— A new strain of H5N1 bird flu has emerged in China that is poised to start yet another global wave of infection.
Nearly three times as many Chinese poultry are infected with H5N1 now than last year, despite Chinas insistence that all poultry be vaccinated. In fact, vaccination may be the reason for the increase in infections, researchers say.
Yi Guan and colleagues at the University of Hong Kong have been testing poultry in markets across southern China for flu for years, the only such long-term monitoring in the world. Between mid-2004 and mid-2005 they found 0.9% of market poultry were carrying H5N1, including 2% of ducks, a major carrier of the virus.
Between then and June 2006, however, they found it in 2.4% of market poultry on average, a near-threefold increase. It now infects 3.3% of ducks. The team found the virus in chickens during 11 months of the year, up from four previously.
The reason, the researchers say, is a new Fujian-like strain of the virus, descended from one first seen in a duck in Fujian, China, in 2005. It caused 3% of poultry infections in September 2005, but 95% by June 2006.
More infected yet apparently healthy birds in Chinese markets for more of the year means more risk for humans. All but one of Chinas reported human cases of H5N1 happened after the Fujian strain started its rise, and some lived far from any known outbreak in poultry, but close to urban poultry markets.
There could be many more unrecognised cases. Serious cases of flu in humans, in China and elsewhere, are only tested for H5N1 if nearby poultry has suddenly died. But seemingly healthy, infected birds may cause human cases that are not tested while the spread of the virus in the poultry also goes unsuspected.
If death of poultry is used as the only indication of H5N1 infection, but the emergence of human cases is ignored, the consequence will be increased transmission of the virus in poultry, says Guan.
The team has no evidence that the virus is more virulent or more likely to transmit among humans than previous strains, he says. But it has caused one human death in Thailand, and the five Chinese cases for which the team has virus samples. As far as I know all 20 human cases recognised since November 2005 were caused by this virus, Guan told New Scientist.
Based on what previous H5N1 viruses in China have done, the team warns, the Fujian strain seems poised to start a third epidemic wave, after the first in 2004, and H5N1s spread across Eurasia in 2005. Fujian virus has so far spread to Thailand, Malaysia and Laos.
In November 2005 China ordered compulsory vaccination of all poultry. The law has been imperfectly applied Guan and colleagues found vaccine-induced antibodies in only 16% of the birds they tested. But they also found that those vaccine-induced antibodies do not recognise the Fujian virus, although they do attack the virus strains that Fujian has now replaced.
This means the Fujian strain has a selective advantage in vaccinated birds. This novel variant may have become dominant because it was not as easily affected as other strains by the current avian vaccine, says Guan. That may also be why H5N1 infection in Chinese poultry has surged, rather than decreased, despite increased poultry vaccination.
Worryingly, the antibodies being used to develop human vaccines for H5N1 have been induced from 2004 strains of the virus these antibodies do not recognise the Fujian strain. This means the current experimental pandemic vaccine would not work against any pandemic virus that emerged equipped with Fujian surface proteins.
Guan and colleagues want comprehensive influenza surveillance in both people and animals throughout the region, both to provide updates for vaccine developers and to track the real spread of the virus.
Journal reference: Proceedings of the National Academy of Sciences (DOI:0:1073/pnas.0608157103)
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