Mar 3, 2004 (CIDRAP News) – More birds have died or been sacrificed in the current avian influenza outbreaks in Asia than in the five largest previous outbreaks of avian flu, the World Health Organization (WHO) reported yesterday.

In a report comparing the current outbreaks with previous ones, the WHO said avian flu until recently was considered a rare disease. Most countries have had no previous experience with it, making it harder for them to cope with the disease now, the agency said.

More than 100 million birds in eight Asian countries have died of avian flu caused by the H5N1 virus or have been killed to contain it. The virus has infected 33 people, 22 of whom have died. The human cases have sparked concern that the avian virus could mix with a human flu virus and generate a new strain that could start the world"e;s first flu pandemic since 1968.

"Up to the end of 2003, highly pathogenic avian influenza (HPAI) has been considered a rare disease," the WHO stated. "Since 1959, only 21 outbreaks had been reported worldwide. The majority occurred in Europe and the Americas. Of the total, only five resulted in significant spread to numerous farms, and only one was associated with spread to other countries."

The largest previous avian flu outbreak was last year"e;s H7N7 outbreak in the Netherlands, Belgium, and Germany, the WHO said. The disease prompted the destruction of more than 30 million birds in the Netherlands, 2.7 million in Belgium, and 400,000 in Germany.

Other major outbreaks occurred in Pennsylvania from 1983 to 1985, Italy in 1999 and 2000, Mexico in 1995, Pakistan in 1994, and Hong Kong in 1997. Seventeen million birds were killed in the Pennsylvania outbreak and 14 million in the Italian one. A mild strain of the H5N2 virus that caused the 1995 Mexican outbreak has not yet been eradicated from the country, despite years of intense efforts, the WHO said.

The outbreaks in Pennsylvania, Italy, and Mexico initially involved mild illness. "When the virus was allowed to continue circulating in poultry, it eventually mutated (within 6 to 9 months) into a highly pathogenic form with a mortality ratio approaching 100%," the WHO reported.

Factors making the Asian outbreaks particularly hard to control include countries"e; lack of experience with the disease and the prevalence of backyard poultry flocks, the agency said. The economic importance of poultry and a shortage of resources also add to the difficulty.

Wild birds can introduce viruses of low pathogenicity into domestic birds, but there is no conclusive evidence that wild birds were the source of the H5N1 virus now circulating in Asia, the WHO said. However, some of the current Asian outbreaks "have been linked to contact between free-ranging flocks and wild birds, including the shared use of water resources."

The WHO said wild birds should not be killed, but farmers should try to prevent contact between poultry and wild birds, especially waterfowl. "An especially risky practice is the raising of small numbers of domestic ducks on a pond in proximity to domestic chicken and turkey flocks," the agency said. "Domestic ducks attract wild ducks, and provide a significant link in the chain of transmission from wild birds to domestic flocks."

Culling of flocks "remains the first line of action" to battle avian flu, the WHO said. Culling is recommended for birds infected with or exposed to H5 and H7 viruses even if they initially show low pathogenicity.