SLIDELL, La. - An outbreak of West Nile virus has infected 58 Louisiana residents and killed four, prompting the governor to declare an emergency yesterday and ask for federal aid for more mosquito spraying across this hot, humid and swampy state.

The West Nile deaths are the first in the country this year and raise the national total to 22 since 1999, when the mosquito-borne virus was first detected in the United States.

The Louisiana victims were three men, ages 53 to 75, and an 83-year-old woman, all of whom died in the past few weeks, state health officials said.

Twelve people remained hospitalized, four of them in intensive care.

"This is only the beginning," warned Dr. Raoult Ratard, the state epidemiologist.

Gov. Mike Foster declared a statewide emergency, hoping to get $3 million to $5 million in federal money for parishes that are rapidly using up their mosquito spraying budgets.

"There ought to be some kind of relief. This is an emergency situation," Foster said Thursday on his weekly Live Mike radio show.

The virus is carried by mosquitoes that feed on infected birds and other animals.

Most people bitten by the infected insects do not get sick, but the virus can cause flu-like symptoms and encephalitis, a potentially fatal swelling of the brain, in the weak and elderly.

Until June, Louisiana's only human West Nile case had been last year, in a homeless man in suburban New Orleans. He survived.

Experts said the virus has now spread to virtually every part of watery Louisiana, where mosquitoes are jokingly called the state bird.

The outbreak here is the deadliest since the virus killed seven people and hospitalized 55 others in the New York City area in 1999.

Across southeastern Louisiana, people have been putting fresh water in birdbaths and dumping water out of flowerpot saucers to deprive mosquitos of the standing water they need to breed.

St. Tammany Parish, north of New Orleans, has sprayed for mosquitoes every night for the past month, three to four times more often than is typical during the summer.

Sales of mosquito traps and insect repellents have been brisk.

In Baton Rouge, where police are investigating a string of three slayings, Louisiana State University employee Patty Scuotto said:

"The joke around here is, if the serial killer doesn't kill you, the mosquitoes will."