Birds seem to be developing the ability to fight off West Nile virus, reducing the likelihood that humans may contract the disease, scientists say.

Studies conducted in eastern North America, where the virus first appeared on the continent, show birds are becoming immune to the virus.

"This virus is maintained in a bird-mosquito life cycle," Margo Pybus, a wildlife disease expert with Alberta Fish and Wildlife, said. "So if you have increasing immunity in birds and mosquitoes, you've got less West Nile virus in Alberta.

Certain types of mosquitoes spread West Nile virus to humans.

"So that has less virus to spill over into humans, when humans might get bitten by mosquitoes."

West Nile is spread by specific types of mosquitoes. Dead birds in the crow family are typically the first indication that the virus has arrived in an area. The virus, which has flu-like symptoms, attacks the nervous system.

There were four confirmed cases of West Nile virus in Canada in 2004 and no deaths, Health Canada reported. In 2003, there were 14 deaths across the country, according to the department's website.

Dr. Karen Grimsrud, Alberta's deputy health officer, said even if birds become immune to the virus, people still need to be cautious.

"In New York, where they've had West Nile since the summer of 1999, they still continue to get human cases reported there," Grimsrud said. "So that tells us there are still birds that haven't got protection.

"So the virus is still circulating, still having human cases. It's not like we'll have this big peak of cases and nothing further."

Pybus said it's difficult to say how quickly birds become immune, and how much of the population will gain protection.

"We can't predict how quickly that immunity would spread to encompass 60 per cent of the birds, or 80 per cent, or 20 per cent of the birds," she said.

Health officials said people can protect themselves by wearing long-sleeved shirts and pants and using insect repellents that contain DEET. They also advise removing standing water from yards, because that's where mosquitoes lay their eggs.