Background on H5N1 in China - are wild birds to blame?

With rampant speculation that wild birds are spreading H5N1 throughout China I thought it important to summarize some of the available information on H5N1 avian influenza in China.

My working hypothesis is that the H5N1 avian influenza outbreak in domestic poultry in Tacheng is a continuation of the 2004 avian influenza epizootic in Chinese domestic poultry. The virus was widespread across China in 2004 with much opportunity for it to circulate without detection after its supposed eradication. Certainlythis happened after the 1997 outbreak in China (see below).

I stand by my previous hypothesis that the outbreak in waterfowl at Qinghai represents a case of spillover from domestic poultry to migratory waterfowl. It remains to be seen whether the spillover occured locally in China or to birds in Southeast Asia which consequently brought it to China.

Although temporally coincident, there is no evidence that the Qinghai waterfowl outbreak and the Tacheng poultry outbreak are related. Certainly the Chinese government promoted the connection in the OIEpost, but I have yet to see evidence.

During outbreaks of highly pathogenic avian influenza, the virus is rarely isolated from wild birds even on affected poultry farms (Swayne and Suarez, 2000; Li et al 2004). In fact, mass die-offs in wild birds associated with avian influenza have only been reported a handful of times (common terns in South Africa in 1961, Becker, 1966; waterfowl and other birds in 2002, Ellis et al. 2004; and the current outbreak in Qinghai, China). Highly-pathogenic strains (HPAI) likely do not circulate in wild populations although wild birds may become infected during outbreaks in domestic poultry (Nestorowicz et al.1987, Melville and Shortridge, 2004).

Surveillance for avian influenza in wild birds is critical to resolving the question of their role in transmission. We need to compare genetic sequences from the viruses isolated at Qinghai to the virus circulating in Southeast Asia as well as the new outbreak inTacheng.

The only effective strategy to prevent transmission of avian influenza from poultry to wild birds (spillover) or from wild birds to poultry (spillback) is improved biosecurity for poultry producers. Contact between wild and domestic birds must be prevented and/or reduced.

Culling of wildlife to control disease is not an effective management strategy. Dr. Wobeser provides some discussion on this topic.

A some background on H5N1 in China:

- H5N1 was first found in Hong Kong and China in domestic waterfowl in 1997. Hong Kong culled all poultry, and the virus was not isolated again in Hong Kong until 2001 (Chen et al. 2004). Although the
outbreak was thought to have ended, the virus circulated in China, from 1999 until 2004 (Cauthen et al 2000, Li et al 2004)

- During these earlier outbreaks, no mortality was seen in wild birds. In 2002, waterfowl and other birds were found to die of H5N1 during an outbreak in captive and semi-captive birds in two zoological parks
in Hong Kong. The outbreak was contained (Ellis et al. 2002).

- By February of 2004, H5N1 was detected in domestic poultry across much of the country (see map), and in pigs. Pigs can be infected with H5N1 without showing clinical signs.

- The H5N1 outbreak continued in 2004, with reports as late as June, however there were no further reports.

References

Becker, W.B. 1966. The isolation and classification of Tern virus:
influenza A-Tern South Africa - 1961. Journal of Hygiene 64:309-320.

Cauthen, A.N., D.E. Swayne, S. Schultz-Cherry, M.L. Perdue, D.L.
Suarez. 2000. Continued circulation in China of highly pathogenic
avian influenza viruses encoding the hemagglutinin gene associated
with the 1997 outbreak in poultry and humans. Journal of Virology 74:
6592-6599.

Chen, H., G. Deng, Z. Li, G. Tian, Y. Li, P. Jiao, L. Zhang, Z. Liu,
R.G. Webster, and K. Yu. 2004. The evolution of H5N1 influenza viruses
in ducks in Southern China. Proceedings of the National Academy of
Sciences 101:10452-10457.

Ellis, T.M. R.B. Bousfield, L.A. Bissett, K.C. Dyrting, G.S.M. Luk,
S.T. Tsim, K. Sturm-Ramirez, R.G. Webster, Y. Guan, and J.S.M. Peiris.
Investigation of outbreaks of highly pathogenic H5N1 avian influenza
in waterfowl and wild birds in Hong Kong in late 2002. Avian Pathology
33:492-505

Li, K.S., Guan, Y., Smith, G.J.D., Xu, K.M., Duan, L., Rahardjo, A.P.,
Puthavathana, P., Buranathal, C., Nguyen, T.D., Estoepangestie,
A.T.S., Chaisingh, A., Auewarakul, P., Long, H.T., Hanh, N.T.H.,
Webby, R.J., Poon, L.L.M., Chen, H., Shortridge, K.F., Yuen, K.Y.,
Webster, R.G., and J.S.M. Peiris. 2004. Genesis of a highly pathogenic
and potentially pandemic H5N1 influenza virus in eastern Asia. Nature
430: 209-213.

Melville, D.S., and K.F. Shortridge. 2004. Influenza: time to come to
grips with the avian dimension. The Lancet Infectious Diseases
4:261-262.

Nestorowicz, A., Y. Kawaoka, W.J. Bean, and R.G. Webster. 1987.
Molecular analysis of the hemagglutin genes of Australian H7N7
influenza viruses: role of passerine birds in maintenance or
transmission. Virology. 160: 411-418.

Swayne, D.E and Suarez, D.L. 2000. Highly pathogenic avian influenza.
Revue of Science and Technology of the Office International des
Epizooties 19:463-482.

Damien
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