Results of a study of four patients in Zhejiang, China, who developed influenza H7N9 virus infection suggests sporadic poultry-to-human transmission:
We diagnosed avian influenza A H7N9 in all four patients (who were epidemiologically unlinked), two of whom died and two of whom were recovering at the time of writing. All patients had histories of occupational or wet market exposure to poultry. The genes of the H7N9 virus in patient 3’s isolate were phylogenetically clustered with those of the epidemiologically linked wet market chicken H7N9 isolate. These findings suggest sporadic poultry-to-person transmission.
The four patients had occupational contact with poultry: one was a chef, one slaughtered and cooked live market poultry, and two bought live market poultry. Each had contact with poultry 3-8 days before onset of disease, and all were positive for influenza H7N9 virus by polymerase chain reaction of sputum or throat swab samples (virus was cultured from three of the four patients). Two of five pigeons and four of 20 chickens from two different wet markets were also positive for influenza H7N9 virus. Sequence analysis of virus recovered from patient 3 revealed that the HA and NA genes are nearly identical with those of two viruses isolated from epidemiologically linked chickens (1673 of 1683 bases for HA, 1394 of 1398 bases for NA).
While these H7N9 infections might have been acquired from poultry, the origin of other infections in different areas of China (>100) is unclear. According to the Ministry of Agriculture, as of 26 April 2013, only 46 of the 68,060 samples collected from poultry markets, habitats, farms and slaughterhouses across the country have tested positive for H7N9 virus, and none of these positive samples have been from poultry farms.
The World Health Organization has been publishing weekly reports on the avian influenza A(H7N9) outbreak which include the geographical location of each case, the cumulative number of cases, and the epidemiological curve. Go to this page at the WHO website for an archive of the weekly reports (there you will also find other useful information on the H7N9 outbreak). Images for report #3 of 24 April 2013 are reproduced below. Click each image for a larger view.
From the Centers for Disease Control in Taiwan:
In the late afternoon of April 24, 2013, the Central Epidemic Command Center (CECC) confirmed the first imported case of H7N9 avian influenza in a 53-year-old male Taiwanese citizen who worked in Suzhou, Jiangsu Province, China prior to illness onset. He developed his illness three days after returning to Taiwan. Infection with avian influenza A (H7N9) was confirmed on April 24, 2013. The patient is currently in a severe condition and being treated in a negative-pressure isolation room.
It’s not clear how the patient acquired the infection in China; he had no contact with birds or poultry and did not eat undercooked poultry or eggs.
The patient has had contact with 139 others, and all but 3 have used the appropriate personal protective equipment to prevent infection.
There is still no evidence for human-to-human transmission of avian influenza H7N9 virus in China. If this trend continues in Taiwan there should be no spread of the virus to others.
As long as people are allowed to travel from China, this probably won’t be the only imported case.
On episode #229 of the science show This Week in Virology, Vincent, Rich, Dickson, and Alan review the current status of human infections with avian influenza H7N9 virus.
You can find TWiV #229 at www.microbe.tv/twiv.
An outbreak of high-pathogenicity avian influenza H7N7 virus that took place on 255 poultry farms in the Netherlands during 2003 has been used to provide clues about the current avian influenza H7N9 viruses in China. During the Dutch outbreak 453 humans showed symptoms of illness and 89 were confirmed to have infection with the virus. Some interesting observations from that outbreak:
- Conjunctivitis (inflammation of the membranes surrounding the eyelids) was observed in many of the human cases, as well as in later human infections with H7 influenza viruses. Apparently these viruses replicate well in the eye, which bears alpha-2,3 sialic acid receptors. From there the viruses could reach the nasal cavity via the nasolacrimal duct.
- There was one fatal infection during the Dutch outbreak, and virus isolated from this individual contained the amino acid change E627K in viral protein PB2, which is associated with higher replication of avian influenza viruses in mammals. This change likely arose during replication of virus in the patient as it was not observed in other isolates. The recent H7N9 isolates from China all have the PB2 E627K mutation.
- In the Dutch outbreak there was no evidence for human to human transmission of H7N7 viruses. This conclusion is in part supported by phylogenetic analysis of viral sequences, which showed that during the outbreak the viruses diversified into multiple lineages with human strains at the ends of the trees (Figure; click to enlarge. Credit: Eurosurveillance).
So far the H7N9 virus does not appear to be spreading from human to human. This observation suggests that the virus is widespread in poultry in China, and that there have been multiple introductions into humans. It seems likely that these novel viruses arose relatively recently in China and some time thereafter had to opportunity to infect humans.
The question on everyone’s mind is whether the avian influenza H7N9 viruses will acquire the ability to transmit among humans. On this subject the authors have the following comment:
Although human infections with H7 influenza viruses have occurred repeatedly over the last decades without evidence of sustained human-to-human transmission, the absence of sustained human-to-human transmission of A(H7N9) viruses does not come with any guarantee.
