Hundreds of people are believed to have caught bird flu from infected poultry, but were not diagnosed because their symptoms were too mild, scientists revealed yesterday. The finding suggests some populations have developed natural resistance to the virus. But experts warn that the virus also appears to spread more easily to humans than they realised.
The study of more than 45,000 people in Vietnam, which has been struggling with an outbreak of the highly pathogenic H5N1 virus in poultry since late 2003, reveals that up to 750 people are very likely to have become infected with the strain after handling sick or ill birds.
Dr Anna Thorson, who led the study at the Karolinska Institute in Stockholm, said the real figure potentially extended into the thousands. The cases were not picked up because, while people developed coughs and a fever, they either failed to seek medical care because their symptoms were mild, received treatment from unregistered practitioners or were missed by the doctors inspecting them. The finding unravels one of the mysteries of the H5N1 strain of bird flu: its lethality. Doctors have estimated the virus kills half of those it infects, but according to Dr Thorson, hundreds to thousands of people may be infected, but have mild symptoms and do not get admitted to hospital, thereby failing to appear in official figures.
The researchers studied interviews with 45,478 people in FilaBavi, a Vietnamese demographic surveillance site with confirmed outbreaks of H5N1 in poulty during April to June 2004. Eighty-four per cent lived in households that kept poultry and 25.9% reported birds falling ill or dying of flu in that time. Of those, between 650 and 750 people suffered flu-like symptoms after handling the birds.
"During the widespread Asian highy pathogenic avian influenza (HPAI) epidemic in poultry, the disease has been reportedly rare in humans. Our findings, however, suggest that in populations living in close contact with poultry, in areas endemic for HPAI, transmission to humans may be frequent," the scientists write in the journal Archives of Internal Medicine.
Professor Peter Dunnill, an expert on vaccines for avian flu at University College London said rural communities in Vietnam have been living with bird flu for more than 10 years and may have developed resistance to it. "It may well be that there is some immunity in Turkey too, but in Britain, we're going to have no exposure to this virus at all, and that is a big worry," he said.
