Virology. Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

[Giuseppe]

Virology. 2009 Jun 4. [Epub ahead of print]

Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

Bahl J, Vijaykrishna D, Holmes EC, Smith GJ, Guan Y. State Key Laboratory of Emerging Infectious Diseases and Department of Microbiology, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Pokfulam, Hong Kong SAR, China.


Geographical separation of host species has shaped the avian influenza A virus gene pool into independently evolving Eurasian and American lineages, although phylogenetic evidence for gene flow and reassortment indicates that these lineages also mix on occasion.

While the evolutionary dynamics of the avian influenza gene pool have been described, the consequences of gene flow on virus evolution and population structure in this system have not been investigated. Here we show that viral gene flow from Eurasia has led to the replacement of endemic avian influenza viruses in North America, likely through competition for susceptible hosts.

This competition is characterized by changes in rates of nucleotide substitution and selection pressures. However, the discontinuous distribution of susceptible hosts may produce long periods of co-circulation of competing virus strains before lineage extinction occurs.

These results also suggest that viral competition for host resources may be an important mechanism in disease emergence.

PMID: 19501380 [PubMed - as supplied by publisher]
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[AlaskaDenise]

Re: Virology. Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

This competition is characterized by changes in rates of nucleotide substitution and selection pressures.
>>>are they saying those calculations to "date" strains aren't constant?

However, the discontinuous distribution of susceptible hosts may produce long periods of co-circulation of competing virus strains before lineage extinction occurs.
>>>so different strains can circulate in different locations that are on the same continent or same country, but don't physically mix often?

These results also suggest that viral competition for host resources may be an important mechanism in disease emergence.
>>>Is this another way of saying that as the human population increases (especially in density), more diseases will emerge?

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[gsgs]

Re: Virology. Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

this is about birds.


but I don't understand ... do American+Eurasian strains mix+compete or not ?

 

[super_flu]

Re: Virology. Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

This competition is characterized by changes in rates of nucleotide substitution and selection pressures.
>>>are they saying those calculations to "date" strains aren't constant?

The method used relaxes the strict molecular clock and allows for rates to vary between branches. This method allows for phylogenetically more accurate and precise trees while inferring a timescale to evolution. This is especially useful when sampling is sparse and missing data is common.

However, the discontinuous distribution of susceptible hosts may produce long periods of co-circulation of competing virus strains before lineage extinction occurs.
>>>so different strains can circulate in different locations that are on the same continent or same country, but don't physically mix often?

Transmission between hosts that primarily reside in different continental regions occurs in regions where these birds co-mingle. It takes time for a virus to spread through a population of susceptible hosts and alter the evolutionary landscape enough for competition to occur

These results also suggest that viral competition for host resources may be an important mechanism in disease emergence.
>>>Is this another way of saying that as the human population increases (especially in density), more diseases will emerge?

Changes in evolutionary pressures may result in the emergence of novel phenotypes, that may have increased virulence. In this paper, the authors show that strains possessing HA genes introduced from Eurasia replaced all endemic H6 viruses in the host population. Eventually these viruses caused disease in Californian poultry. Maybe this change in evolutionary landscape contributed to the emergence of disease.

 

[AlaskaDenise]

Re: Virology. Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

this is about birds.


but I don't understand ... do American+Eurasian strains mix+compete or not ?

Yes.

The results of the 2008 testing done in Alaska are here.

page 17 reports about Northern Pintails:

...From 38 isolates, they reported that 44% had at least one gene segment that was more close related to Asian than North American strains of LPAI. Conversely, several Asian isolates more closely resembled North American Pintail isolates than other Asian viruses. This study provides evidence that intercontinental transfer of influenza viruses occur and that Alaska is a plausible route of H5N1 introduction to North America, should the virus arrive via migratory birds."

87 species of wild birds are being monitored for signs of mixing with Eurasian strains.
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[AlaskaDenise]

Re: Virology. Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

Super flu:

Once again, thank you for taking time to answer our questions. I look forward to reading the entire paper when it becomes available.

It takes time for a virus to spread through a population of susceptible hosts and alter the evolutionary landscape enough for competition to occur

Is this "altering of the evolutionary landscape" referring to the years-long genetic changes directed toward a challenge to a predominant circulating stain?

Changes in evolutionary pressures may result in the emergence of novel phenotypes

...I look forward to reading a more indepth description of that process.

Also...
I looked for Alaskan bird species being tested for their Asian-connections, which also carried H6 influenzas. There was H6N1, H6N4, and H6N8 found in 2006 Northern Pintails testing. I'm not sure which H6 endemic strains you were referencing, but perhaps some migratory birds (like Pintails) still carry a remnant few H6s.

