CALIFORNIA
West Nile could get worse, expert warns
Virus outbreak could turn into major epidemic
Friday, February 24, 2006
America's top expert on West Nile virus warned of growing complacency about the mosquito-borne disease after it completed its predicted sweep across the country last year with cases peaking in Northern and Central California.
As the virus settles in among wild birds and mosquitoes in nearly all the nation's most populated regions, "the potential for future outbreaks has never been greater,'' said Dr. Lyle Petersen, director of vector-borne infectious diseases for the federal Centers for Disease Control and Prevention.
Petersen and 400 other biologists, entomologists and disease prevention specialists are meeting in San Francisco through today to take stock of last year's seasonal outbreak of West Nile virus, which has moved west since it first appeared in New York City in 1999.
Nearly 3,000 cases of the virus were recorded throughout the country in 2005, with the highest concentration in California, where 928 residents contracted the illness and 18 died.
With the numbers trending down from the worst outbreak years in 2002 and 2003 -- when 14,000 Americans were stricken and 548 died in two seasons -- the virus is beginning to seem less fearsome.
"I think people are becoming complacent,'' Petersen said during a meeting with reporters. "Not so much in California, but in the other parts of the country. ... That is a huge mistake. These diseases are episodic.''
West Nile virus was first detected in Uganda in the 1940s and turned up in a more virulent form in Eastern Europe in the 1990s. As the virus marched across the United States, it killed 782 Americans, while more than 8,000 suffered neurological problems ranging from persistent headache to a form of paralysis similar to that caused by polio.
The virus is now considered endemic in the United States, like its cousin St. Louis encephalitis, which has periodically afflicted different parts of the country for 80 years. West Nile virus and St. Louis encephalitis reside primarily in birds and are spread by the same families of mosquitoes. Humans and horses are incidental victims of a biological war between the birds and the bugs.
What worries Petersen is that St. Louis encephalitis has proved to be unpredictable, and occasionally explosive, with a huge national outbreak occurring in 1975. Now that West Nile virus has settled in among us, he is concerned that it, too, will become unpredictable -- with more dire consequences. "It would be a massive epidemic on a scale not seen before,'' he said.
West Nile virus remains a much more lethal bug than its cousin. Birds infected with West Nile have hundreds of times more viruses in their bloodstream than those who contract St. Louis encephalitis. "West Nile virus,'' Petersen said, "is SLE on amphetamines.''
California scientists are carefully studying the outbreaks of the past two years, trying to determine if they can blunt its effects by directing mosquito-control efforts to hot spots as they turn up.
Among the discoveries state health researchers have made is that diabetics may run four times the risk of West Nile virus as the rest of the population. Cynthia Jean, who coordinates research at the state virus lab in Richmond, said that a weakened immune system, or a more porous barrier between the brain and blood vessels that run through it in diabetes patients, may explain why diabetics are more vulnerable.
Dr. Carol Glaser, director of the state virus lab, said research that pinpoints who is likely to be at higher risk for the disease could make it easier to target prevention-education efforts. "It is very difficult to tell 36 million Californians to wear DEET every day,'' she said.
Dead birds have proven to be a remarkably effective predictor of human outbreaks. Using a computer program originally developed in New York and Chicago, the California Department of Health Services is pooling information from citizens calling its dead birds hot line -- (877) 968-2473 -- and tracking West Nile with remarkable accuracy.
Ryan Carney, coordinator for the state's West Nile Virus Dead Bird Surveillance Program, is presenting a poster at the conference today that details how a computer-mapping project has linked clusters of dead bird reports to human cases 83 percent of the time. In half those cases, dead bird hot spots were identified a full month before the first human infections.
Sacramento County, which was hit hardest by West Nile virus last year, served as a testing ground for the system, which divides regions into a detailed grid of quarter-mile squares. After the computer spotted a cluster of reports in Citrus Heights, biologists set mosquito traps and within two days had caught the first infected insects in the county. The first two of what became 177 cases there were found in the same quarter-acre square as the dead-bird reports.
"This year," Ryan said, "we are going to do the entire state."
Ryan's research also provides tantalizing clues that suggest California's intensified mosquito control efforts -- funded by a $12 million supplemental appropriation -- may have saved lives. Using the same gridded maps, he compared two sections of Sacramento County that were subject to controversial aerial spraying of insecticide with two similar gridded neighborhoods that were not. Two weeks after the spraying began, there were no further human cases in the sprayed zones, while infections continued in the two control areas, eventually accounting for 20 percent of the county's total cases.
Vicki Kramer, director of vector-borne diseases for the California Department of Health Services, said that mosquito control is a continuing process in the state and that the information from the computer system can help pinpoint areas that need attention -- either by using larvicides and mosquito fish to kill young mosquitoes or sprays to knock down adults.
"West Nile virus is here to stay in California,'' she said. But early-season mosquito control, in particular, she said can interfere with the life cycle of mosquitoes that feed on infected birds.
E-mail Sabin Russell at srussell@sfchronicle.com.
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