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About viruses and viral disease

aerosol transmission

Inefficient influenza H7N9 virus aerosol transmission among ferrets

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ferretThere have been 131 confirmed human infections with avian influenza H7N9 virus in China, but so far there is little evidence for human to human transmission. Three out of four patients report exposure to animals, ‘mostly chickens‘, suggesting that most of the infections are zoonoses. Whether or not the virus will evolve to transmit among humans is anyone’s guess. Meanwhile it has been found that one of the H7N9 virus isolates from Shanghai can transmit by aerosol among ferrets, albeit inefficiently.

Ferrets were inoculated intranasally with influenza A/Shanghai/02/2013 virus or A/California/07/2009, the 2009 pandemic H1N1 virus. One to two days later the ferrets developed fever, sneezing, coughing, and nasal discharge; both viruses induced similar clinical signs. Virus was shed in nasal secretions for 7 days. Six infected ferrets were then divided among three separate cages, and each group was housed with a naive ferret, and a second uninfected animal was placed in an adjacent cage. Airflow was controlled so that air flowed from the cage of infected animals towards the cage of naive animals. Transmission of infection was measured by observing clinical signs, and measuring virus shedding in nasal secretions and hemagglutination-inhibition antibodies in serum.

Of the three ferrets housed in the same cage with H7N9 virius-infected animals, all three had signs of infection (sneeze, cough, nasal discharge), shed virus in nasal secretions, and developed anti-HA antibodies. All three ferrets in neighboring cages developed signs of infection, but only one shed virus in nasal secretions, and two of three seroconverted. From these data the authors conclude that H7N9 virus is ‘efficiently transmitted between ferrets by direct contact, but less efficiently by airborne exposure’. In contrast, transmission of H1N1 virus to naive ferrets by contact or aerosol was efficient (3/3 animals in both cases).

The authors also found that pigs could be infected intranasally with A/Shanghai/02/2013 virus: the animals shed virus in nasal secretions and developed clinical symptoms. However the infected pigs transmitted infection inefficiently to other pigs by contact or aerosol, or to ferrets by aerosol.

The  authors’ equivocal conclusion that “Under appropriate conditions human to human transmission of the H7N9 virus may be possible” could have been reached even before these experiments were done. Their results provide no information on whether the virus can undergo human to human transmission because animal models are not definitive predictors of what might occur in humans. I disagree with the authors’ statement on page 5, “Efficient transmission of influenza viruses in ferrets is considered as a predictor of human to human transmissibility’. While many influenza virus strains that transmit among humans by aerosol also do so in ferrets, this does not mean that human transmission of a novel virus can be predicted by animal experiments.

Infection of ferrets with A/Shanghai/02/2013 or or A/California/07/2009 virus results in mild disease with no mortality. In contrast, 32 humans infected with H7N9 virus have died, and many humans have died after H1N1 infection. These findings further emphasize the differences in influenza virus pathogenesis in ferrets and humans.

TWiV 233: We’re surrounded

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On episode #233 of the science show This Week in Virology, Vincent, Rich, Alan and Kathy review aerosol transmission studies of influenza H1N1 x H5N1 reassortants, H7N9 infections in China, and the MERS coronavirus.

You can find TWiV #233 at www.microbe.tv/twiv.

Further defense of the Chinese H1N1 – H5N1 study

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Robert Herriman of The Global Dispatch interviewed me this week on the H1N1 – H5N1 reassortant study that has been in the headlines:

There was much written concerning the research published earlier this month in Science, where researchers from China’s Harbin Veterinary Research Institute reported creating an  avian H5N1 (highly pathogenic) and pandemic 2009 H1N1 (easily transmissible) hybrid, that according to them, achieved airborne spread between guinea pigs.

Read the rest of the article at The Global Dispatch.

Influenza H5N1 x H1N1 reassortants: ignore the headlines, it’s good science

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Those of you with an interest in virology, or perhaps simply sensationalism, have probably seen the recent headlines proclaiming another laboratory-made killer influenza virus. From The Independent: ‘Appalling irresponsibility: Senior scientists attack Chinese researchers for creating new strains of influenza virus’; and from InSing.com: ‘Made-in-China killer flu virus’. It’s unfortunate that the comments of several scientists have tainted what is a very well done set of experiments. Let’s deconstruct the situation with an analysis of the science that was done.

It is known that avian influenza H5N1 viruses can occasionally infect but not transmit among humans, while the 2009 pandemic H1N1 virus (which continues to circulate) readily transmits from person to person. The investigators asked whether reassortants of the two viruses – which could arise in nature – might confer transmissibility to H5N1 virus. To answer this question they produced 127 different reassortants of the two viruses, and tested their ability to transmit by aerosol among guinea pigs. The latter have been used for transmission studies on influenza, notably to understand the seasonality of infection. Ferrets have been more famously used for influenza virus transmission studies.

