Anyone can protect a mouse, but the horse result is quite promising

Tom Monath Acambis

Until 1999 it had only ever been recorded in Africa, Asia and some Middle Eastern countries.

But two years ago a wave of infection took New York by surprise, killing seven people and making dozens of others sick.

Many wild birds succumbed to the virus, as did dozens of horses. Last year an outbreak in southern France killed about 20 horses.

Virus family

Because there has been no vaccine against the virus, public health efforts have focused on controlling mosquitoes, which spread the disease.

But New Scientist magazine reports that Dr Jeffrey Chang of the Centers for Disease Control and Prevention in Fort Collins, Colorado, has now successfully developed a vaccine.

West Nile virus is part of a family that includes Japanese encephalitis, and Dr Chang's team was already designing vaccines against that virus.

They were able to modify that vaccine to target West Nile virus.

Genetic material

The vaccine contains genetic material from the West Nile virus.

When injected into animals it triggers an immune response.

Dr Chang said: "It looks like the virus to the immune system. But it is completely harmless."

Within three months, the researchers had showed the vaccine could protect mice from infection, so they moved on to a more relevant species.

Four horses were given a single injection of the DNA. Thirty-nine days later, mosquitoes carrying West Nile virus were allowed to bite them.

None of the horses developed any signs of viral replication, fever, infection or sickness.

Funding search

In contrast the virus replicated rapidly in seven out of eight unvaccinated horses.

Dr Chang thinks his vaccine might also protect people and is looking for funding to test it on monkeys.

He also plans to test his vaccine on American crows to see if it is versatile enough to protect birds as well.

Tom Monath, of Acambis, a vaccine company based in Massachusetts, said: "Anyone can protect a mouse, but the horse result is quite promising.

"Many vaccine approaches don't work in animals of that scale."

Dr Monath's company is also developing a human vaccine based on weakened yellow fever virus containing West Nile genes. But it is yet to report results.