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Wildlife and H5N1 HPAI - Current Knowledge

A recent press release (http://www.fao.org/news/story/en/item/87196/icode/) has raised concerns about the H5N1 Avian Influenza (“bird flu”) and the role of wild birds in the disease ecology.  From the press release:

"...... 2008 marked the beginning of renewed geographic expansion of the H5N1 virus both in poultry and wild birds. The advance appears to be associated with migratory bird movements, according to FAO Chief Veterinary Officer Juan Lubroth. He said migrations help the virus travel over long distances, so that H5N1 has in the past 24 months shown up in poultry or wild birds in countries that had been virus-free for several years. "Wild birds may introduce the virus, but peoples' actions in poultry production and marketing spread it," Lubroth noted.  Recently affected areas are to be found in Israel and the Palestinian Territories, Bulgaria, Romania, Nepal and Mongolia."

Information is presented here to address some confusion regarding the above section of the press statement, clarify the current disease situation and describe ongoing control activities being implemented.

Influenza viruses are continuously evolving through reassortments and mutations.  This constant change requires on-going monitoring and regular vaccine updates to prevent significant mortalities.  There has been an increase in the number of cases identified in the northern hemisphere where H5N1 HPAI had been previously eliminated.  These cases are likely due to a combination of poultry trade and wild bird migration.  However, it is important to remember that wild birds are not considered a reservoir for any known H5N1 virulent virus and the H5N1 virus is rarely isolated from healthy wild birds.

Wildlife, poultry or poultry product movements (legal and illegal), and fomites can all potentially introduce virus into new locations.  Once the infection is introduced, subsequent transmission often occurs through traders, at both markets and slaughterhouses.  Persistence of H5N1 viruses is probably associated with duck production in irrigated rice paddies.  Domestic ducks in post-harvest rice paddies may mingle with wild waterbirds.

H5N1 HPAI viruses belonging to the group are in circulation in Viet Nam, India, China, Bangladesh, and Myanmar.  In 2011 a variant of H5N1 was isolated from poultry in Viet Nam and China belonging to the clade (the variant is currently called 2.3.2B) for which the current available vaccines are not entirely effective. Vaccine failures likely associated with infection from this strain have occurred during the past year.  To date, none of the strains of H5N1 have been isolated from healthy wild birds making it difficult to determine the exact mechanism by which this new strain is spreading, but we recall that with the Qinghai strain of H5N1, wild birds are believed to periodically, or intermittently transmit virus, especially along the central Asia flyway.  For strains, most cases have been associated with poultry with some additional wild bird mortalities.

From the human health perspective, the overwhelming number of human cases can be attributed to contact with poultry since the emergence of HPAI H5N1.  This remains the situation for recent human cases attributed to the strain, although there have not been any human cases identified associated with the new 2.3.2B variant.  Concurrently, Cambodia has experienced more human H5N1 infections during 2011 than in any previous year, but these infections were not related to the clade, but instead caused by an older H5N1 clade 1 virus that has been circulating endemically in the Mekong basin since 2004.  To date, there is no evidence indicating any particular strain of the H5N1 virus to be more virulent or more transmissible to humans.

The evolution of this virus confirms that as a global community, we need to maintain vigilance against this disease including maintaining sensitive surveillance systems, respond rapidly to reported outbreaks, and apply poultry related management measures including good biosecurity practices, quarantine, humane culling, and vaccination when appropriate. H5N1 viruses will likely persist globally and continue to cause significant losses to poultry production, wild bird populations, and threaten human health.  Prevention strategies are already being implemented, including steps to develop and market an updated poultry vaccine that will protect against the 2.3.2B strain that current vaccines are not protective against.   Additionally, encouraging safe practices at poultry holdings in high risk areas for this disease will help prevent the further spread of the virus and other pathogens.

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