It is possible that during replication in birds or humans, the H7N9 viruses might randomly acquire a mutation that allows for transmission. In the right place at the right time, such a virus could spread through the human population. Alternatively, such a transmission-facilitating mutation might interfere with the overall fitness of the virus, thereby preventing it from spreading. I favor the latter hypothesis because the H7N9 viruses have been transmitting since at least February 2013; they have undergone many replication cycles without such a mutation arising. If the virus has not entered wild birds, culling poultry could eradicate it from China – assuming that it has not gone elsewhere.
There have been over 60 human infections with avian influenza virus H7N9 in China, and cases have been detected outside of Shanghai, including Beijing, Zhejiang, Henan, and Anhui Provinces. Information on the first three cases has now been published, allowing a more detailed consideration of the properties of the viral isolates.
The first genome sequences reported were from the initial three H7N9 isolates: A/Shanghai/1/2013, A/Shanghai/2/2013, and A/Anhui/1/2013. These were followed by genome sequences from A/Hongzhou/1/2013 (from a male patient), A/pigeon/Shanghai/S1069/2013), A/chicken/Shanghai/S1053/2013), and A/environment/Shanghai/S1088/2013, the latter three from a Shanghai market.
Analysis of the viral genome sequences reveals that all 8 RNA segments of influenza A/Shanghai/1/2013 virus are phylogenetically distinct from A/Anhui/1/2013 and A/Shanghai/2/2013, suggesting that the virus passed from an animal into humans at least twice. Similar viruses have been isolated from pigeons and chickens, but the source of the human infections is not known. There is as yet no evidence for human to human transmission of the H7N9 viruses, and it seems likely that all of the human infections are zoonotic – transmission of animal viruses to humans. Since the H7N9 viruses are of low pathogenicity in poultry, infected animals may not display disease symptoms, further facilitating transmission to humans.
The RNA sequences reveal that the H7N9 viruses isolated from humans are all triple reassortants, which means that they contain RNA segments derived from three parental viruses. The gene encoding the hemagglutinin protein (HA) is most closely related to the HA from A/duck/Zhejiang/12/2011 (H7N3), while the NA gene is most similar to the NA gene from A/wild bird/Korea/A14/2011 (H7N9). The remaining 6 RNA segments are most related to genes from A/brambling/Beijing/16/2012-like viruses (H9N2). The type of animal(s) in which the mixed infections took place is unknown.
Some observations on the relatedness of these sequences:
- A/Shanghai/2/2013, A/Anhui/1/2013, and A/Hangzhou/1/2013 were isolated in distant cities yet have over 99% identity. The pigeon, chicken, and environmental isolates are also very similar except for one gene of A/pigeon/Shanghai/S1069/2013. Long-range shipping of infected poultry might explain these similarities.
- There are 53 nucleotide differences between A/Shanghai/1/2013 and A/Shanghai/2/2013. Perhaps A/Shanghai/1/2013 and the remaining viruses originated from different sources.
When the gene sequences of these human viral isolates are compared with closely related avian strains, numerous differences are revealed. The locations of the proteins in the influenza virion are shown on the diagram; click for a larger version (figure credit: ViralZone).
- All seven H7N9 viruses do not have multiple basic amino acids at the HA cleavage site. The presence of a basic peptide in this location allows the viral HA to be cleaved by proteases that are present in most cells, enabling the virus to replicate in many organs. Without this basic peptide, the HA is cleaved only by proteases present in the respiratory tract, limiting replication to that site. This is one reason why the H7N9 viruses have low pathogenicity in poultry.
- All seven viruses have a change at HA amino acid 226 (Q226L) which could improve binding of the viruses to alpha-2,6 sialic receptors, which are found throughout the human respiratory tract. Avian influenza viruses prefer to bind to alpha-2,3 sialic acid receptors. This observation suggests that the H7N9 isolates should be able to infect the human upper respiratory tract (alpha-2,3 sialic acid receptors are mainly located in the lower tract of humans). However, viruses which bind better to alpha-2,3 sialic acids still bind to alpha-2,6 receptors and can infect humans.
- All seven viruses have a change at HA amino acid 160 from threonine to alanine (T160A). This change, which has been identified in other circulating H7N9 viruses, prevents attachment of a sugar to the HA protein and could lead to better recognition of human (alpha-2,6 sialic acid) receptors.
- Five amino acids are deleted from the neuraminidase (NA), the second viral glycoprotein, in all seven viruses. In avian H5N1 influenza virus this change may influence tropism for the respiratory tract and enhance viral replication, and might regulate transmission in domestic poultry. This change is believed to be selected upon viral replication in terrestrial birds.
- One of the viruses (A/Shanghai/1/2013) has an amino acid change in the NA glycoprotein associated with oseltamivir resistance (R294K).
- An amino acid change in the PB1 gene, I368V, is known to confer aerosol transmission to H5N1 virus in ferrets.
- An amino acid change in the PB2 gene, E627K, is associated with increased virulence in mice, higher replication of avian influenza viruses in mammals, and respiratory droplet transmission in ferrets.
