West Nile virus on downswing; good recovery rates found

Insect repellent is a first-line defense against the virus. DEET is a good choice, a study finds, because mosquitoes don't like the smell.

By Susan J. Landers — Posted Sept. 15, 2008

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Although patients infected with West Nile virus can contract serious central nervous system diseases, most don't, and long-term prognoses are generally good, according to a new study.

Published in the Aug. 19 Annals of Internal Medicine, the study was described by the authors as the first to examine comprehensively a large population of patients infected by the mosquito-borne illness. "We found that both physical and mental functions, as well as mood and fatigue, seemed to return to normal in about a year," said lead author Mark Loeb, MD, professor of pathology and molecular medicine at McMaster University in Ontario, Canada.

"One of the take-home messages for primary care physicians is that the study gives them a trajectory of the course of the disease in their patients," said Dr. Loeb.

West Nile virus is endemic in the United States, having rapidly spread across the country after it was identified on the East Coast in 1999. Cases of human infections have now been reported in all states except Hawaii, Alaska and Oregon, according to the Centers for Disease Control and Prevention.

The virus can trigger a broad range of symptoms. About 80% of those who become infected never get sick, so the disease can be difficult to diagnose. The remaining 20% have symptoms ranging from a mild flu-like illness to neurological problems. About 1% of those who get sick develop serious issues, including meningitis, encephalitis and acute flaccid paralysis, Dr. Loeb said.

The researchers followed 156 patients from 2003 to 2007 and recorded patterns of the physical and mental effects of the disease. Preexisting health conditions such as peripheral vascular disease, chronic pulmonary disease and diabetes were found to be detrimental to patients' recoveries.

Dr. Loeb is now involved in a study funded by the National Institutes of Health to identify the genetic profiles of those who have become very sick from the disease.

The project, which will ultimately enroll 2,400 people, includes participants in the U.S. and Canada.

"The thing about West Nile is there are people in their 40s and 50s who get sick yet have relatively few comorbidities and are relatively healthy," said Dr. Loeb. "The vast majority don't even have symptoms. So the question is: Are there genes that predispose people to severe complications?"

Dr. Loeb hopes to have some answers in about a year.

The CDC had received reports of 236 cases -- both mild and severe -- of West Nile virus illness from 28 states, as of Aug. 19. Projections estimate the total count for 2008 will be well under the 2007 number of 3,630 and a far cry from the 2003 tally of 9,862 cases. More than 1,000 U.S. deaths have been attributed to West Nile virus since 1999.

Spray mosquitoes away

To continue to keep the incidence rates as low as possible, the CDC promotes the use of repellents for skin and clothing. Among the products recommended are repellents containing DEET and picaridin, considered conventional repellents, or oil of lemon eucalyptus and IR3535, termed biopesticide repellents, which are derived from natural materials.

Researchers recently discovered why DEET works: Mosquitoes don't like the smell. "We found that mosquitoes can smell DEET, and they stay away from it," said study investigator Walter Leal, PhD, professor of entomology at the University of California, Davis.

The finding corrected the long-standing but erroneous belief that an application of DEET jammed the insects' ability to smell, which is accomplished via their antennae, so humans could wander safely in their vicinity without being detected.

The findings are in the Aug. 18 Proceedings of the National Academy of Sciences.

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External links

The Centers for Disease Control and Prevention on West Nile virus (link)

"Prognosis after West Nile Virus Infection," abstract, Annals of Internal Medicine, Aug. 19 (link)

"Mosquitoes smell and avoid the insect repellent DEET," abstract, Proceedings of the National Academy of Sciences, Aug. 18 (link)

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