The finding comes from five years of data on 108 Houston-area residents who came down with severe West Nile disease after infection with the mosquito-borne virus. Murray's team evaluated the patients every six months.
A year after their West Nile virus infection, 60% of these patients still had serious symptoms. Five years later, 42% had not fully recovered – and weren't getting any better.
These lasting symptoms included fatigue, muscle weakness, depression, difficulty walking, memory loss, and personality change.
"For patients with severe West Nile disease, long-term neurological complications are significant and should be expected," Murray tells WebMD. "At around the two-year mark, the recovery that is going to happen has happened. Those with symptoms continue to report them. Some say they get worse at times, not better."
This doesn't happen to the vast majority of people who get bitten by a mosquito carrying West Nile virus.
"Eighty percent of people infected with West Nile virus have no symptoms — they don't even know they are infected," Murray says. "Most of the other 20% have a mild, flu-like illness with no worries in terms of recovery. But here we see the tip of the West Nile iceberg: those with more severe neurological disease."
People over age 50 are more likely than younger people to get severe West Nile disease. But while the average patient in her study was 52 years old, Murray saw severe disease in children, too. One was only 6 months old; another was 11 years old.
Severe West Nile disease includes high fever leading to hospitalization, meningitis, encephalitis, and polio-like paralysis. West Nile meningitis affects the protective membranes covering the brain and spinal cord. West Nile encephalitis is a more serious infection of the brain. Murray and colleagues found that 60% of patients with West Nile encephalitis still have symptoms after five years.
One of the study's most disturbing findings is that 31% of people with serious West Nile disease reported clinical depression. Also disturbing is that 25% of patients reported personality changes. They became more irritable and angry.
"Their families said they had a short fuse, easily lost their tempers, and had decreased socialization," Murray said.
Patients also often had balance problems that made it difficult for them to walk normally. Other nervous system symptoms included loss of strength, sensory loss, hearing loss, posture problems, and tremors.
"People really need to protect themselves against mosquito bites. You never know where on the scale of West Nile infection severity you are going to fall," Murray says. "Just because people are younger does not mean they can't be severely affected. And if they are older, they definitely need to be more careful in terms of exposure."
Murray announced the findings in a report to the 2008 International
Conference on Emerging Infectious Diseases, held March 6-19 in Atlanta