TWiV 230: Gene goes to Washington, flu chickens out

On episode #230 of the science show This Week in Virology, Vincent, Rich, Alan and Kathy review H7N9 infections in China, the debate over patenting genes, and receptor-binding by ferret-transmissible avian H5 influenza virus.

You can find TWiV #230 at www.microbe.tv/twiv.

WHO report on human cases of avian influenza A(H7N9) infection

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.

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TWiV 229: Partly cloudy with a high of H7N9

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.

Avian influenza H7N7 virus outbreak: Lessons for H7N9

Influenza H7 diversityAn 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.

First human infections with avian influenza H7N9 virus

comingled birdsFourteen people in China have been infected with avian influenza H7N9 virus, leading to five deaths. This avian influenza virus has never been isolated from humans.

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.

Circovirus in Shanghai

Circovirus genomeRecently thousands of dead and decaying pigs were pulled from rivers in Shanghai and Jiaxing, China. Apparently farmers dumped the animals into the water after the pigs became ill. Porcine circovirus has been detected in the in pig carcasses and in the water.

Porcine circoviruses are small, icosahedral viruses that were discovered in 1974 as contaminants of a porcine kidney cell line. They were later called circoviruses when their genome was found to be a circular, single-stranded DNA molecule. Upon entry into cells, the viral ssDNA genome enters the nucleus where it is made double-stranded by host enzymes. It is then transcribed by host RNA polymerase II to form mRNAs that are translated into viral proteins. There is some evidence that circoviruses might have evolved from a plant virus that switched hosts and then recombined with a picorna-like virus.

Porcine circoviruses are classified in the Circoviridae family, which contains two genera, Circovirus and Gyrovirus. There are two porcine circoviruses, PCV-1 and PCV-2; only the latter causes disease in pigs. Infection probably occurs via oral and respiratory routes, and leads to various diseases including postweaning multisystemic wasting syndrome, and porcine dermatitis and nephropathy syndrome. Virions are shed in respiratory and oral secretions, urine, and feces of infected pigs. Other circoviruses may cause diseases of birds, including psittacine beak and feather disease, and chicken infectious anemia, the latter caused by the sole member of the Gyrovirus genus. There are also circoviruses that infect canaries, ducks, finches, geese, gulls, pigeons, starlings, and swans.

We have no good evidence that porcine or avian circoviruses can infect humans. In the United States, porcine circovirus sequences can be detected in human feces. These most likely originate from consumption of pork products, most of which also contain porcine circoviruses. Circovirus sequences have also been found in commonly eaten animals such as cows, goats, sheep, camels, and chickens. Outside of the United States, the circoviruses found in human stools do not appear to be derived by meat consumption and might cause enteric infections.

Recently both PCV-1 and PCV-2 sequences were detected in Rotarix and RotaTeq, vaccines for the prevention of rotavirus disease in infants. The source of the contaminant was trypsin, an enzyme purified from porcine pancreas, which is used in the production of cell cultures used for vaccine production. Use of these vaccines was temporarily suspended, but resumed when the Food and Drug Administration concluded that there is no evidence that porcine circoviruses pose a safety risk to humans.

The good news is that porcine circoviruses in Shanghai’s waters are no danger to humans. But it is not a good idea to have rotting pig carcasses in a river that supplies some of Shanghai’s drinking water.