St. Tammany Parish, Louisiana Ð with 50,000 acres of marshland and 300 miles of home front ditches carrying septic tank effluent where the southern house mosquito breeds Ð has become an epicenter of the West Nile virus. Of the 25 people who died of the virus in Louisiana last year, 4 lived in St. Tammany.

West Nile first struck the northern hemisphere in Queens, N.Y., four years ago and killed four people. This year, all 50 states are warning of an outbreak from any of the 30 mosquito species known to carry it. From 62 severe cases in 1999, confirmed human cases of the virus spread to 39 states in 2002, and it killed 284 people.

... It has been found in horses from Alabama and Arkansas to Minnesota and North Dakota, in dozens of dead crows and in a bald eagle near Allentown, Pa. On Wednesday, the Pennsylvania Department of Environmental Protection found infected larva near Pittsburgh, Ronald Ruman, a spokesman, said. In Louisiana, 52 dead birds have tested positive.

... No West Nile vaccine has been developed for humans, although there is one for horses. The virus cannot be stopped by quarantining people, because birds and mosquitoes that carry it cannot be quarantined.

"There are no revolutionary strategies out there that are on the brink of implementation," said Dan O'Leary, an epidemiologist at the infectious disease center.

... Each year, more mosquito species are found to carry the virus. Near La Crosse, Wis., said Linda Glaser, the state's West Nile virus surveillance coordinator, it has been found in the tree hole mosquito, named for its customary habitat. Ms. Glaser found the state's first infected horse in May this year. It survived, but another found this month had to be put to death.

Linn Haramis, entomologist with the Illinois Department of Public Health, said, "There is a certain amount of responsibility for citizens to protect themselves."