|Chickens in Hong Kong|
Experts have renewed their warnings of a bird flu pandemic after a new resistant strain of the H5N1 virus deadly to humans and poultry was found to have spread throughout the region.
Scientists in Hong Kong and the United States believe the new virus -- dubbed the "Fujian-like" strain -- may have mutated in response to vaccination programmes designed to halt the disease in farm flocks.
"The development of highly pathogenic avian H5N1 influenza viruses in poultry in Eurasia, accompanied with the increase in human infections in 2006, suggests the virus has not been effectively contained and that the pandemic threat persists," said a report in the American academic publication "Proceedings of the National Academy of Sciences".
"Studies suggest that H5N1 seroconversion (development of antibodies in blood serum as a result of infection or immunization) in market poultry is low and that vaccination may have facilitated the selection of the Fujian-like sublineage," it added.
The report -- co-written by Hong Kong University's microbiology supremo Yi Guan, who has led worldwide study into the disease -- said the strain had emerged in 2005, and had already spread throughout mainland China, as well as to Hong Kong, Thailand, Laos and Malaysia.
The Fujian-like strain was now the primary variant of the fast-changing virus throughout Asia, and was quickly replacing strains that had emerged in Hong Kong and Vietnam, it said.
"Analyses revealed the emergence and predominance of a previously uncharacterized H5N1 virus sublineage (Fujian-like) in poultry since late 2005," it said.
"The predominance of this virus over a large geographical region within a short period directly challenges current disease control measures," the report added.
The H5N1 strain of bird flu was first reported to have evolved into a form lethal to humans in Hong Kong in 1997, when six people died of the then mysterious disease.
A renewed outbreak in 2003 among poultry flocks in Asia triggered a wave of infections that has left more than 150 people dead throughout the world.
The World Health Organisation has expressed fears that a bird flu pandemic was almost certain in the near future, and in a worst case scenario could kill millions worldwide.