The American crow was hit hardest, loosing 45 percent of its numbers across the United States.

In all, 13 of 20 species studied reached 10-year population lows after a wave of West Nile virus infections among humans in 2002 and 2003, says the study, published in the British journal Nature.

Only two of the seven worst-affected species have since recovered to their previous levels.

There have been more than 24,000 human cases in the United States, with some 1,000 fatalities. Symptoms include headaches, eye pain, skin rashes and extreme fatigue, and in some cases those infected develop meningitis or encephalitis.

Scientists have long suspected that the virus has also decimated certain "peri-domestic" bird species, meaning those that share habitat with human beings. Tens of thousands of specimens found dead in the wild, in zoos and in family bird cages have tested positive for the disease over the last four years.

But the challenge facing a trio of researchers led by Shannon LaDeau of the Smithsonian Migratory Bird Center was how to disentangle the impact of West Nile virus from the other factors, such as development pressures, climate change or long-turn trends in population.

To test the hypothesis that the virus was especially lethal to birds, they constructed a series of predictions for 20 species based on Breeding Bird Survey data going back 26 years, varying sensitivity across species to the disease, and the pattern of human outbreaks.

The results matched up at both ends: there was a significant decline in the population of six species where the virus was predicted to have high impacts, and no decline in five species where low impacts were forecast.

The study "suggest that West Nile virus could potentially change the composition of bird communities across the entire continent," noted environmental biologist Carsten Rahbek in a commentary, also published in Nature.

Rahbek cautioned that the study is not conclusive, and that more research is needed to better understand the impact of West Nile virus on large-scale changes in bird populations.

But he also underlined that delicately balanced ecosystems can be easily disrupted by infectious diseases spread from other parts of the world.

The migratory passenger pigeon of North America, he noted, was once the most abundant bird on the continent with an estimated population of three to five billion. "But it was driven to extinction within a century by human agency and, possibly, diseases."

Other scientists have suggested the spread of the West Nile virus to North America is also a warning sign about the possibly devastating health consequences of global warming, which may be facilitating the spread of tropical, insect-born diseases into more temperate zones.

West Nile virus was first isolated in Uganda in the 1930s, and has since been reported in much of Africa, Europe, the Middle East along with west and central Asia.

It appeared in North America in 1999, causing fatal cases of encephalitis in humans and horses in New York state before spreading across much of the United States.

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Smithsonian Migratory Bird Center
Epidemics on Earth - Bird Flu, HIV/AIDS, Ebola