Millions more birds have died from West Nile virus, and the infectious disease is affecting more species than had previously been believed, researchers say.

The mosquito-borne disease, which can be fatal to both birds and people, arrived in North America 16 years ago and has spread rapidly, they say.

Birds are the virus' primary host, and in just five years, millions of birds have been left dead, victims of the disease.

For the study, the researchers looked at 49 species of North American birds and found West Nile virus had negatively affected almost half of them.

"Many more species of birds than we thought are susceptible to this virus," says researcher T. Luke George at Colorado State University.

One of the findings of the study was that the virus was having long-term effects on the population growth of some species, he says.

"Prior to this study, we generally thought the West Nile virus had a very short-term affect on bird survival," he says.

A team including researchers from CSU, Washington University, UCLA and The Institute for Bird Populations used maps of West Nile virus reports among humans in the U.S., looking for a correlation with bird populations since mosquitoes transmit the virus from birds to humans.

They combined that with data collected from 1992 to 2007 at stations across the United States where birds are regularly trapped, banded and checked for West Nile virus.

That allowed them to determine whether the virus was affecting various species and populations of birds and whether their numbers have recovered or are still declining.

That's when they discovered the virus was capable of inflicting long-term damage on populations.

"For about half of the species that were affected, the impact is not just during the year when West Nile virus arrived, but we saw its influence for many years afterwards," George says. "And we found this occurring all over the United States."

Around half of the species affected by the virus have managed to recover after the initial onslaught of the disease, but the other half was not so lucky and experienced persistent declines.

"Seeing the disease's persistence was a shock," says study co-author Ryan Harrigan, an infectious disease biologist at UCLA. "We don't know why these species are unable to recover. It's alarming."

"Is it because of differences in their immune systems or their habitats or the amount of their exposure to [WNV]?" Harrigan asks. Researchers are looking more closely at the regions they've identified as those where certain avian species are still dying from the disease to see whether they can spot the reason.

One such species is warbling vireos (Vireo gilvus); Harrigan and his study colleagues say they estimate West Nile virus has killed around 15 million vireos, almost a third of the original population of 49 million.

The study suggests West Nile virus is just the first example of what future infectious diseases may do to America's birds, a finding that disturbs George.

"Birds are part of our national heritage," he says.

Photo: Len Blumin | Flickr