Influenza virus in breast milk

Ferret mother-infantDuring breastfeeding, mothers provide the infant with nutrients, beneficial bacteria, and immune protection. Fluids from the infant may also enter the mammary gland through retrograde flux of the nipple. Studies in a ferret model reveal that influenza virus replicates in the mammary gland, is shed in breast milk and transmitted to the infant. Virus may also travel in the opposite direction, from infant to mother.

The role of the mammary gland in influenza virus transmission was studied using a ferret model comprising lactating mothers and nursing infants. Intranasal inoculation of nursing mother ferrets with the 2009 H1N1 influenza virus lead to viral replication and development of influenza in both mother and infant. When the study design was reversed, and 4 week old nursing ferrets were inoculated intranasally with the same virus, viral replication and disease ensued first in the infants, and then in the mothers. Infectious virus was recovered both in the mammary glands and in the nipples at day 4 post infant inoculation, and in mother’s milk from 3-5 days post infant inoculation. Histopathological examination of sections of mammary glands from infected mothers revealed destruction of the mammary architecture.

These results show that nursing infants may pass influenza virus to mothers. It seems clear that influenza virus replicates in the mammary gland and that infectious virus is present in milk. How does this virus infect the mother? One possibility is that infection is transmitted by respiratory contact with virus-containing milk, or by inhalation of aerosols produced by nursing. How influenza virus in the mammary gland would reach the mother’s lung via the blood to cause respiratory disease is more difficult to envision and seems unlikely.

When influenza virus was inoculated into the mammary gland of lactating mothers via the lactiferous ducts, both mother and breast feeding infant developed serious influenza. Infectious virus was detected first in the nasal wash of infants, then later in the nasal wash of mothers. Breast milk contained infectious virus starting on day 2 after inoculation. Histopathological examination of sections from infected mammary glands revealed destruction of glandular architecture and cessation of milk production. This observation is consistent with the results of gene expression analysis of RNA from virus infected mammary glands, which revealed reduction in transcripts of genes associated with milk production.

To determine if human breast cells can be infected with influenza virus, three different human epithelial breast cell lines were infected with the 2009 H1N1 virus strain. Virus-induced cell killing was observed and infectious virus was produced.

Even if we assume that influenza virus can replicate in the human breast, the implications for influenza transmission and disease severity are not clear. Transmission of HIV-1 from mother to infant by breast milk has been well documented. In contrast to influenza virus, HIV-1 is present in the blood from where it spreads to the breast. Most human influenza virus strains do not enter the blood so it seems unlikely that virus would spread to the breast of a mother infected via the respiratory route. However, viral RNA has been detected in the blood of humans infected with the 2009 H1N1 strain, the virus used in these ferret studies. Therefore we cannot rule out the possibility that some strains of influenza virus spread from lung via the blood to the breast, allowing infection of a nursing infant. Some answers might be provided by determining if influenza virus can be detected in the breast milk of humans with influenza.

What would be the implication of a nursing infant infecting the mother’s breast with influenza virus? As I mentioned above, it seems unlikely that this virus would enter the blood, and even if it could, how would the virus infect the apical side of the respiratory epithelium? What does seem clear is that viral replication in the breast could lead to a decrease in milk production which could be detrimental to the infant. If the mother had multiple births, then influenza virus might be transmitted to siblings nursing on the infected mother.

Are you wondering how an infant drinking influenza virus-laded breast milk acquires a respiratory infection? Recently it has been shown that influenza virus replicates in the soft palate of ferrets. The soft palate has mucosal surfaces that face both the oral cavity and the nasopharynx. Ingested virus could first replicate in the soft palate, then spread to the nasopharynx and the lung. A simpler explanation is that nursing produces virus-containing aerosols which are inhaled by the infant.

TWiV 335: Ebola lite

On episode #335 of the science show This Week in Virology, the TWiVumvirate discusses a whole Ebolavirus vaccine that protects primates, the finding that Ebolavirus is not undergoing rapid evolution, and a proposal to increase the pool of life science researchers by cutting money and time from grants.

You can find TWiV #335 at www.microbe.tv/twiv.

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 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.

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.

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 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.

Transmission of Ebola virus

jet nebulizerAs 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 μm 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 μm) 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.

TWiV 303: Borna this way

On episode #303 of the science show This Week in Virology, the TWiV team discusses transmission of Ebola virus, and inhibition of Borna disease virus replication by viral DNA in the ground squirrel genome.

You can find TWiV #303 at www.microbe.tv/twiv.