Rather than describe the results, I’ve made an illustration that shows what I believe to be the most important conclusions of the study (click for a larger version):

h1n1 h5n1 reassortants

The H5N1 virus (red RNAs) is not transmissible among guinea pigs, while the H1N1 virus (green RNAs) has highly efficient transmission. Exchange of the H5N1 RNA coding for PA or NS from H1N1 produces a highly transmissible virus. Exchange of the H5N1 RNA coding for NA or M produces a less efficiently transmitted virus. These are interesting and novel findings. It will be of great interest to determine how the PA, NS, NA, or M genes mechanistically enhance aerosol transmission. This is important information because our understanding of the determinants of transmission is very poor.

All the reassortant viruses shown in the figure have the H5 HA; when only the H1 of the H1N1 virus was substituted with the H5 HA, the reassortant virus transmitted efficiently among guinea pigs. In ferrets the H5 HA is not compatible with aerosol transmission. Therefore guinea pigs are clearly different from ferrets with respect to the determinants of transmissibility.

I cannot understand why some scientists have called these experiments ‘appallingly irresponsible’ and of no scientific use. I can only assume that they are not familiar with the literature on viral transmission and do not appreciate how the results advance our understanding of the field. It also seems irresponsible to predict that these viruses, should they escape from the laboratory, could kill millions of people. If you accept guinea pigs as a predictor of human pathogenicity – which I do not – then there is no reason for fear because none of the reassortants were lethal. I do not believe that any animal model predicts what will occur in humans, and so I am even less concerned about the safety of these experiments. I firmly believe that laboratory-constructed viruses do not have what it takes to be a human pathogen: only viral evolution in nature can produce the right combination of RNA segments and mutations. I also believe that scientists are quite responsible when it comes to safe handling of pathogens. If we worry about every type of transmission experiment involving influenza H5N1 virus, we will never make progress in understanding why this virus does not transmit among humans. The moratorium on H5N1 transmission research is over; let’s move beyond the sensational headlines and get back to the science.

In summary, I believe that these are well designed experiments which show that single RNA exchanges with H1N1 virus can produce an H5N1 virus that transmits via aerosol among guinea pigs. The relevance of these findings to humans is not known; nevertheless understanding how the individual viral proteins identified in this study enhance transmission may be mechanistically informative. I believe that the news headlines depicting these experiments as irresponsible and dangerous are based on uninformed statements made by scientists who are not familiar with the literature on influenza virus transmission. I wonder if they even read the paper in its entirety before making their comments.

Harvard University: Great virology, bad science writing

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Harvard virologyHarvard University is home to some of the world’s finest virologists. But apparently they do not communicate with the writers at Harvard Magazine, where a botched story on the avian H5N1 influenza virus has just been published.

The problems begin with the first paragraph:

But when Dutch researchers recently created an even more deadly strain of the virus in a laboratory for research purposes, they stirred grave concerns about what would happen if it escaped into the outside world.

Readers of virology blog will know by now that the Dutch researchers did not make an ‘even more deadly strain of the virus’ – they made one that could be transmitted by aerosol, but which had lost its lethality.

The title of the article, ‘The Deadliest Virus’, presumably refers to the H5N1 virus that transmits by aerosol among ferrets. This title is simply wrong, because the virus is not deadly to ferrets.

The first paragraph also contains an equally egregious statement by epidemiologist Marc Lipsitch:

If you make a strain that’s highly transmissible between humans, as the Dutch team did, it could be disastrous if it ever escaped the lab.

Dr. Lipsitch seems to be saying that the Dutch group created an H5N1 virus that transmits among humans. As far as I know, ferrets are not humans.

The article is accompanied by a photograph of two scientists working in BSL4 suits. The legend reads:

The modified H5N1 virus could infect a billion people if it escaped a biocontainment lab like the Canadian facility shown above.

And later Lipsitch is quoted as saying:

It could infect millions of people in the United States, and very likely more than a billion people globally, like most successful flu strains do. This might be one of the worst viruses—perhaps the worst virus—in existence right now because it has both transmissibility and high virulence.

For Lipsitch to say that the virus is both transmissible and of high virulence in humans is a misrepresentation of the Dutch group’s findings. He seems to be making up numbers and scenarios.

Perhaps Dr. Lipsitch does not know that ferret studies are not predictive of how viruses will behave in humans. With so many virologists at Harvard, the writer could have checked Dr. Lipsitch’s statements. But he did not, and the result looks as foolish as the New York Times.