More than half of the patients in a recent study of the consequences of West Nile fever reported those and other symptoms more than a year after being diagnosed, according to a report published in the journal Clinical Infectious Diseases.

West Nile fever is a relatively benign result of infection by the virus compared to the full-blown version that can sometimes be fatal.

Dr. Paul Carson of MeritCare Health System and associate professor of medicine with the University of North Dakota School of Medicine and Health Sciences led a team of investigators who looked into the ongoing health problems experienced by patients who had been afflicted by the less severe, flu-like illness.

They evaluated 49 patients for neurological function, quality of life, fatigue and depression about 13 months after they were diagnosed with either West Nile fever or one of the more severe diseases.

The study group with the milder West Nile fever diagnosis were just as likely to suffer health problems later as those who had been hospitalized with related illnesses such as encephalitis or meningitis, the researchers found.

Eighty-four percent of patients continued to report problems with fatigue, 59 percent reported memory problems, and 49 percent reported ongoing weakness or muscle pains.

Nearly half of patients scored low on the physical component of the test for overall health, and a third of patients scored low on the mental component. On the test for depression, one out of four scored in the range of moderate to severe depression.

Tremors were observed in 20 percent of patients. Tests of brain function showed some impairment in memory processing and fine motor skills in up to 30 percent of patients.

The study suggests that patients with West Nile fever and those with the more severe forms of infection may have the same areas of the brain affected, the researchers said.

"We knew before that West Nile encephalitis was a serious health threat, but we didn't appreciate how much ongoing morbidity there is for West Nile fever, which is much more common," remarked Dr. Carson., who called the virus "a substantial public health threat."

West Nile virus infects birds, primarily, but it can also be contracted by humans, horses, cats, bats, chipmunks, skunks, squirrels and domestic rabbits. Humans usually get it from the bite of an infected mosquito.

In most cases, infection withWest Nile virus doesn't result in any symptoms at all. However, about 20 percent of infected individuals develop West Nile fever, while fewer than 1 percent of cases are rated severe, leading to meningitis or encephalitis.

First discovered in the US in 1999, the virus quickly spread across the country. By 2003, the number of human cases of the disease surpassed 9,000, and there were 264 deaths nationwide.