The study found more than half of the patients who had symptoms -- ranging from fatigue and weakness to seizures, paralysis and tremors -- still had symptoms at the end the first year. Recovery -- if any -- appeared to take place in the first two years following infection.

"Once they hit two years it completely plateaus. If a patient has not recovered by that time, it is very likely they will never recover," study lead researcher Kristy Murray of the University of Texas Health Science Center at Houston said in a statement.

About 40 percent of patients in the study continued to experience symptoms five years after infection. Some long-term damage included memory loss, loss of balance and tremors. New-onset depression was reported by 31 percent of patients.

Since 80 percent of those infected with West Nile do not experience symptoms, the study included only those patients with symptoms. Half the 108 patients studied experienced encephalitis due to infection and another third presented with meningitis.

The findings were presented at the International Conference on Emerging Infectious Diseases in Atlanta.