French authorities have been spraying swamps after the disease reached the south

Until last year, it had only ever been recorded in Africa, Asia and some Middle Eastern countries.

But in 1999, it spread. There was an epidemic in Romania, and more than 50 people in New York City became ill.

This year, 30 people have died from West Nile fever in Israel. France has also had cases.

The virus is spread by mosquitoes, which pick it up from wild birds. Dead crows in the United States have been found to be infected.

The revelation that the virus stays active for longer in house sparrows than in many other common birds comes from US expert Dr Nicholas Komar.

He told the annual meeting of the American Society for Tropical Medicine and Hygiene that sparrows also carried a higher concentration of the virus.

There is no cure for West Nile Fever. However, a new study published in the Journal of Infectious Diseases says a drug used to treat Hepatitis C, Ribavirin, could be useful in the short term.