Spreading novel phenotypes may alter the evolutionary landscape and lead to competing strains. Personally, I wonder about the altered real landscape in areas where Asian & North American bird species mix. In 2006, it was reported that Alaskan wetlands had lost about 10,000 small lakes used for habitat, all because of permafrost melting and subsequent water drainage. Those impacted species must alter their nesting locations and associated species mixing.

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[super_flu]

Re: Virology. Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

Super flu:

Once again, thank you for taking time to answer our questions. I look forward to reading the entire paper when it becomes available.


Is this "altering of the evolutionary landscape" referring to the years-long genetic changes directed toward a challenge to a predominant circulating stain?

It's my pleasure, I enjoy discussing literature. - I should thank ironorehopper for scouring all the different sources and putting them up in one place. It's quite handy!

This conversation could potentially fill with jargon. Let's forget 'evolutionary landscape' for a moment.

The evidence presented in this paper is from the interaction of H6 viruses circulating in wild birds. One group consisted of H6 viruses that had circulated in North America for a long time. The second set contained viruses with a H6 hemagluttinin derived from Eurasian ancestors. The consequence of these viral lineages competing was that those viruses containing the H6 gene derived from Eurasia replaced the viruses that had been circulating in wild birds previously. Something changed that exerted evolutionary pressure on the virus.

If I had to venture a guess, the introduced viruses caused mild disease and the host immune response was specific for the endemic virus not the newly introduced one. This increased evolutionary pressures on the endemic virus which started to rapidly diversify. They present a whole bunch of statistical tests and different parameters. But, in general such pressures may cause a shift in the "evolutionary landscape" and select for viruses with greater fitness. - Hmm, maybe I should use fitness landscape instead of evolutionary landscape. I'll look this up.

The consequences are two fold -

1) Eurasian and North American derived viruses are mixing populations - even though these introductions may not become established often.

2) viruses compete for hosts in the natural reservoir and the evolution even in these hosts is dynamic and complex.

...I look forward to reading a more indepth description of that process.

Also...
I looked for Alaskan bird species being tested for their Asian-connections, which also carried H6 influenzas. There was H6N1, H6N4, and H6N8 found in 2006 Northern Pintails testing. I'm not sure which H6 endemic strains you were referencing, but perhaps some migratory birds (like Pintails) still carry a remnant few H6s.

Seems even more interesting in light of this paper. Potentially that virus lineage hasn't been eliminated, or perhaps this will confirm what they present.

I believe the North American endemic H6 lineage has not been detected in North America for several years. It would be interesting if this lineage is discovered dormant in another host, and, how has this virus changed and evolved in that time.

Spreading novel phenotypes may alter the evolutionary landscape and lead to competing strains. Personally, I wonder about the altered real landscape in areas where Asian & North American bird species mix. In 2006, it was reported that Alaskan wetlands had lost about 10,000 small lakes used for habitat, all because of permafrost melting and subsequent water drainage. Those impacted species must alter their nesting locations and associated species mixing.

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Absolutely, it's why surveillance remains a priority in any pandemic preparedness plan and in any conservation efforts as well.

 

[sharon sanders]

Re: Virology. Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

I would also like to say thank you again Super flu and, of course, to: Ironorehopper, who lives in Italy, Gsgs, who lives in Germany, and AlaskaDenise for this thread.

We are very lucky to have such an international team helping to illuminate the facts.

Thanks again!

 

[AlaskaDenise]

Re: Virology. Gene flow and competitive exclusion of avian influenza A virus in natural reservoir hosts.

Yes.

The results of the 2008 testing done in Alaska are here.

page 17 reports about Northern Pintails:

...From 38 isolates, they reported that 44% had at least one gene segment that was more close related to Asian than North American strains of LPAI. Conversely, several Asian isolates more closely resembled North American Pintail isolates than other Asian viruses. This study provides evidence that intercontinental transfer of influenza viruses occur and that Alaska is a plausible route of H5N1 introduction to North America, should the virus arrive via migratory birds."

87 species of wild birds are being monitored for signs of mixing with Eurasian strains.
.

To clarify - those 87 species refer only to the one's being monitored in Alaska. I'm sure there are additional species in NE Canada - another intercontinental mixing area.

An interesting fact , is that of the Northern Wheatears (2 slightly diverse subspecies) that winter in the Senegal-Ivory Coast, etc. area, one subspecies travels up through W. Europe, Greenland, etc. to summer in NE Canada, while the other travels across the Mediterranean, Turkey, Asia, Alaska, to the Yukon area. Parts of their 2 summer nesting areas almost overlap in Northern Canada. Imagine traveling around the world only to meet your winter next-door neighbor. Climate-change-driven lost habitat in the American arctic may create the circumstances for those species to mix twice a year. I wonder how that would alter influenza evolution?

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