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West Nile is a mosquito-transmitted infection that turned up in North America in 1999. Since then, outbreaks have occurred across the United States each summer -- with more than 47,000 reported cases overall, according to the U.S. Centers for Disease Control and Prevention.
However, the true number is estimated to be much higher -- more than 3 million -- largely because most people with West Nile either do not get sick, or have flu-like symptoms such as a fever and headache.
A small number -- fewer than 1 percent -- develop serious inflammation of the brain or surrounding tissues. About 10 percent of those patients die, according to the agency.
The new study suggests that some patients have lingering issues up to eight years later. And that may be true even among people whose initial infection was never recognized, the researchers said.
Of 117 people the study evaluated, nearly half had some kind of symptom during neurological exams. Most often, that meant decreased muscle strength, problems with reflexes or tremors.
In addition, 22 percent showed signs of "impairment" on memory tests.
It's not possible to definitively point the finger at the patients' past West Nile infections, senior researcher Dr. Kristy Murray noted.
However, these patients developed neurological issues at a much higher rate than would be expected in the general population, said Murray, an associate professor at Baylor College of Medicine in Houston.
In addition, her team found evidence of brain-tissue thinning among 30 people who underwent MRI brain scans.
And when the researchers focused on those who'd developed severe West Nile infections, they found tissue degeneration in several areas of the brain. That included regions related to movement and balance, plus essential functions like breathing and speaking.
Murray said the tissue damage was consistent with what could happen after an infection causes severe brain inflammation.
Greg Ebel is a professor of microbiology, immunology and pathology at Colorado State University.
He agreed there are some questions about whether the symptoms in these study patients can be pinned on West Nile itself. However, Ebel called the findings "important," and said more research is needed to understand what's going on.
"It's important for us to know the extent and nature of the damage to the central nervous system," Ebel said.
The findings are based on 262 Houston-area residents who enrolled in a long-term study of West Nile starting in 2002. All had a history of the infection, but had suffered varying degrees of symptoms. Some weren't sickened at all, and had their infection discovered during routine blood-donor screening.
Of the 57 people who showed neurological issues, seven had never noticed any symptoms from their West Nile infection, according to the researchers.
That was a "surprise," Murray said, and it's unclear whether the infection directly caused the neurological symptoms in those people.
Unfortunately, there is no treatment to reverse the damage seen in some of these patients, according to Murray. But "supportive care" might ease symptoms like muscle weakness or tremors, she suggested.
Ebel said the findings underscore the importance of preventing West Nile infections in the first place.
Avoiding mosquito bites is key, he said. The CDC advises Americans to be aware of West Nile "activity" in their local area, and use insect repellent and clothing to ward off mosquitoes when outdoors.
To Murray, the study argues for more investment in finding a West Nile vaccine.
"This is impacting people long term," she said, "and we still have no vaccine for it."
The findings were scheduled for presentation Tuesday at the annual meeting of the American Society of Tropical Medicine and Hygiene, in Baltimore. Studies presented at meetings are generally considered preliminary until published in a peer-reviewed journal.
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