Health experts are warning that a potentially fatal mosquito-borne virus that has struck New York could spread across the western hemisphere.

City officials now say that more than twice as many people as previously thought may have died from the disease. Health experts originally believed that they had to deal with an outbreak of St Louis encephalitis.

But the illness has now been reclassified as West Nile virus, which has never been encountered in the US before.

Birds which carry the infection are now beginning to migrate southwards taking the risk to the rest of the country.

New Yorkers, worried that their city was the target of a killer virus, have been buying insect spray by the crate.

Seventy infected

West Nile-type virus is spread by mosquitoes

And the news that the virus is one that has never before been seen in the US has them even more alarmed - particularly as officials now believe up to eight people may have died from it rather than the three they had previously identified.

They believe that up to 70 more may have been infected.

West Nile Virus has symptoms very similar to those of St Louis encephalitis, which is why officials say they had the two diseases confused.

It is transmitted to humans by mosquitoes which have bitten infected birds. The discovery of dozens of dead crows across the region suggests that it has spread far further than previously thought.

The insects carrying West Nile virus are likely to have travelled in shipping containers or luggage.

A doctor at the Centers for Disease Control (CDC) says this is one of a worrying number of diseases establishing themselves in the United States for the first time.

Spraying insecticide

"It's very easy for pathogens to go from one area to another area, from areas where they naturally occur to places where they shouldn't occur," he said.

Helicopters are spraying insecticide on a daily basis in the hope of bringing this outbreak under control.

But Duane Gubler, director of the CDC's Division of Vector-Borne Infectious Diseases, has said there is "considerable potential" for the virus to spread to Central and South America with the infected birds.

The disease cannot be passed from human to human or from birds to humans.