

sneezeAn otherwise balanced review of selected aspects of Ebolavirus transmission falls apart when the authors hypothesize that ‘Ebola viruses have the potential to be respiratory pathogens with primary respiratory spread.’

The idea that Ebolavirus might become transmitted by the respiratory route was suggested last year by Michael Osterholm in a Times OpEd. That idea was widely criticized by many virologists, including this writer.  Now he has recruited 20 other authors, including Ebola virologists, in an attempt to lend legitimacy to his hypothesis. Unfortunately the new article adds no new evidence to support this view.

In the last section of the review article the authors admit that they have no evidence for respiratory transmission of Ebolavirus:

It is very likely that at least some degree of Ebola virus transmission currently occurs via infectious aerosols generated from the gastrointestinal tract, the respiratory tract, or medical procedures, although this has been difficult to definitively demonstrate or rule out, since those exposed to infectious aerosols also are most likely to be in close proximity to and in direct contact with an infected case.

It is possible that some short-distance transmission of Ebolavirus occurs through the air. But claiming that it is ‘very likely’ to be taking place is an overstatement considering the lack of evidence. As might be expected, ‘very likely’ is exactly the phrase picked up by the Washington Post.

I find the lack of critical thinking in the following paragraph even more disturbing:

To date, investigators have not identified respiratory spread (either via large droplets or small-particle aerosols) of Ebola viruses among humans. This could be because such transmission does not occur or because such transmission has not been recognized, since the number of studies that have carefully examined transmission patterns is small. Despite the lack of supportive epidemiological data, a key additional question to ask is whether primary pulmonary infections and respiratory transmission of Ebola viruses could be a potential scenario for the future.

Why is the possibility of respiratory transmission of Ebolaviruses a ‘key additional question’ when there has been no evidence for it to date? To make matters worse, the authors have now moved from short-range transmission of the virus by droplets, to full-blown respiratory aerosol transmission.

The authors present a list of reasons why they think Ebolavirus could go airborne, including: isolation of Ebolaviruses from saliva; presence of viral particles in pulmonary alveoli on human autopsies; and cough, which can generate aerosols, can be a symptom of Ebolavirus disease. The authors conclude that because of these properties, the virus would not have to change very much to be transmitted by aerosols.

I would conclude the opposite from this list of what Ebolavirus can do: there is clearly a substantial block to respiratory transmission that the virus cannot overcome. Perhaps the virus is not stable enough in respiratory aerosols, or there are not enough infectious viruses in aerosols to transmit infection from human to human. Overcoming these blocks might simply not be biologically possible for Ebolavirus. A thoughtful discussion of these issues is glaringly absent in the review.

The conclusion that Ebolavirus is  ‘close’ to becoming a full-blown respiratory pathogen reveals how little we understand about the genetic requirements for virus transmission. In fact the authors cannot have any idea how ‘close’ Ebolavirus is to spreading long distances through the air.

It is always difficult to predict what viruses will or will not do. Instead, virologists observe what viruses have done in the past, and use that information to guide their thinking. If we ask the simple question, has any human virus ever changed its mode of transmission, the answer is no. We have been studying viruses for over 100 years, and we’ve never seen a human virus change the way it is transmitted. There is no evidence to believe that Ebolavirus is any different.

Viruses are masters of evolution, but apparently one item lacking from their repertoire is the ability to change the way that they are transmitted.

Such unfounded speculation would largely be ignored if the paper were read only by microbiologists. But Ebolavirus is always news and even speculation does not go unnoticed. The Washington Post seems to think that this review article is a big deal. Here is their headline: Limited airborne transmission of Ebola is ‘very likely’ new analysis says.

Gary Kobinger, one of the authors, told the Washington Post that ‘we hope that this review will stimulate interest and motivate more support and more scientists to join in and help address gaps in our knowledge on transmission of Ebola’. Such hope is unrealistic, because few can work on this virus, which requires the highest levels of biological containment, a BSL-4 laboratory.

I wonder if Osterholm endorses Kobinger’s hopes. After all, he opposed studies of influenza virus transmission in ferrets, claiming that they are too dangerous. And the current moratorium on research that would help us understand aerosol transmission of influenza viruses is a direct result of objections by Osterholm and his colleagues about this type of work. The genetic experiments that are clearly needed to understand the limitations of Ebolavirus transmission would never be permitted, at least not with United States research dollars.

The gaps in our understanding of virus transmission are considerable. If virologists are not able to carry out the necessary experiments to fill these gaps, all we will have is rampant and unproductive speculation.

TWiV 312: She sells B cells

On episode #312 of the science show This Week in Virology, the TWiVbolans discuss the finding that human noroviruses, major causes of gastroenteritis, can for the first time be propagated in B cell cultures, with the help of enteric bacteria.

You can find TWiV #312 at www.microbe.tv/twiv.

TWiV 309: Ebola email

On episode #309 of the science show This Week in Virology, the TWiVocytes answer questions about Ebola virus, including mode of transmission, quarantine, incubation period, immunity, and much more.

You can find TWiV #309 at www.microbe.tv/twiv.

Can Ebola virus infect via the skin?

SkinI received this question about Ebola virus infection via email:

Can you become infected if infected droplet lands on your skin even if there is no abrasion on the skin? I am now hearing this, which surprises me. The virus can enter through the actual skin and does not need mucus membrane to enter?

