Two Reassortment Studies To Ponder

 

 

# 5765

 

 

 

More than a year after the declared end of the 2009 H1N1 pandemic, scientists continue to note that the pdmH1N1 virus is still evolving, and that it still has the potential to gain enhanced virulence or transmissibility sometime in the future.

 

Today, we’ve two new studies on the continuing evolution of the H1N1 pandemic virus in pigs.

 

But first, a little background.

 

Flu viruses are notoriously unstable, constantly trying out new mutations, and swapping gene segments (reassorting) with other influenza strains, in an attempt to produce a more efficient pathogen.

 

The reason is simple. 

 

Viruses leave behind antibodies in the hosts they infect. Unless they change enough to evade those antibodies, while remaining biologically `fit’, over time they would run out of susceptible hosts.

 

It’s simply a matter of viral survival.

 

Since pigs can be infected by more than one flu virus at the same time, the potential exists for two viruses to swap genetic material (reassort), resulting in a new hybrid strain.

 

 

The pandemic virus that emerged in the spring of 2009 was the end product of several influenza strains that had kicked around the world’s swine population for many years, trading bits of genetic material back and forth, until they produced a version capable of jumping to humans.

 

 

Last July I wrote about a Review: Evolution & Adaptation Of The 2009 pdmH1N1 Virus by Richard J. Webby PhD, et al. from the Department of infectious Diseases at St Jude Children’s Research Hospital in Memphis, Tennessee. 

 

The study looked at the potential for the pdmH1N1 virus to evolve, and in their conclusion they write:

 

The presence of this virus in swine, the propensity for swine to support reassortment, and the known ability, albeit limited, of viruses to move between swine and humans, create an opportunity for these substantial changes to occur.

 

Pigs are susceptible to a wide variety of influenza viruses, including human, avian, and swine varieties. A trait that many scientists believe makes them ideal `mixing vessels’ for influenza.

 

Complicating matters, in the not-too-distant past, pigs were usually raised and slaughtered locally.

 

Over the past couple of decades, however, the cross country - and even international – shipping of live hogs has become a big business.

 

According to Dr. Michael Greger’s excellent Flu Factories video presentation, pigs raised in the United States often travel hundreds of miles `on the hoof’, with many being `bred in North Carolina,  fattened in the corn belt of Iowa, but slaughtered in California’.

 

Which means that a swine virus that emerges in North Carolina could easily spread across the nation, carried either by live pigs, or on contaminated transport vehicles.

 

Internationally, live hogs are often shipped for breeding purposes, to inject genetic diversity into local herds to improve the breed.  

 

Last May, in You Say You Want An Evolution?, I wrote of a study published recently in the Journal Nature, that took an extensive look at the introduction and evolution of swine flu viruses in China over the past 12 years (supplemented by data going back 34 years).

 

CIDRAP, as part of their daily news scan, had a brief report on this as well (excerpted, and slightly reformatted for readability).

 

Imports helped swine flu in Hong Kong pigs to diversify

Importing pigs from Europe and North America over the past 30 years appears to have increased the genetic diversity of swine influenza in Hong Kong pigs through gene reassortment, according to a study today in Nature.

 

<SNIP>

 

The authors write, "Intercontinental virus movement has led to reassortment and lineage replacement, creating an antigenically and genetically diverse virus population."

 

 

So . . .  not only are swine viruses naturally evolving, by our actions, we are undoubtedly helping the process along.

 

All of which help serve as prelude to a couple of studies on influenza reassortment released yesterday. The first, in the Journal of Virology, looks at reassortments of the 2009 H1N1 virus among swine in Southern China. 

 

This study shares a number of authors with the Nature Journal study mentioned above – including such well known research scientists as  Richard J. Webby, Robert G. Webster, Yi Guan  and  J. S. Malik Peiris.

 

 

Novel reassortment of Eurasian Avian-like and pandemic/2009 influenza viruses in swine: infectious potential to humans

Huachen Zhu, Boping Zhou, Xiaohui Fan, Tommy T.Y. Lam, Jia Wang, Antony Chen, Xinchun Chen, Honglin Chen, Robert G. Webster, Richard Webby, Joseph S.M. Peiris, David K. Smith, and Yi Guan

 

Researchers found that more than half of the pigs tested were seropositive for multiple H1 flu viruses, and they discovered a hybrid virus that contained genes from a Eurasian avian-like H1N1 virus and those of the 2009 pdmH1N1 strain.

 

Lab testing showed this reassorted virus could be transmitted between pigs and ferrets, could replicate ex-vivo in human lung tissue, and ferrets that received the 2009 pandemic vaccine were only partially protected.

 

The authors state:

 

The continuing prevalence of the pdm/09 virus in pigs could lead to the genesis of novel swine reassortant viruses with the potential to infect humans.

 

 

The next stop is the CDC’s EID Journal, which published a study yesterday on multiple reassortments of the pdmH1N1 virus with endemic swine viruses in the United States.

 

 

Multiple Reassortment between Pandemic (H1N1) 2009 and Endemic Influenza Viruses in Pigs, United States

 

Mariette F. Ducatez, Ben Hause, Evelyn Stigger-Rosser, Daniel Darnell, Cesar Corzo, Kevin Juleen, Randy Simonson, Christy Brockwell-Staats, Adam Rubrum, David Wang, Ashley Webb, Jeri-Carol Crumpton, James Lowe, Marie Gramer, and Richard J. Webby Comments to Author

 

Abstract
As a result of human-to-pig transmission, pandemic influenza A (H1N1) 2009 virus was detected in pigs soon after it emerged in humans. In the United States, this transmission was quickly followed by multiple reassortment between the pandemic virus and endemic swine viruses.

 

Nine reassortant viruses representing 7 genotypes were detected in commercial pig farms in the United States. Field observations suggested that the newly described reassortant viruses did not differ substantially from pandemic (H1N1) 2009 or endemic strains in their ability to cause disease.

 

Comparable growth properties of reassortant and endemic viruses in vitro supported these observations; similarly, a representative reassortant virus replicated in ferrets to the same extent as did pandemic (H1N1) 2009 and endemic swine virus.

 

These novel reassortant viruses highlight the increasing complexity of influenza viruses within pig populations and the frequency at which viral diversification occurs in this ecologically important viral reservoir.

 

While none of this means that another pandemic virus is poised and ready to jump out of swine and into the human population, it does mean that the processes that create those kinds of viruses are alive and well.

 

And given the paucity of surveillance – particularly in developing countries, but even here in the United States – we may not get as much advance notice as we really need when the next virus does develop.

 

Whether the next pandemic virus springs forth from a reassortment of the pdmH1N1 virus, a triple reassorted H3N2 or H1N1 swine virus, or one of the many avian viruses (H5N1, H7’s, H9’s, etc.) circulating around the world, the message is clear.

Nature’s laboratory is open 24/7, and the evolution of viruses is not going to stop.

 

And we help contribute to these natural processes through globalization, increased international travel, intensive factory farming, the over-use of poultry vaccines and antivirals in animal feed, and the long distance transportation of live animals.

 

 

All of which tells us, while the last pandemic may be over, we must continue to prepare for the next one.

 

For more background on influenza in swine, and the risks of reassortment, you can’t do better than  Helen Branswell’s excellent Scientific American article from last December called Flu Factories, or her SciAm Podcast interview.

 

You may also wish to revisit these earlier blogs.

 

Hong Kong: Influenza Surveillance In Pigs

EID Journal: Swine Flu Reassortants In Pigs

The (Swine) Influenza Reassortment Puzzle

If You’ve Seen One Triple Reassortant Swine Flu Virus . . .