Symptoms that linger for years. A paralysis reminiscent of polio. Blood donations that deliver an unwanted keepsake.
Five years after arriving in North America, the West Nile virus continues to confound medical researchers as a fast-evolving foe with potentially permanent consequences.
In a recently published survey of 35 New Yorkers hospitalized with a West Nile virus infection in 1999, researchers found that fewer than four in 10 had fully recovered one year later.
Among patients who were 65 or older at the time of their initial hospitalization, only five of 22 had fully recovered.
The results, appearing in the August issue of Emerging Infectious Diseases, suggest that West Nile infection can yield more severe long-term effects in older patients.
But researchers are seeing polio-like symptoms known as "flaccid paralysis" that tend to occur more often in younger patients, a discovery that Dr. Ned Hayes, a medical epidemiologist with the Centers for Disease Control and Prevention, deems one of West Nile's biggest surprises yet.
Other surprises arrived in 2002, Hayes said, when medical researchers learned that the virus could be transmitted not only through blood donations and transplants, but also from a pregnant mother to her child. In one instance, transmission probably occurred through breast milk.
Hayes stressed that the overall risk to babies appears to be quite low, noting that most women known to be infected have delivered healthy babies.
The discovery of transmission via blood donations has enhanced surveillance, with a new CDC-sponsored map tallying the number of West Nile-positive blood donors by state.
Hayes said depending on the timing of the positive test results, they could suggest regions where blood donor screening should be strengthened, or where human clinical cases may pop up.
In Mexico, a lack of human cases has raised eyebrows, especially since the virus has long since reached Latin America and the Caribbean.
As of Nov. 4, Mexico had confirmed only one human case out of 226 people tested nationwide.
An ahead-of-print study published online in Emerging Infectious Diseases could offer an intriguing explanation.
Researchers from Texas and Mexico City isolated a strain of West Nile virus from a raven in the Mexican state of Tabasco, and tested it and several variants for their ability to infect mice.
One variant, in particular, displayed a greatly reduced virulence.
Although the existence of related viruses in Mexico could have spurred greater West Nile immunity among its citizens, as Hayes notes, the study raises a tantalizing, though as yet unproven, possibility: the version spreading through that nation could be less lethal than the one sweeping across its neighbor to the north.
Nationally, the virus has claimed 576 lives since 1999, with 14,662 total infections. Maryland public health authorities have reported two cases of human West Nile fever in Maryland this year.
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