A virus new to the continent travels across the country in the bodies of mosquitoes, making the insects' bite potentially deadly to birds, horses and humans.
People strike back by spraying pesticides to kill mosquitoes.
Is the battlefield a mess? How has the ecosystem borne the attack and counterattack?
The answer, as far as anyone knows, is that there's been no major environmental upheaval.
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In the six years since West Nile virus was recognized to be in North America, wildlife scientists say they can point to no dramatic disruptions to the environment, either as a direct result of the virus or as a result of pesticide treatments to stem its spread.
Birds are most susceptible, but no hard evidence exists that the disease has driven any species toward extinction. No predators of mosquitoes have been known to starve en masse because chemical fogs killed their meal.
That's not to say disruptions haven't occurred. Some local populations of birds, especially crows and magpies, seem to have been hit hard by the pathogen, and some beneficial insects caught in spray zones probably have been poisoned along with mosquitoes.
But for the most part, effects among wildlife are unseen - largely because few people have looked in methodical fashion.
"Basically, we don't really know," said Emi Saito, West Nile virus surveillance coordinator at the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis.
The effort to measure effects is scattered, Saito said. "There are very few wildlife populations that have long-term monitoring," she said.
To get a grip on whether changes in the environment are related to the virus or pesticide treatments, Saito said, scientists need to know historical patterns.
The National Wildlife Health Center keeps a list of animals that have been found infected with the West Nile virus. The list encompasses more than 300 species of birds, 34 species of mammals and two species of reptiles.
The animals on the list are not equally vulnerable. For example, the pathogen is highly lethal to birds in the corvid family - including crows, magpies and jays - but isn't known to sicken dogs or cats, though it may be in their blood.
West Nile virus is, above all, a disease of birds - with humans and horses as incidental victims - but most bird monitoring efforts have been geared toward protecting human health, not trying to understand the illness in birds.
"The dead-bird surveillance programs are biased. They're geared toward those large species that occur near people," said Walter Boyce, director of the Wildlife Health Center at the University of California, Davis.
As it turns out, though, some of the birds that live around people also are exquisitely susceptible to the virus. The yellow-billed magpie is a case in point.
The boisterous birds with showy black and white plumage make their home mostly in the Central Valley - which happens to be the nation's West Nile virus hot spot this year.
Boyce and Holly Ernest, a UC Davis wildlife veterinarian, say they're very concerned about the yellow-billed magpie because of its limited range.
"They're a separate species unto themselves, found only in California and found nearly exclusively in prime mosquito habitat and nowhere else," Ernest said.
While the scientists have no proof that the magpies' numbers are in serious decline, they know that dead magpies by the thousands have been reported.
Dale Steele, program manager for the state Department of Fish and Game's species conservation and recovery program, said that before the arrival of West Nile virus, the yellow-billed magpie population was considered stable at 100,000 to 200,000 birds. The department plans to convene experts to discuss whether the magpie merits formal monitoring.
Boyce said his concern for birds has led him to consider the possibility that spraying, although it is being done to protect people, may protect birds as well.
He also said the things that eat mosquitoes, including bats and dragonflies, are unlikely to go hungry as a consequence of this summer's anti-mosquito treatments, because mosquito predators tend to eat any number of other things.
However, Boyce said, night-flying insects such as moths and bugs that hang out on top of leaves rather than under them are vulnerable to the spraying done on recent evenings by the Sacramento-Yolo Mosquito and Vector Control District.
That's one of the reasons lovers of natural landscapes object to or question the spraying.
Sharon Lawler, an entomologist at UC Davis, has researched the fallout from mosquito pesticide treatments in Northern California wetlands. In a study published in 1999 in the Journal of the Mosquito Control Association, Lawler and colleagues monitored the effects on fish and non-target species of three kinds of pesticides, including one similar to that being used in Sacramento.
They found that the abundance of flying insects shrank immediately after pesticide treatments but rebounded in 48 hours. They concluded that mosquito control efforts using "ultra-low volume" sprays were unlikely to have substantial effects on aquatic insects or fish in seasonal wetlands.
Lawler acknowledged the researchers didn't look at whether beneficial insects may have been more severely hit than pests.
"It's really hard to prove that there's no effect," she said. "You could do studies from here until the next century looking at everything ... . I can't say we're not losing some pollinators or some beneficial predators."
However, she said, "Applications (of pesticide) that are limited in time and space don't appear to have lasting effects."
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