Just over a year ago, Kevin Parks and a friend were fishing a calm section of western Colorado's Gunnison River in a two-man pontoon. An insurance agent and avid outdoorsman in his 50s, Parks had slathered on mosquito repellant earlier in the day, but by dusk, he had sweated most of it off.

He never felt the bite, but within a week, he felt its effects. He came down with flu-like symptoms, including high fever and vomiting. A rash appeared on his chest, and his wife took him to the nearest emergency room, 30 miles away. There, he lost his vision and became unconscious. He was diagnosed with encephalitis - inflammation of the brain, caused by the mosquito-borne West Nile virus.

For days, an IV pumped morphine into him to alleviate the excruciating pain in his head and right shoulder. Finally he began to improve, moved home and over several months worked his way from a wheelchair to a walker to a cane. Today, Parks walks without support, but continues to have what he calls "West Nile days," when his back hurts, or he feels mentally fuzzy.

Parks was one of 345 people in Colorado diagnosed with West Nile virus in 2006 - and one of 66 who contracted one of the more serious strains that affect the nervous system. After entering the U.S. via New York City in 1999, the virus has spread across the country and seems to have found a home in the West. This year, there were almost 1,600 cases in the 11 Western states - more than half the total for the entire country.

The West offers a favorable habitat and climate for the West Nile virus-bearing Culex mosquito, particularly in areas where flood irrigation provides suitable breeding grounds. "There are always holes, divots and pockets where water settles and is covered by vegetation," making it difficult to kill the larvae, according to John Pape of the Colorado Department of Health and Environment. His state leads the West with 544 reports of West Nile this year, including 95 neuroinvasive cases and six fatalities.

The Culex tarsalis mosquito, the species most common in the rural West, is also one of the most effective transmitters of West Nile - good not only at replicating the virus, but also at "giving it away," says Emily Zielinski-Gutierrez, with the Vector-Borne Infectious Diseases Division of the Centers for Disease Control.

Westerners are also probably more likely to be exposed to mosquitos, and the virus, says Zielinski-Gutierrez, because they spend more time outdoors. After a peak in 2003, when cases nationwide totaled 9,862, WNV numbers are trending downward, even, generally, in the West. But there is no discernible pattern and no single cause, according to Zielinski-Gutierrez.

In Idaho, cases plummeted from 996 in 2006 to 104 this year. Dr. Leslie Tengelsen, Idaho's deputy state epidemiologist, said that although nothing has been studied scientifically, the decrease is likely due to drier weather, stronger abatement efforts, better public education, and fewer crows and magpies. The birds amplify the virus: A mosquito gives the virus to the bird, other mosquitoes bite the bird and catch the virus and then spread it on to other hosts. But birds also die from the disease, which possibly accounts for their population declines.

About 20 percent of people bitten by an infected mosquito will have some symptoms of West Nile fever. Less than 1 percent of those will suffer from a neuroinvasive form of the virus, including encephalitis, meningitis (inflammation of the membrane around the brain and spinal cord) or a polio-like disease called "acute flaccid paralysis." People over 50 or with a compromised immune system are most vulnerable to the neuroinvasive strains, with those who have had an organ transplant at particular risk.

There is no cure for West Nile, but a vaccine for horses has brought disease numbers down. A human vaccine is likely to be developed in the next year, but Pape and Zielinski-Gutierrez question how readily available it will be, as drug companies may not find it profitable enough to manufacture in quantity.

In response to West Nile, abatement programs across the West have kicked into high gear. Idaho nearly doubled the number of mosquito control districts this year, from 13 to 23. Integrated abatement programs - including the use of larvicides and adulticides, bolstered by surveillance efforts to pinpoint "hot spots" - are generally deemed effective in keeping the numbers of mosquitoes down.

For the most part, communities have accepted the pesticide use. But in Delta County, where Kevin Parks was bitten, insecticide spraying has sparked a conflict between those who support it and those more concerned about the downsides of the chemicals and their ultimate effectiveness.

As for Parks, he thinks any risk posed by insecticides is outweighed by the agony of the virus.

The author is HCN's online editor.

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