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Migratory birds: an easy explanation or sound epid
« on: Sep 7th, 2005, 1:34pm » |
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Migratory birds: an easy explanation or sound epidemiology?
Posted on ProMed on 5 september 2005
Dr. George Arzey
Senior Veterinary Officer (Avian Health)
NSW Department of Primary Industries
Elizabeth Macarthur Agricultural Institute
Australia
Considering the reported widespread infection with H5N1 in SE Asia and the recent findings in Central Asia attributed to migratory birds, Australia and NZ are indeed an enigma: 2 islands endowed with wild aquatic birds, on the migration zone of several flyways in and out of SE Asia, and yet untouched by the subtype ravaging this continent.
Since the emergence of the AIV Goose/GD/96 in the province of Guangdong in 1996, it is estimated that approximately 27 million migratory birds have visited Australian shores, approximately 3 million birds per year (according to Wetlands International 2002, Waterbirds population estimates 3rd Ed., Global Series 2002 no 12).
Another intriguing aspect is that while both countries are on the migration route of several flyways, Australia has experienced 5 outbreaks in chicken flocks but NZ has experienced none.
In the 5 outbreaks in Australia, all in intensive poultry units, the H7 subtype was involved, and in all 5 outbreaks migratory/aquatic birds were considered the source. This conclusion was largely based on (1) the premise that aquatic wild birds are a significant reservoir of AIV and (2) anecdotal evidence that suggested presence of aquatic wild birds in the vicinity of an infected farm or inhabiting a body of water that supplied water to the infected farm. AIV of the H7 subtypes were never reported in Australia or NZ in wild waterfowl before, during, or after the outbreaks (Arzey G. The role of aquatic wild birds in the epidemiology of avian influenza in Australia. Aust Vet J 2004; 82(6), Jun). Several other AIV subtypes were found in wild waterfowl during surveys over the years, but none of those was ever found in the infected chicken flocks. During the last outbreak in Australia (1997), the same AIV subtype which was found in chicken (but of lower virulence) was isolated from farmed emus adjacent to the infected biosecured index chicken shed. The emu as a possible source of infection was largely ignored in favour of the water from the river being the source of the infection in the chicken, despite the fact that it was chlorinated (albeit with fluctuating levels of chlorine).
Considering the repeated outbreaks in Australia with H7 and the uniqueness of emus to Australia, this population should perhaps receive more consideration as a potential reservoir of AIV infection in Australia.
While it is acknowledged that absence of evidence is not necessarily evidence of absence of infection with H7 subtype in Australian and NZ aquatic wild birds, it is highly significant that the H7 subtypes isolated from the AI outbreaks in Australia between 1985 to 1994 were all phylogenetically delineated from H7 subtypes found in other regions of the globe and the Australian H7 subtype formed a separate sublineage (Aust Vet J 2004; 82(6), Jun). Would this delineation persist for such a long period if migratory birds were responsible for the AI outbreaks in Australia?
Unlike Europe, Australian and NZ Anatides are considered nomadic within their respective countries and generally do not migrate internationally or intercontinentally. While the European data (see Avian influenza - Europe
(03): migratory birds, northern Europe 20050821.2463) may point to a strong association between migratory birds and outbreaks in domestic poultry, this has not been consistently the case, for example, in North America, where the infections were, in significant numbers of outbreaks, related to local bird markets. When the spread of the current epidemic in SE Asia occurred, migratory waterfowl were almost instantaneously blamed as the source, although the timing and distribution of several new outbreaks did not fit any known migratory pattern for any species including terrestrial birds (Melville DS & KF Shortridge. Reflection and reaction. Lancet Infect Dis 2004; 4: May). The presence of H5N1 in live bird markets as early as 1999 (Hong Kong in geese) and 2001 in Vietnam in Geese imported from China (Nguyen, et al. Isolation and characterisation of Avian Influenza viruses, including HP H5N1 from poultry in live bird markets in Hanoi, Vietnam in 2001. J Virology 2005; 79(7): 4201-12), provided a credible alternative explanation for H5N1 outbreaks in domestic poultry. The paper by Nguyen et al (2005) identified the domestic duck as being the major reservoir of the AIV pool in nature and the live bird markets in Asian countries as a suitable environment for reassortment and transmission.
Perhaps the Australia and NZ scenario provides another reason to examine the possible association between migratory birds and outbreaks in domestic poultry with an open mind.
Requiring a thorough examination of the evidence that links migratory birds or other wild waterfowl and AI outbreaks in domestic poultry is not aimed to weaken or to question the concept of biosecurity. The promotion of the concept of biosecurity and exclusion of wild birds from poultry enterprises should be viewed as a tool to reduce disease risks rather than as an undisputed epidemiological association and acceptance of the direct role of wild birds in all AI outbreaks on poultry farms.
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| « Last Edit: Sep 7th, 2005, 1:37pm by HKBWS Project » |
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