H5N1 and Bird Migration

Many people who follow H5N1, specifically HP (Highly Pathogenic) H5N1, and its steady progress across the globe believe that it is only a matter of time before a migrating bird brings it to North America. I believed this as well until I attended the National Pandemic Conference held in Conover, NC in September 2006 and heard Dr. Michael Osterholm's beliefs on this.

It was he that first brought to my attention that there are numerous Avian Influenza strains that have existed for a very long time in the Far East, and China specifically, that have not made it to North America, even after decades of circulation there.

First defining Low Pathogenic vs. Highly Pathogenic avian influenza:

A snip from the CDC's Avian Influenza web page:

Low Pathogenic versus Highly Pathogenic Avian Influenza A Viruses

Avian influenza A virus strains are further classified as low pathogenic (LPAI) or highly pathogenic (HPAI) on the basis of specific molecular genetic and pathogenesis criteria that require specific testing. Most avian influenza A viruses are LPAI viruses that are usually associated with mild disease in poultry. In contrast, HPAI viruses can cause severe illness and high mortality in poultry. More recently, some HPAI viruses (e.g., H5N1) have been found to cause no illness in some poultry, such as ducks. LPAI viruses have the potential to evolve into HPAI viruses and this has been documented in some poultry outbreaks.

In general, direct human infection with avian influenza viruses occurs very infrequently, and has been associated with direct contact (e.g., touching) infected sick or dead infected birds (domestic poultry).

 

Now we have a paper published on PlosPathogens (Hat Tip Crof of H5N1) that supports what Dr. Osterholm intuited early on in the debate.

Influenza in Migratory Birds and Evidence of Limited Intercontinental Virus Exchange

Scott Krauss1, Caroline A. Obert2, John Franks1, David Walker1, Kelly Jones1, Patrick Seiler1, Larry Niles3, S. Paul Pryor4, John C. Obenauer2, Clayton W. Naeve2, Linda Widjaja5, Richard J. Webby1, Robert G. Webster1*

Migratory waterfowl of the world are the natural reservoirs of influenza viruses of all known subtypes. However, it is unknown whether these waterfowl perpetuate highly pathogenic (HP) H5 and H7 avian influenza viruses. Here we report influenza virus surveillance from 2001 to 2006 in wild ducks in Alberta, Canada, and in shorebirds and gulls at Delaware Bay (New Jersey), United States, and examine the frequency of exchange of influenza viruses between the Eurasian and American virus clades, or superfamilies. Influenza viruses belonging to each of the subtypes H1 through H13 and N1 through N9 were detected in these waterfowl, but H14 and H15 were not found. Viruses of the HP Asian H5N1 subtypes were not detected, and serologic studies in adult mallard ducks provided no evidence of their circulation. The recently described H16 subtype of influenza viruses was detected in American shorebirds and gulls but not in ducks. We also found an unusual cluster of H7N3 influenza viruses in shorebirds and gulls that was able to replicate well in chickens and kill chicken embryos. Genetic analysis of 6,767 avian influenza gene segments and 248 complete avian influenza viruses supported the notion that the exchange of entire influenza viruses between the Eurasian and American clades does not occur frequently. Overall, the available evidence does not support the perpetuation of HP H5N1 influenza in migratory birds and suggests that the introduction of HP Asian H5N1 to the Americas by migratory birds is likely to be a rare event.

Continues…

 

As I am known to say: "Rarely doesn't mean never", or some iteration thereof; but this paper lends credence to the belief that instead of HP H5N1's arrival into North America via migratory birds is a foregone conclusion, it would instead be an outside happenstance.

 

If we see an introduction of HP H5N1 into North America, still not a certainty, we are much more likely to see it via the illegal trade of birds or the introduction of it by the poultry industry as happened in the UK recently.

Or, there is always the possibility that our "native" clade of LP H5N1 (genetically distinct virus) will spontaneously morph into an HP version, but that doesn't invalidate this paper's finding. In that case it would be an independent "birthing", still genetically distinct, and not an introduction via the migrating birds.

 

All-in-all it gives me a modicum of comfort as I watch the Canada Geese that reside around the lake that abuts my home.

 

SZ