That worrisome finding comes from a study of 49 North Dakotans who suffered at least some symptoms after a West Nile infection in 2003.
Four of five people who get West Nile virusdon't have any symptoms.
One in five, however, gets a fluflu-like illness called West Nile fever. It's supposed to be what doctors call a "benign, self-limited illness."
But it may be much worse, University of North Dakota researcher Paul J. Carson, MD, and colleagues find.
"Our patients continued to show clinical abnormalities one year after illness," the researchers report. "Patients with 'milder' illness -- that is, non-hospitalized patients with West Nile fever -- demonstrated equivalent rates of these abnormalities."
This means West Nile fever may leave lasting damage in its wake, Carson and colleagues suggest. They report their findings in the Sept. 15 issue of Clinical Infectious Diseases.
Lingering West Nile Symptoms
The year 2003 was a big one for West Nile virus in the U.S. Nearly 10,000 confirmed infections were reported to the CDC -- 617 of them in North Dakota.
Carson's team tried to track down the 122 North Dakota patients known to the state at the time of their study. All of these people reported some kind of West Nile illness -- either West Nile fever or more severe West Nile encephalitisencephalitis.
The researchers were able to persuade 49 of these patients to submit to a battery of tests, including measures of psychological and neurological function. What they found was scary.
One patient had the most severe kind of West Nile illness: polio-like paralysis.
About a fifth of the study's patients had a confirmed West Nile infection of the brain or lining of the brain and spinal cord (encephalitis or meningitis).
The other four-fifths had West Nile fever.
About a third of the patients had been hospitalized.
A year after their infection, the West Nile patients commonly reported fatigue, memory problems, weakness, headache, joint pain, and balance problems.
Half reported poor physical health. Nearly one in four reported depression. One in five developed a tremor.
However, fewer than one in 10 reported moderate-to-severe disability.
West Nile Brain Damage Common?
Perhaps the most chilling findings came from psychological and neurological tests.
Among the patients whose West Nile illness wasn't severe enough to put them in the hospital, 15% had moderate-to-severe impairment of executive function. That's the ability to plan, inhibit behavior, and pay sustained attention.
More alarming was the finding that nearly 70% of the patients had abnormal results on a finger-tapping test to look at motor speed. In 43% of the patients, this impairment was severe. These patients were more likely to suffer depression than were the other patients.
Despite these problems, most of the patients were able to return to a "reasonable level" of daily function. This may be why previous studies -- which mostly looked for disabilities -- find more positive long-term outcomes for West Nile virusinfection.
Carson and colleagues suggest West Nile fever "is not a self-limited benign illness, as previously thought." They suggest it may be a brain infection that leaves behind long-lasting damage.
West Nile virus is spread by mosquitoes. There's no treatment or cure for the infection, so avoiding mosquito bites is the best way to deal with West Nile disease.
August and September are the biggest months for West Nile virus.
This year has been relatively quiet for West Nile so far, with 388 cases of human illness reported in 26 states as of Aug. 15.
But if previous years are any guide, those numbers are sure to go up before mosquitoes wane. In 2003, for example, there were 393 human infections as of Aug. 13 -- and 715 by Aug. 20.