WASHINGTON - West Nile virus may well complete its coast-to-coast spread this summer, infecting large numbers.

There's no good way to predict, as the deadly virus is from a family that's notoriously fickle. But during last year's record-setting epidemic - more than 4,000 people became ill and 274 died - only a handful of states escaped human illness. Even some of those harbored infected mosquitoes and birds.

And no, the harsh winter in much of the country probably won't lead to a reprieve. Many mosquitoes can survive the cold by hiding out in places such as sewers, ready to start spreading infection once it's warm enough to re-emerge.

Another myth: that if crows aren't dying in your neighborhood, West Nile probably hasn't spread there yet. Dead birds of any species are suspicious. But of more concern are birds West Nile doesn't easily kill, such as common house sparrows. They harbor far more of the virus in their blood than crows do, yet few die - offering a highly infectious feeding trough for mosquitoes who bite them and then bite us.

Tackling this virus "is unbelievably complex," says Dr. Lyle Petersen of the Centers for Disease Control and Prevention, the nation's chief West Nile specialist. The one sure discovery is that "where West Nile has been, it stays."

He cautions that he has no crystal ball to accurately predict if West Nile, part of a family of mosquito-borne flaviviruses that can rapidly wax and wane, will prove as bad this year.

But Petersen says another large epidemic "would not be surprising," with West Nile hitting each of the 48 contiguous states. Not counting Alaska and Hawaii, only nine states have escaped human illness so far.

Of particular concern is the West, where the virus was just encroaching when winter hit.

Only four states - Oregon, Nevada, Utah and Arizona - appear free of the virus, which first appeared in New York in 1999. Washington, Idaho and New Mexico, along with Maine and New Hampshire, have confirmed infected animals, birds or mosquitoes, but have reported no human cases, according to the CDC.

Unable to stop West Nile's inexorable march, health officials' challenge now is to prevent serious illness.

The CDC is analyzing some dismal data showing that's a hard job: During the height of last summer's epidemic, less than half of people surveyed took any precaution to avoid mosquito bites - and only about a third used repellent containing DEET, which provides the best protection against bites.

Nor are mosquito bites the sole threat: Last fall, scientists discovered West Nile could spread through blood transfusions and organ transplants. By July, two companies hope to have begun nationwide testing of a way to detect West Nile in donated blood.

It also can spread to a developing baby when a pregnant woman becomes infected. In the one documented case so far, the infected baby was born with severe brain defects. The CDC plans more pregnancy tracking this year, and cautions pregnant women to be especially vigilant against mosquitoes.

West Nile can cause potentially fatal brain inflammation, either meningitis or encephalitis. It can strike at any age, but those most at risk are over 50.

For every case of serious disease, 150 more people are thought to be mildly infected - they have either a flu-like illness or, in most cases, no symptoms at all.

There is no treatment, just supportive care. A vaccine for people will require years more research, although there is one for horses - also hard hit by West Nile - and some zoos are testing one for endangered birds.

Once infected, symptoms or not, you're thought to be immune for life. So eventually, West Nile epidemics should become rare here.

Meanwhile, local governments are gearing up programs to kill mosquitoes as eggs and after they hatch.

The average person can take some simple steps to avoid mosquito bites, says Dr. George Pankey of the Ochsner Clinic Foundation in Louisiana, where warming weather has him already doling out anti-bug advice.

•Wear a mosquito repellent containing DEET, usually labeled as "N,N-diethyl-m-toluamide." DEET-free types aren't nearly as effective. DEET is considered very safe, but follow instructions for the amount to apply, especially for children.

•Police flower pots, unused pools, blocked rain gutters, tires and other mosquito breeding grounds. Eggs can hatch in only a tiny amount of water left standing a few days.

•Make sure windows have screens in good repair.

•Stay inside at dawn and dusk, prime biting times, or wear long pants and sleeves; use special mosquito netting when infants are outdoors.

•Don't forego repellent for anti-mosquito gadgets, which haven't yet been proven to reduce bites.

On the Net:

West Nile virus basics: http://www.cdc.gov/ncidod/dvbid/westnile/index.htm

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