- Changes of P42S in NS1 protein, and N30D and T215A in M1 are associated with increased virulence in mice, but these changes are also observed in circulating avian viruses.
- All seven viruses have an amino acid change in the M2 protein known to confer resistance to the antiviral drug amantadine.
- All seven viruses lack a C-terminal PDZ domain-binding motif which may reduce the virulence of these viruses in mammals.
For the most part we do not know the significance of any of the amino acid changes for viral replication and virulence in humans.
I believe that these H7N9 viruses might take one of two pathways. If they are widespread in birds, they could spread globally and cause sporadic zoonotic infections, as does avian influenza H5N1 virus. Alternatively, the H7N9 viruses could cause a pandemic. Influenza H7N9 virus infections have not occurred before in humans, so nearly everyone on the planet is likely susceptible to infection. Global spread of the virus would require human to human transmission, which has not been observed so far. Some human to human transmission of avian H7N7 influenza viruses was observed during an outbreak in 2003 in the Netherlands, but those viruses were different from the ones isolated recently in China. Whether or not these viruses will acquire the ability to transmit among humans by aerosol is unknown and cannot be predicted. If a variant of H7N9 virus that can spread among humans arises during replication in birds or humans, it might not have a chance encounter with a human, or if it did, it might not have the fitness to spread extensively.
What also tempers my concern about these H7N9 viruses is the fact that the last influenza pandemic (H1N1 virus) took place in 2009. No influenza pandemics in modern history are known to have taken place 4 years apart, although only 11 years separated the 1957 (H2N2) and 1968 (H3N2) pandemics. I suppose that is not much consolation, as there are always exceptions, especially when it comes to viruses.
Meanwhile a vaccine against this H7N9 strain is being prepared (it will be months before it is ready), surveillance for the virus continues in China and elsewhere, and health agencies ready for a more extensive outbreak. These are not objectionable courses of action. But should this be our response to every zoonotic influenza virus infection of less than 100 cases?
On episode #227 of the science show This Week in Virology, the complete TWiV team reviews the controversial publication of the HeLa cell genome, a missing vial of Guanarito virus in a BSL-4 facility, and human infections with avian influenza H7N9 virus.
You can find TWiV #227 at www.microbe.tv/twiv.
Influenza A viruses with the H7 hemagglutinin protein circulate among birds, and some, such as H7N2, H7N3, and H7N7, have been previously found to infect humans. It is not known how the individuals in China acquired the H7N9 virus. Some of the infections have occurred in Shanghai, where a similar virus was found in pigeon samples collected at a marketplace in that city. It is not clear what types of pigeon samples tested positive for the virus, nor is it known whether the virus spread from poultry to pigeons or vice versa. In response the city has begun mass slaughter of poultry to stem further spread of the virus.
Influenza H7N9 virus is typically a low-pathogenicity virus, which means that infection of chickens causes mild respiratory disease, depression, and decrease in egg production. The virus does not have a basic peptide between HA1 and HA2. The presence of a basic peptide in this location allows the viral hemagglutinin glycoprotein to be cleaved by proteases that are present in most cells, enabling the virus to replicate in many organs. Without this basic peptide, the HA is cleaved only by proteases present in the respiratory tract, limiting replication to that site.
According to Brian Kimble on Google+, the nucleotide sequence reveals that the H7N9 human isolate is a reassortant* with 6 RNA segments encoding the internal proteins PB1, PB2, PA, NP, M, and NS derived from H9N2 virus, and the HA and NA from H7N9 virus. The significance of this observation is not clear, because I do not know if H7N9 viruses isolated from birds are also reassortants. One possibility is that reassortment produced a virus that can infect humans. It is known that reassortants of H9N2 viruses with the 2009 pandemic H1N1 strain can transmit via aerosols in ferrets.
An important question is whether this H7N9 virus isolated from humans has pandemic potential. So far there is no evidence for human to human transmission of the virus. There is no vaccine for this subtype of influenza virus, but the virus is susceptible to neuraminidase inhibitors oseltamivir and zanamivir. WHO has released the following statement:
Any animal influenza virus that develops the ability to infect people is a theoretical risk to cause a pandemic. However, whether the influenza A(H7N9) virus could actually cause a pandemic is unknown. Other animal influenza viruses that have been found to occasionally infect people have not gone on to cause a pandemic.
*Because the influenza virus genome occurs as 8 segments of RNA, when multiple viruses infect a single cell, new viruses can be produced with combinations of the parental segments, a process known as reassortment.
Update: Peter Palese notes that the human H7N9 isolates do not have a serine in position 61 (as does the 1918 virus). This change is a human virulence marker for some animal influenza viruses. Brian Kimble notes that the H7N9 isolates possess a L226 equivalent in the HA, which confers human-like receptor binding in other viruses. Human influenza viruses prefer to bind to alpha-2,6 sialic acid receptors, while avian strains bind alpha-2,3 sialic acids. If the human H7N9 viruses can bind alpha-2,6 sialic acid receptors then they are adapted to infect the human upper respiratory tract.
Harvard 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.