BEIJING, June 6 (Xinhua) -- The Chinese government should review the strategies and effects of the bird flu control efforts of the past two years and improve them to cope with the epidemic which is still a serious threat, said a Chinese scientist here on Monday.

    "When, and to what extent, the current avian influenza virus could evolve into a human pandemic is unpredictable. We should do our best to reduce the risk of a human pandemic influenza breaking out and make necessary preparations before such a risk becomes reality," said Chinese bird flu control expert Liu Xiufan.

    Liu, a member of the Chinese Academy of Engineering (CAE), said at a national conference of the CAE members that controlling the H5N1 virus in poultry at its source is the best way to reduce or even eliminate the risk of a human pandemic virus.

    He said the full range of control measures should include enforced bio-security of poultry farms and restriction on the movement of poultry and products, culling of infected poultry, quarantine, disinfection, and prudent use of vaccines.

    Some changes in the H5N1 virus have taken place recently. The virus has increased virulence to ducks, and the currently available vaccines are not effective for protecting poultry, said Liu.

    The H5N1 viruses isolated during the 2004-2006 period have increased their ability to replicate in mammalian cell culture. The transmission mode of the viruses is changing from fecal-oral to aerosol, said the scientist, adding that the viruses have increased resistance to the environment, especially to temperature.

    He noted that it is a big challenge for China to eradicate the H5N1 viruses because the viruses have been circulating in poultry in China for some time.

    The outbreaks of bird flu have affected vast areas of China. The extensive presence of waterfowls and vaccinated birds as the carriers of the H5N1 virus has increased the difficulty of effective control and eradication, Liu said.

    China produces 3.7 billion waterfowls each year, more than 75 percent of the world´s total.

    Huge numbers of small poultry holders scattered all over China increase the difficulty of disease prevention and control, Liu added.

    He said that in some Asian countries, such as Japan, Republic of Korea and Malaysia, the outbreaks of bird flu in poultry did occur several times during 2004-2005, but no human cases appeared.

    The reason is that the epidemics in poultry were stamped out very quickly in a short time, the scientist said.

    Currently the transmission efficiency of H5N1 virus from avian species to human is very low, as the species barrier still exists. However, the H5N1 virus is changeable, and might acquire the ability to cross the species barrier to transmit infection to mammals and humans through gene mutation and re-assortment, Liu said.

    "Therefore, the better the avian flu in poultry is prevented and controlled, the less in quantity and the shorter in time the H5N1 virus exists in the environment, the less the risk that avian flu might evolve into human pandemic flu," said Liu.

    The government should sponsor a review of the strategy and its implementation and to evaluate the effectiveness of the prevention and control of bird flu in the last two years, Liu said.

    Because bird flu infection has become endemic in some areas in China and cannot be stamped out in a short time, the government should draw up a short-term plan of prevention and control and a long-term program of eradication, Liu added.

    More than 80 outbreaks of bird flu have been reported in China since February 2004, affecting 24 provinces, municipalities and autonomous regions. Eighteen confirmed human cases with 12 fatalities have been reported since last September.