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Biosecurity Briefing
New Doubts Emerge about Migratory Birds’ Role in Spread of H5N1 By Jennifer Nuzzo, August 17, 2007 A perspective article published in the August issue of Emerging Infectious Diseases (EID) examines the role of migratory birds the long-distance spread of the H5N1 highly pathogenic avian influenza (HPAI) virus. The authors report that, contrary to frequent assertion, there is “no convincing evidence” that wild migratory birds are capable of spreading the H5N1 virus over long distances.1 The belief that migratory birds are responsible for the long-distance spread of H5N1 has been based on an assumption that “infected wild birds can remain asymptomatic and migrate long distances unhampered.” However, after reviewing many studies the authors conclude that the prolonged intense physical demands of long-distance migration can suppress the immune systems of birds, making it unlikely that birds could complete the trip and remain free of the symptoms of highly pathogenic H5N1. Rather, it is more likely that the ability to migrate long distances is “negatively affected by infections.”1 The authors concede that although the results of their analysis “do not offer a direct rebuttal,…they cast some serious doubts on the frequently repeated claim that wild birds can easily act as long-distance vectors for influenza A viruses.” Moreover, the studies they examined do not “rule out the possibility that HPAI (H5N1) may be transported relatively short distances by wild birds.” The role of wild migratory birds in spreading H5N1 is also questioned in a new report by the United Nation’s Food and Agricultural Organization (FAO), which analyzes recent H5N1 HPAI infections in wild that occurred in Germany, France, and the Czech Republic.2 The FAO notes that mortalities in these areas either occurred in non-migratory resident birds or in birds that would have migrated back into Europe at least two months prior to the outbreaks. Both of these observations indicate that the virus may not have arrived in these locations via wild bird migration. Rather, the FAO suggests that “a much more classical explanation for the initial outbreak is importation of contaminated egg shells or their crates, poultry, or unintentional spread of disease by workers, equipment, or other fomites.” Still, the FAO report stresses that additional “epidemiological investigations that take into consideration a full evaluation of biosecurity as well as production, trade, and wildlife mechanisms of disease introduction” are needed for more conclusive evidence.2 References
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