Lead author David Crowder, a Washington State University entomologist, said roughly 1-in-5 infected people experience a fever, headache, body aches and, in some cases, a skin rash and swollen lymph glands. One in 150 people can get a high fever, headache, neck stiffness, disorientation and neurological problems.
Most efforts to figure out the ecological workings of the virus have focused on reports of infected people, "a crude indicator at best," Crowder said. Almost all victims have no symptoms or are misdiagnosed, while others can be infected far from where they file a report, he said.
Crowder and fellow entomologist Jeb Owen, other Washington State University colleagues and the state Department of Health, merged data from a variety of sources, including West Nile infections in humans, horses and birds, surveys of virus-bearing mosquitoes, breeding bird surveys and detailed land use maps and climate data from around the Northwest.
The study, published in the journal PLOS ONE, found that habitats with high instances of the disease in horses and birds also have significantly more mosquitoes -- as well as American robins and house sparrows, the two bird species implicated the most in the disease's transmission.
"These same habitats are also resulting in much higher rates of infection within mosquitoes themselves," Crowder said in a statement.
It's still unclear why the habitats would create such a perfect storm for the virus, the study said.