The skin of most animals is an effective barrier against viral infections. The outer layer of human skin, called the stratum corneum, consists of a layer of dead, keratinized cells (illustrated). Viruses cannot replicate in, or be transported across, dead cells. Therefore any virus that lands on the skin cannot simply replicate in the outer layer or be transported to the underlying living cells.

However, viruses can pass through the dead layer of the skin through cuts or abrasions. Many activities, such as shaving, or even scratching, lead to microabrasions. It is relatively easy to breach the dead layer of cells with a fingernail, and such abrasions cannot be seen.

A patient in the late stages of Ebola virus infection (such as the Dallas patient) is shedding high amounts of virus particles in body fluids. If virus-laden droplets land on the skin, the virus can readily enter via cuts or abrasions. Even if the skin is intact, the droplets could be inadvertently transferred to mucous membranes of the eye, nose, or mouth, initiating infection. For this reason it is important that the skin be entirely covered when caring for Ebola virus infected patients.

TWiV 307: Ebola aetiology

On episode #307 of the science show This Week in Virology, Tara Smith joins the TWiEBOVsters to discuss the Ebola virus outbreak in west Africa, spread of the disease to and within the US, transmission of the virus, and much more.

You can find TWiV #307 at www.microbe.tv/twiv.

TWiV 306: This Week in Ebolavirus

On episode #306 of the science show This Week in Virology, the Grand Masters of the TWiV discuss Ebola virus transmission, air travel from West Africa, Ebola virus infectivity on surfaces, the Dallas Ebola virus patient, and Ebola virus in dogs.

You can find TWiV #306 at www.microbe.tv/twiv.

The incubation period of a viral infection

Incubation periodThe time before the symptoms of a viral infection appear is called the incubation period. During this time, viral genomes are replicating and the host is responding, producing cytokines such as interferon that can have global effects, leading to the classical symptoms of an acute infection (e.g., fever, malaise, aches, pains, and nausea). These symptoms are called the prodrome, to distinguish them from those characteristic of infection (e.g. paralysis for poliovirus, hemorrhagic fever for Ebolaviruses, rash for measles virus).

Whether or not an infected person is contagious (i.e. is shedding virus) during the incubation period depends on the virus. For example, Ebola virus infected patients do not pass the virus on to others during the incubation period. This fact explains why Tom Frieden said there was ‘zero chance’ that the passenger from Liberia who was diagnosed with Ebola virus infection in Dallas would have infected others while on an airplane. He had no symptoms of infection because he was still in the incubation period of the disease.

In contrast to Ebolaviruses, poliovirus and norovirus are shed during the incubation period – in the feces, where they can infect others.

Remarkably, viral incubation periods can vary from 1 or 2 days to years (Table; click to magnify). Short incubation times usually indicate that actions at the primary site of infection produce the characteristic symptoms of the disease. Longer incubation times indicate that the host response, or the tissue damage required to reveal the symptoms of infection, take place away from the primary site of infection.

The table was taken from the third edition of Principles of Virology. Missing from the table (which will be corrected in the next edition) is the incubation period of Ebola virus, which is 2 to 21 days. I would also argue that the incubation period of HIV is not 1-10 years, but 2-4 weeks, the time until the prodromal symptoms occur. The characteristic symptom of HIV-1 infection, immunosuppression, occurs much later.

WHO on Ebola virus transmission

The World Health Organization has issued a situation assessment entitled ‘What we know about transmission of the Ebola virus among humans‘. WHO is rather late entering the transmission discussion which began on 12 September 2014 with the suggestion that Ebola virus transmission could go airborne. WHO is a big organization and moves slowly; nevertheless their voice may reassure those who are not convinced by what virologists have to say. Here are the salient points (voiced here and by many others in the past few weeks).

The Ebola virus is transmitted among humans through close and direct physical contact with infected bodily fluids, the most infectious being blood, faeces and vomit.

Ebola virus disease is not an airborne infection. Airborne spread among humans implies inhalation of an infectious dose of virus from a suspended cloud of small dried droplets.

This mode of transmission has not been observed during extensive studies of the Ebola virus over several decades.

Moreover, scientists are unaware of any virus that has dramatically changed its mode of transmission*. For example, the H5N1 avian influenza virus, which has caused sporadic human cases since 1997, is now endemic in chickens and ducks in large parts of Asia.

That virus has probably circulated through many billions of birds for at least two decades. Its mode of transmission remains basically unchanged.

Speculation that Ebola virus disease might mutate into a form that could easily spread among humans through the air is just that: speculation, unsubstantiated by any evidence.

The last sentence is the key point:

To stop this outbreak, more needs to be done to implement – on a much larger scale – well-known protective and preventive measures. Abundant evidence has documented their effectiveness

*Sounds familiar?

TWiV 305: Rhymes with shinola

On episode #305 of the science show This Week in Virology, Vincent, Alan, and Kathy continue their coverage of the Ebola virus outbreak in West Africa, with a discussion of case fatality ratio, reproductive index, a conspiracy theory, and spread of the virus to the United States.

You can find TWiV #305 at www.microbe.tv/twiv.

TWiV 304: Given X, solve for EBOV

On episode #304 of the science show This Week in Virology, the TWiV team consults an epidemiologist to forecast the future scope of the Ebola virus epidemic in West Africa.

You can find TWiV #304 at www.microbe.tv/twiv.